Mercury Battery Replacement
There are many older battery-powered instruments on the surplus market that require mercury batteries but these batteries have been banned for environmental reasons. Simply substituting another battery type may give unsatisfactory results; mercury batteries have an unusually stable output voltage over their life and many designs rely on this stability for proper operation.
The following circuit simulates a single mercury battery cell (1.35 volts) for low current loads and is powered by a couple of ordinary cells. (Two, 1.5 volt alkaline cells in series would be a typical power source.) The circuit automatically turns on when power is demanded so no power switch is necessary.
The reference portion of the LM10 supplies the desired output voltage to the op-amp section and this voltage may be adjusted by changing the 4.7 meg and 180k in the collector of the 2N4401. The output current flows through the emitter-base junction of the 2N4403, turning it and the 2N4401 on, activating the regulator. The 22 ohm, .01 uF and 4.7k help to stabilize the circuit for large capacitive loads. If the load is resistive, those three components may be replaced with a short but check for oscillations on the output. The 22 ohm may be reduced for higher current loads but below about 10 ohms, stability may become a problem if the load includes a large shunt capacitor. Increasing the input voltage will increase the available current but the LM10 will limit the ultimate current to about 20 mA so this circuit will not drive motors!
Also consider a shunt regulator when you can get to the other side of the power switch. The following circuit was used to supply about 500uA to a pH meter/simulator. Notice how the output of the op-amp is connected directly to the power pin. When the voltage across the op-amp exceeds 1.35 volts, the output stage sinks current, lowering the voltage. The 100 ohm resistor determines how much current is consumed. Determine how much current your circuit needs then select this value to supply the required current with about .05 volts across it (R = .05/I). For my 500uA load and with a fresh battery at about 1.6 volts, the voltage across the 100 ohm resistor is 1.6 - 1.35 = .25 volts and the current is about 2.5 mA, easily handled by an alkaline AA cell. The voltage on the battery can drop down to 1.4 volts before regulation is lost. For a higher current load or a lower drop-out voltage, lower the 100 ohm resistor value. The 57.5 k resistor was selected to achieve exactly 1.35 volts and consisted of two resistors in parallel.
This circuit can only supply low currents, too, since the LM10 must shunt whatever is not used by the load and it can only sink about 20 mA. So the load current can only change by 20 mA, max. That is fine for most gadgets but some devices might have momentary current demands in excess of that amount (especially if a motor is involved). One could add a super-cap across the output of these regulators to supply high transient currents.
For higher voltage situations, consider a simple voltage regulator:
The zener is selected to give the desired 8.4 volts output. The J108 can supply a few mA with virtually no voltage drop so this circuit will work in many mercury battery applications until the 9 volt battery drops below 8.4 volts. It is true that 8.4 volts is a little soon to be tossing a 9 volt battery. A 1.5 volt AAA cell could be added in series with the 9 volt battery to extend the life of the battery but a fresh alkaline will serve well for most of its useful life; just put the depleted battery in a radio or voltmeter to get the last bit of life. Keep in mind that in many of the devices that use a couple of mercury batteries, the higher voltage battery does not need to exhibit a precise voltage. My pH meter/simulator relies on the precision of the 1.35 volt battery, for example and the 8.4 volt battery simply powers an op-amp. This circuit loses regulation gracefully and the output voltage simply equals the battery voltage below 8.4 volts.
Thursday, July 30, 2009
split supply generator
Split Supply Generator
Occasionally a designer needs a dual power supply to power a circuit that is operating with signals near or at ground but the only available supply is a single polarity, usually positive. Many excellent IC solutions are available but a suitable solution for many projects may be constructed from "junk box" parts. The simple circuit below will generate about 9 volts and -4 volts from a single 5 volt supply with sufficient current to power a simple op-amp circuit. The positive voltage drops to about 7 volts when supplying 7 mA and the negative voltage drops to about 3.5 volts when supplying 3.5 mA (1k loads). Although this isn't exactly a +- 15 volt supply, this is plenty of voltage and current for many op-amp circuits and will allow the output of most op-amps to swing below zero volts and will allow most op-amp inputs to measure voltages below zero volts. This circuit uses the CD4049 which is a high current version of the CD4069 which will also work with somewhat lower current capability.
wpeF.jpg (120497 bytes)
The two inverters on the left generate a square wave and the other four inverters are connected in parallel to increase the current drive to the diodes. The diode on top clamps the voltage on the top capacitor at about 4.5 volts when the inverters go low. When the inverters go high, their output voltage is added to the 4.5 volts to give about 9.5 volts. The second diode rectifies this voltage to give a little over 9 volts on the output. The bottom two diodes work in the same way only the voltage on the first capacitor is clamped to about 0.5 volts on the positive swing and then goes down to about -4.5 volts on the negative swing of the inverters, giving about -4 volts out.
The prototype is operating at only 500 Hz to allow for the use of some old-fashioned germanium rectifiers that I have in large numbers. If more modern schottky rectifiers are used the frequency may be set higher by lowering the .001uF capacitor or the 1 megohm resistors. The 4, 330 uF capacitors are larger than necessary and a few uF will suffice if the frequency is raised to, say, 5 kHz (try 100k resistors or a 100pF capacitor). Yep, I have a lot of those 330 uF capacitors, too. In fact, I have a few thousand of the CD4049, if you would like a few. (charles@wenzel.com)
wpe11.jpg (20333 bytes)
This little circuit is going into a sub-picoampere leakage meter for characterizing JFETs and other components for extremely high impedance circuits.
Occasionally a designer needs a dual power supply to power a circuit that is operating with signals near or at ground but the only available supply is a single polarity, usually positive. Many excellent IC solutions are available but a suitable solution for many projects may be constructed from "junk box" parts. The simple circuit below will generate about 9 volts and -4 volts from a single 5 volt supply with sufficient current to power a simple op-amp circuit. The positive voltage drops to about 7 volts when supplying 7 mA and the negative voltage drops to about 3.5 volts when supplying 3.5 mA (1k loads). Although this isn't exactly a +- 15 volt supply, this is plenty of voltage and current for many op-amp circuits and will allow the output of most op-amps to swing below zero volts and will allow most op-amp inputs to measure voltages below zero volts. This circuit uses the CD4049 which is a high current version of the CD4069 which will also work with somewhat lower current capability.
wpeF.jpg (120497 bytes)
The two inverters on the left generate a square wave and the other four inverters are connected in parallel to increase the current drive to the diodes. The diode on top clamps the voltage on the top capacitor at about 4.5 volts when the inverters go low. When the inverters go high, their output voltage is added to the 4.5 volts to give about 9.5 volts. The second diode rectifies this voltage to give a little over 9 volts on the output. The bottom two diodes work in the same way only the voltage on the first capacitor is clamped to about 0.5 volts on the positive swing and then goes down to about -4.5 volts on the negative swing of the inverters, giving about -4 volts out.
The prototype is operating at only 500 Hz to allow for the use of some old-fashioned germanium rectifiers that I have in large numbers. If more modern schottky rectifiers are used the frequency may be set higher by lowering the .001uF capacitor or the 1 megohm resistors. The 4, 330 uF capacitors are larger than necessary and a few uF will suffice if the frequency is raised to, say, 5 kHz (try 100k resistors or a 100pF capacitor). Yep, I have a lot of those 330 uF capacitors, too. In fact, I have a few thousand of the CD4049, if you would like a few. (charles@wenzel.com)
wpe11.jpg (20333 bytes)
This little circuit is going into a sub-picoampere leakage meter for characterizing JFETs and other components for extremely high impedance circuits.
stepper motor
Stepper Motor Experiments
Stepper motors are most commonly controlled by microprocessors or custom controller ICs and the current is often switched by stepper motor driver ICs or power transistors. Precise motion is possible but the complexity usually lands the hobbyist's stepper motors in the "maybe someday" parts bin. But steppers may be used for a variety of applications without complex circuitry or programming. At first glance the stepper motor looks a bit intimidating since there are at least four wires and often there are six. Most steppers have two independent windings and some are center-tapped, hence the four or six wires. A quick ohmmeter check will determine which wires belong together and the center-tap may be identified by measuring the resistance between the wires; the center-tap will measure 1/2 the total winding resistance to either end of the coil. Tie the wires that belong together in a knot and tie another knot in the center-tap wire for easy identification later. Stepper motors have become quite abundant and are available in all shapes and sizes from many surplus dealers. Experimenters can also salvage excellent steppers from old office and computer equipment.
Steppers move in small increments usually indicated on the label in degrees. To make a stepper motor spin in one direction current is passed through one winding, then the other, then through the first winding with the opposite polarity, then the second with flipped polarity, too. This sequence is repeated for continuous rotation. The direction of rotation depends upon which winding is the "leader" and which is the "follower". The rotation will reverse if either winding is reversed. The center-tapped versions simplify the reversal of current since the center-tap may be tied to Vcc and each end of the coil may be alternately pulled to ground. Non-tapped motors require a bipolar drive voltage or a bit more switching circuitry. If current is applied to both windings, the stepper will settle between two steps (this is often called a "half-step"). Taking the half-step idea to the extreme, one could apply two quadrature sinewaves to the windings and get very smooth rotation. This technique would not be particularly efficient since the controller would be dissipating at least as much power as the motor but, if smooth motion is required, it might be worth a try! Or, for those who don't mind complexity, the sinewaves could be efficiently approximated by using variable duty-cycle pulses. But the purpose here is to get those motors out of the junk box, not to think of more reasons to leave them alone! So here are some simple things to try.
Steppers make excellent low power generators and surprisingly efficient low power motors for low RPM applications. As a starting point, try connecting the windings of two steppers together. Pick steppers that turn freely so that internal friction doesn't spoil the experiment. When you spin one motor shaft, the other will follow. Admittedly, there is little torque. But it does illustrate that steppers may be used to generate electricity. Here are a couple of sketches showing how to connect stepper motors as generators:
Step2.gif (4581 bytes)
The AC windings are not directly connected since the voltages are 90 degrees out of phase and the resulting voltage would be somewhat lower. Dual isolated outputs are possible if the grounds for each winding are not connected together. Here are a few application ideas:
bullet Add a hand crank and bulb for kids' science demonstration. A 24 volt stepper will light a 120 volt nightlight or Christmas tree light.
bullet Make a wind-powered anemometer.
bullet Generate a higher voltage by turning a high voltage stepper with a low voltage motor.
bullet Run various gadgets including radios, calculators, multimeters, flashers, garage door openers, remote controls, LED flashlights, bike flashers.
bullet Achieve extreme isolation - use a long plastic shaft to turn the generator for high voltage isolation or use a line-powered motor to spin a stepper for very high line isolation.
A large capacitor may be charged to run low current devices for some time after a few quick spins of the crank. A voltage regulator may be required for some devices but use an efficient series regulator to conserve power if the affair is hand-powered.
Many steppers make excellent low power motors despite the markings on the label. For example, a stepper rated at 16.8 volts, 280 mA consumed only 20 mA when unloaded and driven with bipolar 5 volt squarewaves. Such low currents may be directly extracted from many op-amps and logic devices without any additional drivers! Obviously the mechanical load must be light and the speed will be low or the motor will stall but many useful applications exhibit little load. Here are some examples:
bullet Use vanes to mechanically chop light, electrostatic fields or other radiation.
bullet Spin a sign or attention-getting symbol on an advertising display.
bullet Spin a toy radar antenna or make a highly efficient electric toy car or train.
bullet Make a spinning Christmas tree ornament.
bullet Make a laborator stirrer.
The following circuit is commonly called a quadrature oscillator or a sine - cosine oscillator and may be built using almost any op-amp with reasonably high output current. An LM358 actually worked in this circuit, but only barely! A better choice would be an LM833 or any one of many higher current op-amps.
The two transistors generate Vcc/2 so that the current in the windings will reverse when the op-amps go high and low. They are not needed if a dual polarity power supply is used. Simply ground the windings that go to the emitters. A motor rated at 16.8 volts and 280 mA consumed only 30mA in the above circuit when unloaded. Try whatever stepper motor is available and adjust the power supply voltage for proper operation. Don't expect to get much torque from this circuit! For more drive capability connect two transistors to the op-amp outputs in the same manner as the Vcc/2 circuit above. Leave out the resistors and simply connect the two bases to the op-amp output. The two emitters connect together and to the motor winding.
The above circuit is all that is needed for many applications but the following circuit has a bit more flexibility.
stepper2.gif (14575 bytes)
In this circuit the 74HC74 directly drives the stepper motor for low power applications. The two flip-flops are alternately clocked to give the desired "follow-the-leader" pulse train. The 16.8 volt motor (1.8 degree step size) described above draws only 20 mA in this circuit and a tiny 15 degree step size, 12 volt motor only draws 30 mA. The unused inverters are wired to form a slow pulse generator which may be used to randomly change the direction of rotation. Actually, the change of direction will often synchronize with the spin oscillator giving a back-and-forth action unless the oscillator frequency is just right. This change of direction will add interest to moving displays.
The circuit generates four control signals and adding circuitry for high power operation is fairly easy. If a center-tapped motor is used then the following connection will work:
You will need one of these circuits for each motor winding. If the motor doesn't have a center-tap then try the circuit below. The values are only representative and may vary depending upon the motor and transistor gain. No provisions are shown to protect against inductive kickback but small motors don't seem to generate much. However, some care may be required if larger motors and higher currents are used. Consider adding diodes from each motor winding to Vcc (the cathode goes to Vcc). Also, the 2N440Xs are only good for a few hundred milliamps so choose heftier transistors and use lower value resistors if higher motor currents are expected.
The flip-flop shown is an HC device and its power supply should be limited to 5 or 6 volts but the flip-flop could be a 4000 series device if driver transistors are added. The 4000 series devices cannot supply much current but they will run on +15 volts. Increase the resistors to 10k and use darlington transistors (or mosfets such as the VN10KM) in place of the NPNs shown.
Here is a note sent in by Garin :
I appreciate your experimenters internet home site for new electronics nuts. A good source of a ready made "robot" stepper motor system is an old dot matrix computer printer. Not only does it contain several motors, but it also has in it the electronics to drive them which can be interfaced to your control computer via the parallel port. Old manuals list control codes which can be translated into your application much faster and cheaper than designing your own system. In fact with the advent of color inkjet printers, many old dot matrix PC computer printer can be had for free. Hopefully this note will be of help.
Stepper motors are most commonly controlled by microprocessors or custom controller ICs and the current is often switched by stepper motor driver ICs or power transistors. Precise motion is possible but the complexity usually lands the hobbyist's stepper motors in the "maybe someday" parts bin. But steppers may be used for a variety of applications without complex circuitry or programming. At first glance the stepper motor looks a bit intimidating since there are at least four wires and often there are six. Most steppers have two independent windings and some are center-tapped, hence the four or six wires. A quick ohmmeter check will determine which wires belong together and the center-tap may be identified by measuring the resistance between the wires; the center-tap will measure 1/2 the total winding resistance to either end of the coil. Tie the wires that belong together in a knot and tie another knot in the center-tap wire for easy identification later. Stepper motors have become quite abundant and are available in all shapes and sizes from many surplus dealers. Experimenters can also salvage excellent steppers from old office and computer equipment.
Steppers move in small increments usually indicated on the label in degrees. To make a stepper motor spin in one direction current is passed through one winding, then the other, then through the first winding with the opposite polarity, then the second with flipped polarity, too. This sequence is repeated for continuous rotation. The direction of rotation depends upon which winding is the "leader" and which is the "follower". The rotation will reverse if either winding is reversed. The center-tapped versions simplify the reversal of current since the center-tap may be tied to Vcc and each end of the coil may be alternately pulled to ground. Non-tapped motors require a bipolar drive voltage or a bit more switching circuitry. If current is applied to both windings, the stepper will settle between two steps (this is often called a "half-step"). Taking the half-step idea to the extreme, one could apply two quadrature sinewaves to the windings and get very smooth rotation. This technique would not be particularly efficient since the controller would be dissipating at least as much power as the motor but, if smooth motion is required, it might be worth a try! Or, for those who don't mind complexity, the sinewaves could be efficiently approximated by using variable duty-cycle pulses. But the purpose here is to get those motors out of the junk box, not to think of more reasons to leave them alone! So here are some simple things to try.
Steppers make excellent low power generators and surprisingly efficient low power motors for low RPM applications. As a starting point, try connecting the windings of two steppers together. Pick steppers that turn freely so that internal friction doesn't spoil the experiment. When you spin one motor shaft, the other will follow. Admittedly, there is little torque. But it does illustrate that steppers may be used to generate electricity. Here are a couple of sketches showing how to connect stepper motors as generators:
Step2.gif (4581 bytes)
The AC windings are not directly connected since the voltages are 90 degrees out of phase and the resulting voltage would be somewhat lower. Dual isolated outputs are possible if the grounds for each winding are not connected together. Here are a few application ideas:
bullet Add a hand crank and bulb for kids' science demonstration. A 24 volt stepper will light a 120 volt nightlight or Christmas tree light.
bullet Make a wind-powered anemometer.
bullet Generate a higher voltage by turning a high voltage stepper with a low voltage motor.
bullet Run various gadgets including radios, calculators, multimeters, flashers, garage door openers, remote controls, LED flashlights, bike flashers.
bullet Achieve extreme isolation - use a long plastic shaft to turn the generator for high voltage isolation or use a line-powered motor to spin a stepper for very high line isolation.
A large capacitor may be charged to run low current devices for some time after a few quick spins of the crank. A voltage regulator may be required for some devices but use an efficient series regulator to conserve power if the affair is hand-powered.
Many steppers make excellent low power motors despite the markings on the label. For example, a stepper rated at 16.8 volts, 280 mA consumed only 20 mA when unloaded and driven with bipolar 5 volt squarewaves. Such low currents may be directly extracted from many op-amps and logic devices without any additional drivers! Obviously the mechanical load must be light and the speed will be low or the motor will stall but many useful applications exhibit little load. Here are some examples:
bullet Use vanes to mechanically chop light, electrostatic fields or other radiation.
bullet Spin a sign or attention-getting symbol on an advertising display.
bullet Spin a toy radar antenna or make a highly efficient electric toy car or train.
bullet Make a spinning Christmas tree ornament.
bullet Make a laborator stirrer.
The following circuit is commonly called a quadrature oscillator or a sine - cosine oscillator and may be built using almost any op-amp with reasonably high output current. An LM358 actually worked in this circuit, but only barely! A better choice would be an LM833 or any one of many higher current op-amps.
The two transistors generate Vcc/2 so that the current in the windings will reverse when the op-amps go high and low. They are not needed if a dual polarity power supply is used. Simply ground the windings that go to the emitters. A motor rated at 16.8 volts and 280 mA consumed only 30mA in the above circuit when unloaded. Try whatever stepper motor is available and adjust the power supply voltage for proper operation. Don't expect to get much torque from this circuit! For more drive capability connect two transistors to the op-amp outputs in the same manner as the Vcc/2 circuit above. Leave out the resistors and simply connect the two bases to the op-amp output. The two emitters connect together and to the motor winding.
The above circuit is all that is needed for many applications but the following circuit has a bit more flexibility.
stepper2.gif (14575 bytes)
In this circuit the 74HC74 directly drives the stepper motor for low power applications. The two flip-flops are alternately clocked to give the desired "follow-the-leader" pulse train. The 16.8 volt motor (1.8 degree step size) described above draws only 20 mA in this circuit and a tiny 15 degree step size, 12 volt motor only draws 30 mA. The unused inverters are wired to form a slow pulse generator which may be used to randomly change the direction of rotation. Actually, the change of direction will often synchronize with the spin oscillator giving a back-and-forth action unless the oscillator frequency is just right. This change of direction will add interest to moving displays.
The circuit generates four control signals and adding circuitry for high power operation is fairly easy. If a center-tapped motor is used then the following connection will work:
You will need one of these circuits for each motor winding. If the motor doesn't have a center-tap then try the circuit below. The values are only representative and may vary depending upon the motor and transistor gain. No provisions are shown to protect against inductive kickback but small motors don't seem to generate much. However, some care may be required if larger motors and higher currents are used. Consider adding diodes from each motor winding to Vcc (the cathode goes to Vcc). Also, the 2N440Xs are only good for a few hundred milliamps so choose heftier transistors and use lower value resistors if higher motor currents are expected.
The flip-flop shown is an HC device and its power supply should be limited to 5 or 6 volts but the flip-flop could be a 4000 series device if driver transistors are added. The 4000 series devices cannot supply much current but they will run on +15 volts. Increase the resistors to 10k and use darlington transistors (or mosfets such as the VN10KM) in place of the NPNs shown.
Here is a note sent in by Garin :
I appreciate your experimenters internet home site for new electronics nuts. A good source of a ready made "robot" stepper motor system is an old dot matrix computer printer. Not only does it contain several motors, but it also has in it the electronics to drive them which can be interfaced to your control computer via the parallel port. Old manuals list control codes which can be translated into your application much faster and cheaper than designing your own system. In fact with the advent of color inkjet printers, many old dot matrix PC computer printer can be had for free. Hopefully this note will be of help.
power control
Power Control
Sequential Flasher
Here is a simple circuit for sequentially flashing Christmas light strings or other similar low-power lamps. The socket symbols may be single bulbs or sockets for lamp strings. The load must be capable of operating from DC since the SCRs rectify the line voltage. The SCRs must be sensitive-gate types and must be able to handle the load current and line voltage. The 1uF capacitors are non-polar film types. A fuse is indicated and a GFI type of outlet is recommended.
Warning: This circuit should be constructed only by persons with the qualifications to work on and design high voltage circuitry. Necessary safety considerations are not indicated. All parts of the circuit should be considered "hot".
schemit...schemut...wiring diagram
The circuit may be extended by repeating the pattern with the last neon lamp connecting back to the first through a capacitor. Although a quick breadboard was tested, this circuit is "experimental" and may require some tweaking for best performance. The flashing speed may be controlled by connecting a 100k ohm (or larger) potentiometer across the 10 uF capacitor and connecting the 1 meg resistors to the wiper. Make sure that the shaft is an insulated type. The 1 meg resistors may be lowered in value for faster flashing.
Line Power Flasher
Here is an unusual flasher circuit for 120VAC loads. The circuit is similar to the two-transistor flasher seen in several circuits in techlib.com except that an SCR is used.
Warning: This circuit should be constructed only by persons with the qualifications to work on and design high voltage circuitry. Necessary safety considerations are not indicated. All parts of the circuit should be considered "hot".
schematic diagram
A little circuit trick is hiding in the selection of the 0.1uF capacitor in series with the 1k resistor. When the SCR is off, this capacitor smoothes the ripple from the bridge sufficiently for the transistor flasher circuitry to work properly but when the SCR turns on, the capacitor immediately discharges and will not provide enough current to keep the SCR on. Also,the capacity is low enough to leave quite a bit of ripple voltage causing the circuit to trigger near zero volts - a desirable feature!
Parts are not particularly critical:
bullet The 0.1uf capacitor should be a non-polar type with a voltage rating of at least 200 volts. The value is not critical but stay below about 1uF.
bullet The power handling device is a sensitive-gate SCR with a rated voltage of at least 200 volts and with a current rating sufficient to handle the expected load. Sensitive-gate SCRs typically have a gate trigger current near 200uA.
bullet The full-wave bridge may be made from four rectifier diodes rated for 200 volts or more and with a current rating well above the load current.
bullet The resistors all dissipate low power and ordinary carbon-film 1/4 watt types are fine.
bullet The PNP transistor may be just about any general-purpose type or a high voltage type may be selected to reduce the chances of zapping it while experimenting! Ordinarily, the transistor sees about 35 volts.
bullet The 22uF capacitor is a 50 volt or greater aluminum electrolytic or similar.
bullet The load sees AC voltage so most line-powered devices may be "flashed" with this circuit.
bullet Not shown are necessary fuses and line filtering. A power input module with everything built in is hard to beat. Low power loads will not generate much line noise but a fuse is always recommended.
Here is how it works:
When power is first applied, the SCR is off and the 0.1uF capacitor charges to about 160 volts with several volts of ripple. The base voltage sees a divided-down version of this rippling voltage, near 35 volts. The 22uF capacitor begins charging through the 1meg resistor and the emitter voltage begins to rise towards the base voltage. When the capacitor voltage reaches about 35 volts and exceeds the base voltage (typically a low point in the ripple voltage), the transistor conducts causing the SCR to trigger. The triggering pulls the voltage on the base low and the transistor turns on hard because of the now-higher voltage on the emitter. The transistor stays on until the capacitor is discharged to the point that the gate current cannot keep the SCR triggered. The SCR turns off at the next voltage zero-crossing and the process repeats. Notice how the circuit tends to turn on and off near zero-crossings! The 1k in series with the 0.1uF capacitor protects the SCR from high discharge currents. (Most SCRs can handle the current surge without the resistor.)
The 1meg resistor connected to the 22uF may be varied to change the flash rate or the 22uF capacitor value may be changed. Do not drop the resistance much below 330k or the transistor may be susceptible to overvoltage damage in the event the SCR doesn't trigger. The length of the flash is set by the 27k resistor; a higher value gives longer on-time. The 2.2k is included to ensure that the SCR turns off and this value may be higher or eliminated if the SCR doesn't show a tendency to stay on. Some sensitive-gate SCRs become too sensitive for this circuit when they become warm, they actually require that current be pulled out of the gate to prevent triggering. A typical TO-92 type SCR will handle up to about 25 watt loads without a heating problem but for heavier loads choose a TO-202 or other power package. Heat-sinking may be required for heavy loads. If the circuit stays on after a few flashes, the SCR is self-triggering due to heating.
A GFI breaker is recommended and always turn off power before making modifications. A double-insulated or grounded housing is required for safety. If you don't understand, don't build!
Sequential Flasher
Here is a simple circuit for sequentially flashing Christmas light strings or other similar low-power lamps. The socket symbols may be single bulbs or sockets for lamp strings. The load must be capable of operating from DC since the SCRs rectify the line voltage. The SCRs must be sensitive-gate types and must be able to handle the load current and line voltage. The 1uF capacitors are non-polar film types. A fuse is indicated and a GFI type of outlet is recommended.
Warning: This circuit should be constructed only by persons with the qualifications to work on and design high voltage circuitry. Necessary safety considerations are not indicated. All parts of the circuit should be considered "hot".
schemit...schemut...wiring diagram
The circuit may be extended by repeating the pattern with the last neon lamp connecting back to the first through a capacitor. Although a quick breadboard was tested, this circuit is "experimental" and may require some tweaking for best performance. The flashing speed may be controlled by connecting a 100k ohm (or larger) potentiometer across the 10 uF capacitor and connecting the 1 meg resistors to the wiper. Make sure that the shaft is an insulated type. The 1 meg resistors may be lowered in value for faster flashing.
Line Power Flasher
Here is an unusual flasher circuit for 120VAC loads. The circuit is similar to the two-transistor flasher seen in several circuits in techlib.com except that an SCR is used.
Warning: This circuit should be constructed only by persons with the qualifications to work on and design high voltage circuitry. Necessary safety considerations are not indicated. All parts of the circuit should be considered "hot".
schematic diagram
A little circuit trick is hiding in the selection of the 0.1uF capacitor in series with the 1k resistor. When the SCR is off, this capacitor smoothes the ripple from the bridge sufficiently for the transistor flasher circuitry to work properly but when the SCR turns on, the capacitor immediately discharges and will not provide enough current to keep the SCR on. Also,the capacity is low enough to leave quite a bit of ripple voltage causing the circuit to trigger near zero volts - a desirable feature!
Parts are not particularly critical:
bullet The 0.1uf capacitor should be a non-polar type with a voltage rating of at least 200 volts. The value is not critical but stay below about 1uF.
bullet The power handling device is a sensitive-gate SCR with a rated voltage of at least 200 volts and with a current rating sufficient to handle the expected load. Sensitive-gate SCRs typically have a gate trigger current near 200uA.
bullet The full-wave bridge may be made from four rectifier diodes rated for 200 volts or more and with a current rating well above the load current.
bullet The resistors all dissipate low power and ordinary carbon-film 1/4 watt types are fine.
bullet The PNP transistor may be just about any general-purpose type or a high voltage type may be selected to reduce the chances of zapping it while experimenting! Ordinarily, the transistor sees about 35 volts.
bullet The 22uF capacitor is a 50 volt or greater aluminum electrolytic or similar.
bullet The load sees AC voltage so most line-powered devices may be "flashed" with this circuit.
bullet Not shown are necessary fuses and line filtering. A power input module with everything built in is hard to beat. Low power loads will not generate much line noise but a fuse is always recommended.
Here is how it works:
When power is first applied, the SCR is off and the 0.1uF capacitor charges to about 160 volts with several volts of ripple. The base voltage sees a divided-down version of this rippling voltage, near 35 volts. The 22uF capacitor begins charging through the 1meg resistor and the emitter voltage begins to rise towards the base voltage. When the capacitor voltage reaches about 35 volts and exceeds the base voltage (typically a low point in the ripple voltage), the transistor conducts causing the SCR to trigger. The triggering pulls the voltage on the base low and the transistor turns on hard because of the now-higher voltage on the emitter. The transistor stays on until the capacitor is discharged to the point that the gate current cannot keep the SCR triggered. The SCR turns off at the next voltage zero-crossing and the process repeats. Notice how the circuit tends to turn on and off near zero-crossings! The 1k in series with the 0.1uF capacitor protects the SCR from high discharge currents. (Most SCRs can handle the current surge without the resistor.)
The 1meg resistor connected to the 22uF may be varied to change the flash rate or the 22uF capacitor value may be changed. Do not drop the resistance much below 330k or the transistor may be susceptible to overvoltage damage in the event the SCR doesn't trigger. The length of the flash is set by the 27k resistor; a higher value gives longer on-time. The 2.2k is included to ensure that the SCR turns off and this value may be higher or eliminated if the SCR doesn't show a tendency to stay on. Some sensitive-gate SCRs become too sensitive for this circuit when they become warm, they actually require that current be pulled out of the gate to prevent triggering. A typical TO-92 type SCR will handle up to about 25 watt loads without a heating problem but for heavier loads choose a TO-202 or other power package. Heat-sinking may be required for heavy loads. If the circuit stays on after a few flashes, the SCR is self-triggering due to heating.
A GFI breaker is recommended and always turn off power before making modifications. A double-insulated or grounded housing is required for safety. If you don't understand, don't build!
magic lamp
Magic Lamp
You have probably seen "magic lamp" circuits in which an ordinary incandescent bulb is lit by a match. These circuits rely on a hidden temperature or light sensor and are not particularly interesting. I decided to make a magic lamp, too! But, to make it more interesting, I decided to just use plain old magic for this circuit instead of resorting to any hidden components.
Warning: This circuit should be constructed only by persons with the qualifications to work on and design high voltage circuitry. Necessary safety considerations are not indicated. All parts of the circuit should be considered "hot". The use of magic is frowned upon in higher places.
schematic
The components are not critical and substitutions are possible. The SCR is an ordinary sensitive-gate type like the 2N5064 or other type designed for 200 volts or more, the PNP transistor is a high-voltage type like a MPSA-92 or a 2N6520, the 1N4003 diode is an ordinary rectifier with a breakdown voltage of 200 volts or more. The 1uF capacitor is any low voltage type and the resistors are ordinary 1/4 watt types. The 200k potentiometer could be a trimmer type but a panel-mount type may be more desirable since this circuit does require a lot of tweaking. Use an insulated shaft, however. All points in the circuit can shock. The lamp is a clear 7-watt bulb typically used in night lights. White or frosted bulbs will not work as well.
Note: If the lamp ever lights during the following adjustment process, remove the power for several minutes before starting over and wait a couple of minutes after applying power before making adjustments. This is an "expert tweaker" level circuit and might drive the beginner crazy. A beginner shouldn't build line-powered devices anyway!
The DC voltmeter is an ordinary digital VOM set to the 2 volt scale and is used for adjusting the potentiometer. Set the pot to the highest resistance and then apply power to the circuit. If the bulb lights immediately, you will need to increase the 100k resistor in parallel with the pot. If it doesn't light immediately, slowly reduce the resistance on the pot until the voltmeter starts to show a voltage. The SCR will trigger and the lamp will light when the voltage reaches about 250mV. Remember the exact voltage. Turn off the circuit, wait 5 minutes, reapply power with the pot set to the high end and begin approaching this trigger voltage again. Stop when the voltage is about 50mV below the triggering point. Hold a flame near the bulb and the voltage should begin to climb toward the trigger point. (You can use a flashlight instead of a flame for testing purposes.) After a few seconds the lamp will light. Magic!
Unfortunately, this "magic" isn't particularly stable and frequent adjustment may be required. Some bulbs may require a larger or smaller resistance across the SCR than can be obtained with the values shown. If the bulb will not light at any pot setting, reduce the 100k in parallel with the pot to 47k. If it won't stay out, increase the 100k and possibly the 47k. After a little experience you will be able to make the adjustments without the meter. The bulb in the prototype measures 150 ohms at room temperature. Another identical looking bulb measured 300 ohms which is more typical of the little night-light bulbs that vary from 4 to 7 watts.
By now you probably get how it works! The tungsten in the bulb has a pretty steep temperature coefficient and since it is in the good thermal insulation of a vacuum, it is easy to heat with moderate light levels. In fact, a small flashlight can heat the filament to 50 C and a nearby 60 watt bulb can raise the temperature to over 200 C! (I recently changed these numbers after additional testing. The original value of 400 C rise for a 60 watt bulb seemed too high to be possible and it apparently was. Maybe I switched the two bulbs or I was working in Fahrenheit! I've also noticed that some filaments don't absorb the radiant energy as well as others, perhaps due to surface color or texture. I wonder if a used bulb is more sensitive due to a darkened surface.) A tiny penlight will raise the temperature of the prototype's bulb over 1/2 degree from across the room! These temperature increases result in significant resistance changes as the chart below indicates:
chart
Tungsten Filament 7-watt Lamp
The resistors and potentiometer across the SCR cause a couple of mA to flow in the dark lamp and when the value is set just right, the transistor is on the verge of conducting at the peaks of the line voltage. A bright light heats the filament and, as the bulb's resistance increases, the voltage across the bulb goes up. The transistor starts passing pulses at the peaks of the line voltage that charge the 1uF triggering the SCR. Once the SCR triggers the lamp lights and shoots up in resistance, and the circuit latches on. Power must be removed for several minutes to reset the circuit because it takes quite a while for the filament to cool to room temperature. The 1uF capacitor prevents the SCR from triggering due to the turn-on transient.
This fun demonstration of some properties of the ordinary incandescent bulb suggest other projects. The filament is a low resistance and will exhibit little noise so it should be possible to place a couple of bulbs in a bridge to achieve an extremely sensitive radiometer (one bulb shielded from light). Some other ideas include a light meter, optical isolator, beam-break detector, solar radiometer (Just how hot does the sun get a filament?), flame monitor, and solar panel positioner. Or an unusual gain control; many years ago Hewlett Packard used a similar bulb to stabilize their audio oscillator but the energy to heat the filament came from current flow, not light. One bulb illuminating another for gain control would be unique, if not practical. Many photoelectric projects will seem amazing with a light bulb as a sensor in place of a photocell! Other type bulbs may be worth investigating, too. Choose a high voltage, low wattage bulb to get a high resistance filament. For example, a tiny #387 type (28 volts at 40 mA) has about 1/2 the resistance of the larger night light bulb and it makes an excellent detector. For most light sensing applications, apply only a volt or two across the bulb to keep self-heating down.
More experimental results:
Using two #387s in the bridge configuration described above and feeding a differential amplifier with a gain of 500, a radiometer with surprising stability and sensitivity was realized. It drifts around a little but this detector can easily detect the infrared from an IR LED and a soldering iron and the sensitivity to light is also excellent. (It is fascinating to see.)
A 100uA meter was connected across the bridge in place of the differential amplifier and a 60 watt lamp held near one bulb gave a near full-scale reading. (Exposure meter!) The bridge voltage was increased to 9 volts for this experiment. The bridge circuit is nothing more than the two lamps in series across 9 volts for one leg and a 1k ohm potentiometer connected across the 9 volts for the other leg. The diff amp or meter connect between the wiper of the pot and the point where the two bulbs connect together. The pot is adjusted for a zero reading.
Here is a somewhat impractical but interesting idea:
Car taillights have two filaments in one bulb. How about a circuit that watches the turn signal filament to determine if the taillight is functioning properly? It would only work when the brakes and turn signals were not in use but it would still be handy and could be done remotely without running additional wires or fibers as done in some cars. A bad brake/turn signal filament is already easy to spot since the rate of flashing changes when one is bad.
Update - I just checked a taillight bulb and determined the higher current filament increases from about 0.5 ohm to near 1.5 ohm when the taillight filament is illuminated. A 3 to 1 change should be easy to detect! Reversing the roles of the filaments results in a 2 ohm to 9 ohm change.
Reader Tom Bruhns suggests using a light bulb to measure and possibly control your oven's temperature! In this application the bulb is acting as a simple temperature sensor. Ordinary light bulbs can take the extreme temperatures in an oven - there is already one in there, after all. Come to think of it, other lamps might serve as thermometers when not "lit", the refrigerator being one example. The use of the lamp for light might degrade the calibration due to filament aging but temperature measurement/control applications could benefit from the use of a dedicated light bulb sensor. Bulbs, after all, are made from corrosion and temperature resistant materials and the filament is sealed inside a protective vacuum. I can't think of a superior temperature sensor available at the local convenience store!
You have probably seen "magic lamp" circuits in which an ordinary incandescent bulb is lit by a match. These circuits rely on a hidden temperature or light sensor and are not particularly interesting. I decided to make a magic lamp, too! But, to make it more interesting, I decided to just use plain old magic for this circuit instead of resorting to any hidden components.
Warning: This circuit should be constructed only by persons with the qualifications to work on and design high voltage circuitry. Necessary safety considerations are not indicated. All parts of the circuit should be considered "hot". The use of magic is frowned upon in higher places.
schematic
The components are not critical and substitutions are possible. The SCR is an ordinary sensitive-gate type like the 2N5064 or other type designed for 200 volts or more, the PNP transistor is a high-voltage type like a MPSA-92 or a 2N6520, the 1N4003 diode is an ordinary rectifier with a breakdown voltage of 200 volts or more. The 1uF capacitor is any low voltage type and the resistors are ordinary 1/4 watt types. The 200k potentiometer could be a trimmer type but a panel-mount type may be more desirable since this circuit does require a lot of tweaking. Use an insulated shaft, however. All points in the circuit can shock. The lamp is a clear 7-watt bulb typically used in night lights. White or frosted bulbs will not work as well.
Note: If the lamp ever lights during the following adjustment process, remove the power for several minutes before starting over and wait a couple of minutes after applying power before making adjustments. This is an "expert tweaker" level circuit and might drive the beginner crazy. A beginner shouldn't build line-powered devices anyway!
The DC voltmeter is an ordinary digital VOM set to the 2 volt scale and is used for adjusting the potentiometer. Set the pot to the highest resistance and then apply power to the circuit. If the bulb lights immediately, you will need to increase the 100k resistor in parallel with the pot. If it doesn't light immediately, slowly reduce the resistance on the pot until the voltmeter starts to show a voltage. The SCR will trigger and the lamp will light when the voltage reaches about 250mV. Remember the exact voltage. Turn off the circuit, wait 5 minutes, reapply power with the pot set to the high end and begin approaching this trigger voltage again. Stop when the voltage is about 50mV below the triggering point. Hold a flame near the bulb and the voltage should begin to climb toward the trigger point. (You can use a flashlight instead of a flame for testing purposes.) After a few seconds the lamp will light. Magic!
Unfortunately, this "magic" isn't particularly stable and frequent adjustment may be required. Some bulbs may require a larger or smaller resistance across the SCR than can be obtained with the values shown. If the bulb will not light at any pot setting, reduce the 100k in parallel with the pot to 47k. If it won't stay out, increase the 100k and possibly the 47k. After a little experience you will be able to make the adjustments without the meter. The bulb in the prototype measures 150 ohms at room temperature. Another identical looking bulb measured 300 ohms which is more typical of the little night-light bulbs that vary from 4 to 7 watts.
By now you probably get how it works! The tungsten in the bulb has a pretty steep temperature coefficient and since it is in the good thermal insulation of a vacuum, it is easy to heat with moderate light levels. In fact, a small flashlight can heat the filament to 50 C and a nearby 60 watt bulb can raise the temperature to over 200 C! (I recently changed these numbers after additional testing. The original value of 400 C rise for a 60 watt bulb seemed too high to be possible and it apparently was. Maybe I switched the two bulbs or I was working in Fahrenheit! I've also noticed that some filaments don't absorb the radiant energy as well as others, perhaps due to surface color or texture. I wonder if a used bulb is more sensitive due to a darkened surface.) A tiny penlight will raise the temperature of the prototype's bulb over 1/2 degree from across the room! These temperature increases result in significant resistance changes as the chart below indicates:
chart
Tungsten Filament 7-watt Lamp
The resistors and potentiometer across the SCR cause a couple of mA to flow in the dark lamp and when the value is set just right, the transistor is on the verge of conducting at the peaks of the line voltage. A bright light heats the filament and, as the bulb's resistance increases, the voltage across the bulb goes up. The transistor starts passing pulses at the peaks of the line voltage that charge the 1uF triggering the SCR. Once the SCR triggers the lamp lights and shoots up in resistance, and the circuit latches on. Power must be removed for several minutes to reset the circuit because it takes quite a while for the filament to cool to room temperature. The 1uF capacitor prevents the SCR from triggering due to the turn-on transient.
This fun demonstration of some properties of the ordinary incandescent bulb suggest other projects. The filament is a low resistance and will exhibit little noise so it should be possible to place a couple of bulbs in a bridge to achieve an extremely sensitive radiometer (one bulb shielded from light). Some other ideas include a light meter, optical isolator, beam-break detector, solar radiometer (Just how hot does the sun get a filament?), flame monitor, and solar panel positioner. Or an unusual gain control; many years ago Hewlett Packard used a similar bulb to stabilize their audio oscillator but the energy to heat the filament came from current flow, not light. One bulb illuminating another for gain control would be unique, if not practical. Many photoelectric projects will seem amazing with a light bulb as a sensor in place of a photocell! Other type bulbs may be worth investigating, too. Choose a high voltage, low wattage bulb to get a high resistance filament. For example, a tiny #387 type (28 volts at 40 mA) has about 1/2 the resistance of the larger night light bulb and it makes an excellent detector. For most light sensing applications, apply only a volt or two across the bulb to keep self-heating down.
More experimental results:
Using two #387s in the bridge configuration described above and feeding a differential amplifier with a gain of 500, a radiometer with surprising stability and sensitivity was realized. It drifts around a little but this detector can easily detect the infrared from an IR LED and a soldering iron and the sensitivity to light is also excellent. (It is fascinating to see.)
A 100uA meter was connected across the bridge in place of the differential amplifier and a 60 watt lamp held near one bulb gave a near full-scale reading. (Exposure meter!) The bridge voltage was increased to 9 volts for this experiment. The bridge circuit is nothing more than the two lamps in series across 9 volts for one leg and a 1k ohm potentiometer connected across the 9 volts for the other leg. The diff amp or meter connect between the wiper of the pot and the point where the two bulbs connect together. The pot is adjusted for a zero reading.
Here is a somewhat impractical but interesting idea:
Car taillights have two filaments in one bulb. How about a circuit that watches the turn signal filament to determine if the taillight is functioning properly? It would only work when the brakes and turn signals were not in use but it would still be handy and could be done remotely without running additional wires or fibers as done in some cars. A bad brake/turn signal filament is already easy to spot since the rate of flashing changes when one is bad.
Update - I just checked a taillight bulb and determined the higher current filament increases from about 0.5 ohm to near 1.5 ohm when the taillight filament is illuminated. A 3 to 1 change should be easy to detect! Reversing the roles of the filaments results in a 2 ohm to 9 ohm change.
Reader Tom Bruhns suggests using a light bulb to measure and possibly control your oven's temperature! In this application the bulb is acting as a simple temperature sensor. Ordinary light bulbs can take the extreme temperatures in an oven - there is already one in there, after all. Come to think of it, other lamps might serve as thermometers when not "lit", the refrigerator being one example. The use of the lamp for light might degrade the calibration due to filament aging but temperature measurement/control applications could benefit from the use of a dedicated light bulb sensor. Bulbs, after all, are made from corrosion and temperature resistant materials and the filament is sealed inside a protective vacuum. I can't think of a superior temperature sensor available at the local convenience store!
flasher circuits
Flasher Circuits
Two Transistor Flasher Ideas
The basic two-transistor flasher shown below has found its way into dozens of applications due to its simplicity and versatility. Applications have included such diverse circuits as a micropower low battery indicator, a lightning detector, a off-line switching power supply, a micropower high voltage supply, an unusual beeping capacitance probe, a windshield wiper controller, a lamp dimmer, a police siren, and several others. The simple circuit can be used at very low frequencies, RF frequencies, low voltages, or even very high voltages with careful selection of transistors. The power handling capability and power consumption are also easily modified to suit the requirement.
This circuit is great for beginners! If you build it, it will flash. And you can easily change the on-time and flash rate.
The basic flasher is shown below. Notice that it is a "two-wire" circuit and simply connects in series with the load and battery. The two resistors on the base of the PNP set a threshold voltage and when power is applied the capacitor begins charging toward this voltage. When the capacitor voltage is high enough the two transistors begin to conduct. The current flow causes the voltage across the circuit to drop slightly and this drop causes a drop in the threshold voltage. The lower threshold voltage causes even more current and this positive feedback causes the circuit to rapidly turn on. It stays on until the capacitor discharges at which point a reverse process causes the circuit to suddenly switch off.
Power transistors may be added for handling higher current loads. The two circuits below are typical connections. In the first circuit a flasher circuit in series with a 220 ohm resistor turns on a power transistor. In the second circuit, a power FET is used in place of the NPN. A pull-down resistor is added to pull the gate low when the circuit turns off.
Don't hesitate to modify this basic circuit to meet your specific requirements. It is easy to troubleshoot and almost always works! Here are a few more ideas for the experimenter to try:
bullet A diode may be inserted in series with the capacitor charging resistor so that discharge current is blocked which gives a longer "on" time for a given flash rate. The NPN base resistor determines how fast the capacitor discharges.
bullet A signal can be coupled into the base of the PNP to modulate the flashing rate for FM applications.
bullet The PNP base divider resistors can be adjusted so that the voltage is just a little too high for a flash to occur when the capacitor fully charges. Then, a very tiny AC signal applied to the base will cause the circuit to "trigger". The frequency response of this detector can be surprisingly high.
bullet The capacitor charging current may come from any source making a simple current to frequency converter.
bullet You can reverse the polarity of everything and switch the transistor types.
The circuit below is a "silent" metronome that keeps the beat without becoming a member of the band. The circuit flashes the 6 volt lamp at a rate set by the 20k potentiometer which can have a dial for setting the desired tempo. Alternately, the potentiometer could be replaced with a rotary switch and selected resistors. The lamp is an ordinary #47 bulb which will give good omni directional brightness but an LED and resistor could be used instead - try a 100 ohm in series with a high-intensity LED. The batteries could be three C or D cells for good life. This circuit could be used to generate "clicks" in a speaker but such metronomes are not particularly pleasing. The ambitious might replace the lamp with a solenoid which taps on the wall of a hardwood box or wooden chime for a "professional" sound.
schematic
Here is a low battery indicator that flashes a lamp when the battery voltage falls below about 5 volts. The circuit draws about 25 microamps when not flashing so battery life is not significantly shortened by the circuit. The two 1 megohm resistors set the switching point at V/2 (plus a little due to the emitter-base diode drop) and when this voltage is above the zener voltage the circuit cannot turn on. When the battery voltage drops below 5 volts, the base voltage drops to 2.5 volts and the emitter can reach a voltage sufficient to turn on the PNP (2N4403 or similar). When the PNP conducts, the NPN also conducts dropping the voltage across the circuit even more and the circuit snaps on. When the 4.7 uF capacitor has discharged, the circuit turns off and the capacitor begins charging again.
schematic
The zener is a "4.7 volt" type but in this circuit it is operating at a very low current and is limiting the emitter voltage to about 2.5 volts. Some experimentation may be necessary if another zener series is used.
The following circuit uses the flasher circuit to drive a complementary output stage and step-up audio transformer. This circuit is used in a high voltage breakdown tester but it would be useful for a variety of applications.
schematic
The transformer may be an audio type connected for step-up or step-down depending upon the desired output voltage. An old tube radio output transformer with the speaker winding connected to the circuit gave about 250 VRMS on the secondary and the voltage multiplier may be extended to reach thousands of volts DC.
Warning! This thing can produce lethal shocks when used to generate high voltages! Don't build it unless you are experienced and qualified to work with dangerous voltages.
Power transformers will also work but some experimentation may be necessary. The output transistors are shown as small-signal types but power transistors may be necessary if the load current is high. The duty cycle is not exactly 50/50 and other circuits would probably be better for high power inverters. This circuit is easily controlled, however. Pulling the 0.02 uF capacitor low is a good way to stop or reduce the output of the circuit. See the Geiger counter supply for an example that produces a regulated output voltage.
The AC out frequency at the secondary is several hundred Hz which may be changed by changing the 0.02 uF cap or the 6.8k resistor. The high frequency is useful for driving diode voltage multipliers as shown or D.C. rectifiers since smaller capacitors are needed then when using 50 or 60 Hz.
Super-simple Flasher
Here is a simple flasher circuit that uses no resistors! However, it relies on leakage in the base of the PNP germanium transistor and only some will work; be prepared to try a few. If you add a 100k resistor from the base to the collector of the PNP, the circuit will work with most germanium transistors and will work down to 1 VDC! The NPN should be a silicon type. The 100 uF may be replaced with a 22uF in series with a 5k resistor and it would be a good idea to add 39 ohms in series with the base of the NPN (but then the circuit starts to lose its charming simplicity).
schematic
Requiring a few more parts, this low voltage flasher uses ordinary silicon transistors and is powered by two cells. The circuit will work down to about 1.6 volts.
schematic
To flash a 600 mA bulb, change the 330k to 22k, the 100 ohm to 39 ohm, the 4.7k to 1k and the 4.7uF to 100uF.
Fig. 1 shows a versatile LED flasher circuit that works with smaller capacitor values. Note that this circuit is significantly different from the circuits above; the capacitor is in the base circuit. This configuration can give a long delay with much smaller capacitors than other flashers but the 2N4403 will not "saturate" so a few volts will remain across the circuit during the flash.
schematic
The circuit is shown as a "two-wire" flasher which is simply connected in series with the load but a slight modification might prove more satisfactory when several lamps will be operated from the same battery. When the battery begins to lose its charge and its series resistance increases, the lights may tend to synchronize. By connecting the capacitor to the positive terminal of the battery instead of the negative as shown in fig.2 the sudden voltage drop caused by other flashers will not tend to trigger the circuit.
schematic
This flasher circuit is an excellent addition to the experimenter's bag of tricks because it offers a surprising level of performance for its simplicity. For example, increase the 1 megohm charging resistor up to 100 megohms (5, 22 megohms in series), increase the discharge resistor from 100k up to 1 megohm, and reduce the capacitor down to 0.01 uf and the circuit will flash an LED at about one flash per second. That's pretty slow for only 0.01uf. Increase the capacitor to 1uf (non-electrolytic) and the delay will reach 100 seconds. High gain transistors are best for this circuit and an MPSD-54 or similar PNP darlington is a great choice for the output transistor when driving higher current loads. Electrolytic capacitors may be used in this circuit but they often exhibit a little leakage so charging resistor values below 1 megohm are recommended.
A nice Christmas surprise can be constructed by building about five blinkers into a small, red felt stocking. Decorate the stocking with a glitter Christmas tree and poke the LEDs through holes in the stocking to light the tree. The battery can be dropped to the bottom of the stocking and held in place with a wad of paper. Glue a heavy piece of paper over the circuitry on the inside of the stocking to protect the wiring. The circuit will run for many days so it can be sent to Grandma and Grandpa with the battery installed and the lights twinkling.
Marx Flasher
flasher
Here is a strange-looking flasher that uses an unusual form of the Marx high voltage multiplier. The traditional Marx multiplier uses spark gaps to repetitively charge capacitors from a high voltage supply (in parallel) then to suddenly connect them in series to generate a much higher voltage, near N times the supply voltage where N is the number of capacitors. This multiplier uses Lumex gas tube transient suppressors (GT-RLSA3230D) as the spark gap, providing reliable and repeatable triggering at about 250 volts (much lower than the typical spark gap). The 120 volt line voltage is rectified and doubled to provide enough voltage to trigger the suppressors and to reduce the required number of stages. The high voltage output builds up to just under 1000 volts when a miniature fluorescent tube fires. The tube firing discharges the output capacitor and the process begins again. The prototype is built into a clear plastic tube and hangs next to a bookshelf, looking pretty strange, flashing about every minute.
Picture 078.jpg (429603 bytes)Picture 077.jpg (407978 bytes)
The circuit is constructed on a long phenolic tube with solder terminals installed on opposite sides but any construction technique will work. Remember that the circuit is line powered without any isolation so insulation is mandatory and the device should be plugged into a GFI protected outlet. Everything in an experimenter's lab should be on GFI circuits anyway!
As the large capacitor charges, the surge suppressors will dimly flicker with a blue light. In case you are wondering, ordinary neon lamps will also work but you will only get about 25 volts per bulb; it is hard to beat these suppressors. Notice that the Marx generator uses only one capacitor per stage instead of two as with the Cockcroft-Walton multiplier. Other values may be used in just about every case. Another prototype used .01 uF capacitors instead of 5000 pF, 100k instead of 1 meg and 1meg in place of the 3 meg so don't hesitate to experiment with what is on hand. This circuit can shock the begeebers out of you even when it is off so keepa your hands off!
Two Transistor Flasher Ideas
The basic two-transistor flasher shown below has found its way into dozens of applications due to its simplicity and versatility. Applications have included such diverse circuits as a micropower low battery indicator, a lightning detector, a off-line switching power supply, a micropower high voltage supply, an unusual beeping capacitance probe, a windshield wiper controller, a lamp dimmer, a police siren, and several others. The simple circuit can be used at very low frequencies, RF frequencies, low voltages, or even very high voltages with careful selection of transistors. The power handling capability and power consumption are also easily modified to suit the requirement.
This circuit is great for beginners! If you build it, it will flash. And you can easily change the on-time and flash rate.
The basic flasher is shown below. Notice that it is a "two-wire" circuit and simply connects in series with the load and battery. The two resistors on the base of the PNP set a threshold voltage and when power is applied the capacitor begins charging toward this voltage. When the capacitor voltage is high enough the two transistors begin to conduct. The current flow causes the voltage across the circuit to drop slightly and this drop causes a drop in the threshold voltage. The lower threshold voltage causes even more current and this positive feedback causes the circuit to rapidly turn on. It stays on until the capacitor discharges at which point a reverse process causes the circuit to suddenly switch off.
Power transistors may be added for handling higher current loads. The two circuits below are typical connections. In the first circuit a flasher circuit in series with a 220 ohm resistor turns on a power transistor. In the second circuit, a power FET is used in place of the NPN. A pull-down resistor is added to pull the gate low when the circuit turns off.
Don't hesitate to modify this basic circuit to meet your specific requirements. It is easy to troubleshoot and almost always works! Here are a few more ideas for the experimenter to try:
bullet A diode may be inserted in series with the capacitor charging resistor so that discharge current is blocked which gives a longer "on" time for a given flash rate. The NPN base resistor determines how fast the capacitor discharges.
bullet A signal can be coupled into the base of the PNP to modulate the flashing rate for FM applications.
bullet The PNP base divider resistors can be adjusted so that the voltage is just a little too high for a flash to occur when the capacitor fully charges. Then, a very tiny AC signal applied to the base will cause the circuit to "trigger". The frequency response of this detector can be surprisingly high.
bullet The capacitor charging current may come from any source making a simple current to frequency converter.
bullet You can reverse the polarity of everything and switch the transistor types.
The circuit below is a "silent" metronome that keeps the beat without becoming a member of the band. The circuit flashes the 6 volt lamp at a rate set by the 20k potentiometer which can have a dial for setting the desired tempo. Alternately, the potentiometer could be replaced with a rotary switch and selected resistors. The lamp is an ordinary #47 bulb which will give good omni directional brightness but an LED and resistor could be used instead - try a 100 ohm in series with a high-intensity LED. The batteries could be three C or D cells for good life. This circuit could be used to generate "clicks" in a speaker but such metronomes are not particularly pleasing. The ambitious might replace the lamp with a solenoid which taps on the wall of a hardwood box or wooden chime for a "professional" sound.
schematic
Here is a low battery indicator that flashes a lamp when the battery voltage falls below about 5 volts. The circuit draws about 25 microamps when not flashing so battery life is not significantly shortened by the circuit. The two 1 megohm resistors set the switching point at V/2 (plus a little due to the emitter-base diode drop) and when this voltage is above the zener voltage the circuit cannot turn on. When the battery voltage drops below 5 volts, the base voltage drops to 2.5 volts and the emitter can reach a voltage sufficient to turn on the PNP (2N4403 or similar). When the PNP conducts, the NPN also conducts dropping the voltage across the circuit even more and the circuit snaps on. When the 4.7 uF capacitor has discharged, the circuit turns off and the capacitor begins charging again.
schematic
The zener is a "4.7 volt" type but in this circuit it is operating at a very low current and is limiting the emitter voltage to about 2.5 volts. Some experimentation may be necessary if another zener series is used.
The following circuit uses the flasher circuit to drive a complementary output stage and step-up audio transformer. This circuit is used in a high voltage breakdown tester but it would be useful for a variety of applications.
schematic
The transformer may be an audio type connected for step-up or step-down depending upon the desired output voltage. An old tube radio output transformer with the speaker winding connected to the circuit gave about 250 VRMS on the secondary and the voltage multiplier may be extended to reach thousands of volts DC.
Warning! This thing can produce lethal shocks when used to generate high voltages! Don't build it unless you are experienced and qualified to work with dangerous voltages.
Power transformers will also work but some experimentation may be necessary. The output transistors are shown as small-signal types but power transistors may be necessary if the load current is high. The duty cycle is not exactly 50/50 and other circuits would probably be better for high power inverters. This circuit is easily controlled, however. Pulling the 0.02 uF capacitor low is a good way to stop or reduce the output of the circuit. See the Geiger counter supply for an example that produces a regulated output voltage.
The AC out frequency at the secondary is several hundred Hz which may be changed by changing the 0.02 uF cap or the 6.8k resistor. The high frequency is useful for driving diode voltage multipliers as shown or D.C. rectifiers since smaller capacitors are needed then when using 50 or 60 Hz.
Super-simple Flasher
Here is a simple flasher circuit that uses no resistors! However, it relies on leakage in the base of the PNP germanium transistor and only some will work; be prepared to try a few. If you add a 100k resistor from the base to the collector of the PNP, the circuit will work with most germanium transistors and will work down to 1 VDC! The NPN should be a silicon type. The 100 uF may be replaced with a 22uF in series with a 5k resistor and it would be a good idea to add 39 ohms in series with the base of the NPN (but then the circuit starts to lose its charming simplicity).
schematic
Requiring a few more parts, this low voltage flasher uses ordinary silicon transistors and is powered by two cells. The circuit will work down to about 1.6 volts.
schematic
To flash a 600 mA bulb, change the 330k to 22k, the 100 ohm to 39 ohm, the 4.7k to 1k and the 4.7uF to 100uF.
Fig. 1 shows a versatile LED flasher circuit that works with smaller capacitor values. Note that this circuit is significantly different from the circuits above; the capacitor is in the base circuit. This configuration can give a long delay with much smaller capacitors than other flashers but the 2N4403 will not "saturate" so a few volts will remain across the circuit during the flash.
schematic
The circuit is shown as a "two-wire" flasher which is simply connected in series with the load but a slight modification might prove more satisfactory when several lamps will be operated from the same battery. When the battery begins to lose its charge and its series resistance increases, the lights may tend to synchronize. By connecting the capacitor to the positive terminal of the battery instead of the negative as shown in fig.2 the sudden voltage drop caused by other flashers will not tend to trigger the circuit.
schematic
This flasher circuit is an excellent addition to the experimenter's bag of tricks because it offers a surprising level of performance for its simplicity. For example, increase the 1 megohm charging resistor up to 100 megohms (5, 22 megohms in series), increase the discharge resistor from 100k up to 1 megohm, and reduce the capacitor down to 0.01 uf and the circuit will flash an LED at about one flash per second. That's pretty slow for only 0.01uf. Increase the capacitor to 1uf (non-electrolytic) and the delay will reach 100 seconds. High gain transistors are best for this circuit and an MPSD-54 or similar PNP darlington is a great choice for the output transistor when driving higher current loads. Electrolytic capacitors may be used in this circuit but they often exhibit a little leakage so charging resistor values below 1 megohm are recommended.
A nice Christmas surprise can be constructed by building about five blinkers into a small, red felt stocking. Decorate the stocking with a glitter Christmas tree and poke the LEDs through holes in the stocking to light the tree. The battery can be dropped to the bottom of the stocking and held in place with a wad of paper. Glue a heavy piece of paper over the circuitry on the inside of the stocking to protect the wiring. The circuit will run for many days so it can be sent to Grandma and Grandpa with the battery installed and the lights twinkling.
Marx Flasher
flasher
Here is a strange-looking flasher that uses an unusual form of the Marx high voltage multiplier. The traditional Marx multiplier uses spark gaps to repetitively charge capacitors from a high voltage supply (in parallel) then to suddenly connect them in series to generate a much higher voltage, near N times the supply voltage where N is the number of capacitors. This multiplier uses Lumex gas tube transient suppressors (GT-RLSA3230D) as the spark gap, providing reliable and repeatable triggering at about 250 volts (much lower than the typical spark gap). The 120 volt line voltage is rectified and doubled to provide enough voltage to trigger the suppressors and to reduce the required number of stages. The high voltage output builds up to just under 1000 volts when a miniature fluorescent tube fires. The tube firing discharges the output capacitor and the process begins again. The prototype is built into a clear plastic tube and hangs next to a bookshelf, looking pretty strange, flashing about every minute.
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The circuit is constructed on a long phenolic tube with solder terminals installed on opposite sides but any construction technique will work. Remember that the circuit is line powered without any isolation so insulation is mandatory and the device should be plugged into a GFI protected outlet. Everything in an experimenter's lab should be on GFI circuits anyway!
As the large capacitor charges, the surge suppressors will dimly flicker with a blue light. In case you are wondering, ordinary neon lamps will also work but you will only get about 25 volts per bulb; it is hard to beat these suppressors. Notice that the Marx generator uses only one capacitor per stage instead of two as with the Cockcroft-Walton multiplier. Other values may be used in just about every case. Another prototype used .01 uF capacitors instead of 5000 pF, 100k instead of 1 meg and 1meg in place of the 3 meg so don't hesitate to experiment with what is on hand. This circuit can shock the begeebers out of you even when it is off so keepa your hands off!
astable flip flop circuits
Astable Flip-Flop Circuits
The familiar astable flip-flop circuit is a handy configuration for making flashers or generating squarewaves. Here is a typical alternating LED flasher with the LEDs in the emitters instead of collectors as is normally done. (There is another good reason to put them in the emitters - see Karen's note below. ) The bias resistors are directly connected to the supply and are chosen to have a value about 100 times the collector resistor for ordinary gain transistors. The flashing period is approximately the product of this resistance and the capacitance which is about 1 second for the circuit as shown. The 470 ohm resistors set the LED current and may be reduced for lower battery voltage but remember to also reduce the bias resistors. If no LEDs are desired, the emitters may be directly connected to ground and two out-of-phase voltage squarewaves are available on the collectors. Flip-flop Flasher
This is another version of the circuit that uses negative feedback for the bias. This technique is generally more desirable because the feedback ensures that both transistors are in a high-gain, linear mode when power is applied. In actual practice the first circuit will often work "better" with ordinary bipolar transistors since there is no negative feedback slowing the switching. The feedback makes the circuit more immune to parameter variations due to temperature changes, gain variations, or even component substitution. flip-flop oscillator with negative feedback bias
This version will work with just about any NPN darlington transistor. The bias resistor may be much larger due to the high gain of the darlington so much lower value, non-polar capacitors will give a suitable flash rate. Of coures, other applications may require different oscillation rates which are easily achieved by changing the capacitor value. Other voltages and currents may be accommodated by changing the collector resistor value. PNP versions of all of these circuits may be built by reversing the polarity of the battery and polarized capacitors. The high gain of the darlingtons makes it feasible to handle heavy loads either in the emitters as shown or in place of the collector resistors as is commonly done. Lower value bias resistors may be necessary depending upon the load current and the gain of the transistors. darlington version
Here is an unusual way to get more power out of the astable flip-flop without resorting to huge capacitors. The emitter current flows through the base-emitter junction much like the LEDs above saturating the output transistors. The 2N4401 can handle up to 600mA in this circuit but a higher current transistor may be substituted. The base current of the output transistor may be adjusted by changing the 470 ohm resistor, as needed. power flasher
very high power flasher Adding another power transistor on each side brings this flasher into the 10 amp range using ordinary bipolar transistors. Only one side needs the extra transistors if only one flashing lamp is required. Just ground the emitter of the low power side.
The loads may be placed in the collector circuits as this darlington flasher illustrates. The lamps should be rated near the voltage of the power supply. darlington with collector load
Mosfet power transistors will work in most of these circuits as long as the negative feedback biasing is used. A capacitor is needed across the mosfet circuit and is generally a good idea in all of the circuits. Some mosfets will exhibit RF oscillations in this circuit (the VN67, for example). Transistors that worked well were: VN10KM, VN88, SK3165, and IRF531. Most power mosfets that require only a couple of volts to turn them on will work up to their current and power ratings. mosfet version
Here is a way to use the circuit at high voltages. The voltage divider resistors in the gate circuit limit the gate voltage to safe levels. The circuit shown flashes two ordinary 7 watt nightlights but the input voltage must be only 90 VRMS. If the flasher is to be operated directly of off the rectified line voltage, add an 820 ohm, 2 watt resistor in series with each lamp. One lamp may be replaced with a 10k resistor if only one flashing lamp is required.
Warning: This circuit uses potentially deadly voltages and should be constructed only by qualified persons.
high voltage mosfet
These circuits are useful for purposes other than flashing lamps. Here is a simple tone generator driving a 16 ohm speaker at about 2.5 kHz with plenty of volume (set by the 22 ohm). Note the non-symmetrical values. There is no need to waste power in the transistor that isn't driving the load. To get a 50/50 squarewave the product of the bias resistor and capacitor values connecting to one base should be close to the product of the others. (47k X 0.01 is close to 2.2k X 0.22.) tone generator
Since the circuit uses non-symmetrical values, the total current drain will be a squarewave, too. This circuit for a code-practice oscillator oscillates near 1kHz. Notice that the speaker, key, battery, and circuit are all connected in series. This "two-wire" feature can be handy in some situations. A fairly large capacitor is connected across the circuit to make it work properly and this capacitor has a low-pass effect on the squarewave the speaker sees and its value will depend upon the desired frequency. two-wire tone generator
These circuits shown so far are basically a two-stage AC-coupled amplifier with the output fed back to the input through another capacitor. Redrawing the circuit and using DC-coupling between the stages gives this circuit. Emitter degeneration was added to one stage and the resistor values were modified to get both transistors into the active region. The problem with DC-coupled amplifiers is that the high gain can result in the last transistor being fully on or off unless care is taken. one capacitor version
Here is the standard flasher circuit seen in many hobby books with the exception that the bias resistor is connected from collector to base for better reliability. Note that it is also a two-stage amplifier with DC coupling but by switching to a PNP, the biasing is a little simpler. Most engineers looking at this circuit want to add a resistor from base to emitter on the PNP or from the collector to plus on the NPN but the circuit works OK without either. modified traditional circuit
Have fun designing your own flasher. The circuit will need sufficient non-inverting gain to achieve oscillation which probably means at least two transistors. Make sure that the two or more stages are "alive" by biasing them away from ground or the power supply voltage. Then apply the feedback and try to figure out what happens when the circuit switches from being a linear amplifier into a switching, non-linear flip-flop. If the gain is sufficient and non inverting, something will happen!
Here are some notes from readers:
Karen mentions that, " there is a practical supply limit on the good old fashioned two transistor astable, and in fact any timing ciruit that uses a reverse-biased BE junction. Between 5V and 9V most reverse-biased BE junctions go into zener mode. Explains why they were never shown with supplies in excess of about 6V!" Good point, Karen! When one collector pulls down, the voltage on the base of the other transistor will be pulled below the emitter voltage by nearly the supply voltage. I would recommend adding diodes in the emitters much like the LEDs in the first circuits for operation above 6 volts.
Thought I would pass this along. I was using the "Astable Flip Flop Circuit" page and setting up the first example, the "flip flop flasher". I added a potentiometer between the power supply and the two 47K resistors and came up with a single component modification that will adjust the flash rate. I removed the 47K resistors from the power supply and connected them to one end of a 500K VR. I connected the opposite end of the 500K VR to the power supply. The center tap gets connected to either side of the VR producing a 0 - 500K (slow to fast) or 500K - 0 (fast to slow) range.
Trial and error shows that the second circuit, "With negative feedback bias", can be adjustable using a 10k resistor and a 0-100k variable resistor in series between the two transistor bases.
Jay Herde Louisville KY
Thanks Jay!
Jay's first modification connects the bases together somewhat but since one is directly connected to the collector of the other transistor through the capacitor, the switching still occurs. There might be a problem if the potentiometer is much higher in resistance than the resistors, especially if the transistors do not have similar characteristics. A small capacitor, maybe 10% of the timing capacitors, connected to ground at the junction of the bias resistors and potentiometer might fix any problem. Jay's worked with a pot 10 times bigger than the resistor so perhaps the problem is minimal.
The familiar astable flip-flop circuit is a handy configuration for making flashers or generating squarewaves. Here is a typical alternating LED flasher with the LEDs in the emitters instead of collectors as is normally done. (There is another good reason to put them in the emitters - see Karen's note below. ) The bias resistors are directly connected to the supply and are chosen to have a value about 100 times the collector resistor for ordinary gain transistors. The flashing period is approximately the product of this resistance and the capacitance which is about 1 second for the circuit as shown. The 470 ohm resistors set the LED current and may be reduced for lower battery voltage but remember to also reduce the bias resistors. If no LEDs are desired, the emitters may be directly connected to ground and two out-of-phase voltage squarewaves are available on the collectors. Flip-flop Flasher
This is another version of the circuit that uses negative feedback for the bias. This technique is generally more desirable because the feedback ensures that both transistors are in a high-gain, linear mode when power is applied. In actual practice the first circuit will often work "better" with ordinary bipolar transistors since there is no negative feedback slowing the switching. The feedback makes the circuit more immune to parameter variations due to temperature changes, gain variations, or even component substitution. flip-flop oscillator with negative feedback bias
This version will work with just about any NPN darlington transistor. The bias resistor may be much larger due to the high gain of the darlington so much lower value, non-polar capacitors will give a suitable flash rate. Of coures, other applications may require different oscillation rates which are easily achieved by changing the capacitor value. Other voltages and currents may be accommodated by changing the collector resistor value. PNP versions of all of these circuits may be built by reversing the polarity of the battery and polarized capacitors. The high gain of the darlingtons makes it feasible to handle heavy loads either in the emitters as shown or in place of the collector resistors as is commonly done. Lower value bias resistors may be necessary depending upon the load current and the gain of the transistors. darlington version
Here is an unusual way to get more power out of the astable flip-flop without resorting to huge capacitors. The emitter current flows through the base-emitter junction much like the LEDs above saturating the output transistors. The 2N4401 can handle up to 600mA in this circuit but a higher current transistor may be substituted. The base current of the output transistor may be adjusted by changing the 470 ohm resistor, as needed. power flasher
very high power flasher Adding another power transistor on each side brings this flasher into the 10 amp range using ordinary bipolar transistors. Only one side needs the extra transistors if only one flashing lamp is required. Just ground the emitter of the low power side.
The loads may be placed in the collector circuits as this darlington flasher illustrates. The lamps should be rated near the voltage of the power supply. darlington with collector load
Mosfet power transistors will work in most of these circuits as long as the negative feedback biasing is used. A capacitor is needed across the mosfet circuit and is generally a good idea in all of the circuits. Some mosfets will exhibit RF oscillations in this circuit (the VN67, for example). Transistors that worked well were: VN10KM, VN88, SK3165, and IRF531. Most power mosfets that require only a couple of volts to turn them on will work up to their current and power ratings. mosfet version
Here is a way to use the circuit at high voltages. The voltage divider resistors in the gate circuit limit the gate voltage to safe levels. The circuit shown flashes two ordinary 7 watt nightlights but the input voltage must be only 90 VRMS. If the flasher is to be operated directly of off the rectified line voltage, add an 820 ohm, 2 watt resistor in series with each lamp. One lamp may be replaced with a 10k resistor if only one flashing lamp is required.
Warning: This circuit uses potentially deadly voltages and should be constructed only by qualified persons.
high voltage mosfet
These circuits are useful for purposes other than flashing lamps. Here is a simple tone generator driving a 16 ohm speaker at about 2.5 kHz with plenty of volume (set by the 22 ohm). Note the non-symmetrical values. There is no need to waste power in the transistor that isn't driving the load. To get a 50/50 squarewave the product of the bias resistor and capacitor values connecting to one base should be close to the product of the others. (47k X 0.01 is close to 2.2k X 0.22.) tone generator
Since the circuit uses non-symmetrical values, the total current drain will be a squarewave, too. This circuit for a code-practice oscillator oscillates near 1kHz. Notice that the speaker, key, battery, and circuit are all connected in series. This "two-wire" feature can be handy in some situations. A fairly large capacitor is connected across the circuit to make it work properly and this capacitor has a low-pass effect on the squarewave the speaker sees and its value will depend upon the desired frequency. two-wire tone generator
These circuits shown so far are basically a two-stage AC-coupled amplifier with the output fed back to the input through another capacitor. Redrawing the circuit and using DC-coupling between the stages gives this circuit. Emitter degeneration was added to one stage and the resistor values were modified to get both transistors into the active region. The problem with DC-coupled amplifiers is that the high gain can result in the last transistor being fully on or off unless care is taken. one capacitor version
Here is the standard flasher circuit seen in many hobby books with the exception that the bias resistor is connected from collector to base for better reliability. Note that it is also a two-stage amplifier with DC coupling but by switching to a PNP, the biasing is a little simpler. Most engineers looking at this circuit want to add a resistor from base to emitter on the PNP or from the collector to plus on the NPN but the circuit works OK without either. modified traditional circuit
Have fun designing your own flasher. The circuit will need sufficient non-inverting gain to achieve oscillation which probably means at least two transistors. Make sure that the two or more stages are "alive" by biasing them away from ground or the power supply voltage. Then apply the feedback and try to figure out what happens when the circuit switches from being a linear amplifier into a switching, non-linear flip-flop. If the gain is sufficient and non inverting, something will happen!
Here are some notes from readers:
Karen mentions that, " there is a practical supply limit on the good old fashioned two transistor astable, and in fact any timing ciruit that uses a reverse-biased BE junction. Between 5V and 9V most reverse-biased BE junctions go into zener mode. Explains why they were never shown with supplies in excess of about 6V!" Good point, Karen! When one collector pulls down, the voltage on the base of the other transistor will be pulled below the emitter voltage by nearly the supply voltage. I would recommend adding diodes in the emitters much like the LEDs in the first circuits for operation above 6 volts.
Thought I would pass this along. I was using the "Astable Flip Flop Circuit" page and setting up the first example, the "flip flop flasher". I added a potentiometer between the power supply and the two 47K resistors and came up with a single component modification that will adjust the flash rate. I removed the 47K resistors from the power supply and connected them to one end of a 500K VR. I connected the opposite end of the 500K VR to the power supply. The center tap gets connected to either side of the VR producing a 0 - 500K (slow to fast) or 500K - 0 (fast to slow) range.
Trial and error shows that the second circuit, "With negative feedback bias", can be adjustable using a 10k resistor and a 0-100k variable resistor in series between the two transistor bases.
Jay Herde Louisville KY
Thanks Jay!
Jay's first modification connects the bases together somewhat but since one is directly connected to the collector of the other transistor through the capacitor, the switching still occurs. There might be a problem if the potentiometer is much higher in resistance than the resistors, especially if the transistors do not have similar characteristics. A small capacitor, maybe 10% of the timing capacitors, connected to ground at the junction of the bias resistors and potentiometer might fix any problem. Jay's worked with a pot 10 times bigger than the resistor so perhaps the problem is minimal.
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