Audio Oscillators
Here is a phase-shift audio oscillator with excellent distortion characteristics thanks to "softened" diode limiting provided by the 1N914 and resistor divider and degenerated gain provided by the 68 ohm emitter resistor. For minimum distortion, increase the 68 ohm resistor to a point just below where oscillation stops. A simple buffer may be added for driving lower impedance loads. The output amplitude will be about 5 volts p-p but one of the 1N914's 10k divider resistors may be changed for a different output amplitude. The circuit will work well with a power supply voltage other than 9 volts but the 68 ohm resistor may need adjustment.
schematic
The circuit can be built in the "blobular cluster" style and potted with epoxy mixed with a little model airplane paint. Use quick-setting epoxy and, holding the circuit by the legs, keep rotating the blob near the end of the cure cycle to get an even coat - quite an art form! The finished module looks quite professional, not unlike many dipped caps.
schematic
Here is a two-transistor Wien bridge oscillator using an ordinary night-light bulb for stabilization. The output is about 6 volts p-p and can drive fixed loads as low as 2 or 3 thousand ohms without additional buffering. A 10 k amplitude potentiometer with the wiper going to a high input impedance output amplifier would make an excellent load.
Excellent distortion is achieved by adjusting the 1 k feedback potentiometer until the output amplitude is about a volt less than the maximum level (with the pot set to the highest resistance). Wait a few seconds between adjustments to give the bulb time to stabilize; the audio signal actually heats the bulb's filament causing the resistance to go up which controls the loop gain. You will see the signal bounce a little as the bulb gains control. This simple version of the popular Wien bridge oscillator uses feedback to hold the junction of the two RC networks (base of first transistor) near zero volts (100 mV p-p) and the ends of the RC networks move in opposite directions like a see-saw.
With the resistor values shown, the frequency may be varied from a few Hz to over 60 kHz by selecting a value for C between 1 uF and 47 pF. The frequency will be reasonably close to 1/ (6.28 x RC). R may be varied also for additional range but values too low or high may cause problems. The 7-watt bulb may be replaced by a smaller type with similar resistance (more than 50 ohms) but the long time constant of the larger filament is helpful when generating very low frequencies.
bullet
The 100 uF output capacitor may be smaller if very low frequencies are not generated.
bullet
The 22 pF is added for stability and may be eliminated depending on the transistor types and circuit layout. A larger value may be needed in some cases.
bullet
The output is ground referenced with no DC offset.
bullet
A load change may require readjustment of the feedback potentiometer.
bullet
The series RC may be switched if it is desired to have the resistors connected together.
bullet
A high gain transistor like the MPSA-18 for the first transistor will allow a much larger value for R, up to 1 Megohm.
The circuit should draw between 18 and 45 mA, a value determined by the transistor gain and the value of R. Current outside of this range may cause distortion. The 1.2 k emitter resistor may be varied slightly to adjust the current consumption; shoot for 25 to 30 mA.
It should be noted that op-amps make great Wien bridge oscillators without significant impedance and bias concerns! There are dozens on the web and in manufacturers' application notes. But sometimes a couple of friendly transistors fit the bill perfectly.
Thursday, July 30, 2009
seismic alarms
Seismic Alarms
Seismic Alarm
My original "Deer Repellent/ Seismic Sensor" recommended using a speaker with a weight glued to the cone as a vibration sensor (see bottom of page) but it recently occurred to me that the speaker could supply the mass itself. By gluing a standoff to the center of the cone, an inexpensive 2", 8 ohm speaker becomes a vibration sensor with a natural resonance below 100Hz which is quite good. My implementation is shown below:
After the epoxy cured, the speaker was hung in the middle of the lid from an empty instant ice tea jar. Be careful mounting the speaker! The cone is weak and cannot tolerate a lot of torque when you tighten the screw. Put some epoxy on the screw threads near the head and hand-tighten only. The epoxy will provide a firm mount and will seal the hole, too. Flexible wires lead from the speaker terminals to the circuit:
The circuit is similar in operation to my original design but without the now-obsolete CA3094 op-amp. This design is every bit as sensitive and uses ordinary discrete components. The output will drive any variety of buzzers, relays or other devices but my favorite is a Radio Shack wireless doorbell:
schematic
This transmitter for this "wireless remote door chime" has a couple of connections on the back that are in parallel with the pushbutton and shorting these contacts causes the bell to ring. One of the contacts has a positive voltage near 9 volts and this contact should go to the drain of the FET. The other should go to the negative battery connection. I used a couple of brass standoffs to space the circuit board above these terminals to mount the board. If you want a different load (like a 9 volt buzzer), connect the negative end to the drain and the positive end to the positive battery terminal. You can use a different battery to power the load, if desired. I decided to power my circuit from the battery in the transmitter so I drilled a small hole in the back and ran two wires out from the PCB where the battery wires attached. If you use a separate battery, the transmitter can be left "unmolested".
I wedged the transmitter into the jar with a piece of styrofoam then screwed on the lid. It will take a minute or so before the circuit reaches full sensitivity. But then the slightest jarring of the jar will ring the bell! The circuit will take a while to recover after a trigger, too. To protect a large area with the sensor set it over a pipe buried just beneath the surface. A footstep anywhere along the pipe will shake the sensor.
Parts Considerations:
*
Just about any modern bipolar transistors will work including the 2N3904 (NPN) and 2N3906 (PNP).
*
The output mosfet transistor may be just about any N-channel enhancement type including the common 2N7000 or older VN10KM.
*
The resistors are not critical at all and any type or size should work fine and reasonably close values are OK.
*
The 1uF capacitors may be just about any type, with a 16 volt aluminum electrolytic being the most likely choice. The amplifier stages will work with smaller values. Increase the value of the 1uF on the gate of the mosfet for a longer on-time, if desired.
*
The battery is an ordinary 9 volt rectangular type and no power switch is needed because the current drain is extremely low. You may share the battery in the transmitter.
*
The doorbell transmitter is a Radio Shack wireless doorbell but others should be adaptable. If you find one without the rear contacts, connect two wires to the back of the pushbutton and use a voltmeter to determine which wire has the positive voltage.
*
The speaker is an ordinary 2", 8 ohm speaker but others will work, too.
*
The jar is chosen to be tall enough to accommodate the long doorbell transmitter.
Theory of Operation:
The first transistor is a very low current, high gain audio amplifier that boosts the tiny signal from the speaker. When the signal is large enough, due to vibration, it pulls down the PNP base enough to turn it on slightly. Once that happens, regenerative action cause it and the third NPN to turn on fully. The voltage rises on the gate of the mosfet, turning it on. The capacitor in the emitter of the PNP discharges and the transistor turns off suddenly due to reverse regenerative action. It takes many seconds for the circuit to recover and be ready for another trigger.
Other Ideas:
The circuit is a very sensitive "changing voltage" detector and the input could be any number of sensors besides a speaker.
*
For sensing vibration, a piezo speaker could be used in place of the dynamic speaker.
*
An electret microphone could make a sensitive sound detector.
*
A photocell or pin photo diode could be used to sense a sudden change in light level.
*
A diode detector could alert you to the presence of a transmitter or cell phone.
The following circuit is my original design an is supplied for historical purposes. If you happen to have a few of the CA3094s, this is a great sensor!
Deer Repellent/ Seismic Sensor
Here is a simple sensor which can detect the seismic vibrations caused by a person or large animal walking nearby. A representative application for the sensor is a deer repellent for the vegetable garden. When a deer steps near the sensor a loud buzzer or beeper sounds for a few seconds startling the would-be vegetable thief away. The sensor also makes an effective intruder detector to catch trespassers as soon as they step on the property!
The unit is designed to
Seismic Alarm
My original "Deer Repellent/ Seismic Sensor" recommended using a speaker with a weight glued to the cone as a vibration sensor (see bottom of page) but it recently occurred to me that the speaker could supply the mass itself. By gluing a standoff to the center of the cone, an inexpensive 2", 8 ohm speaker becomes a vibration sensor with a natural resonance below 100Hz which is quite good. My implementation is shown below:
After the epoxy cured, the speaker was hung in the middle of the lid from an empty instant ice tea jar. Be careful mounting the speaker! The cone is weak and cannot tolerate a lot of torque when you tighten the screw. Put some epoxy on the screw threads near the head and hand-tighten only. The epoxy will provide a firm mount and will seal the hole, too. Flexible wires lead from the speaker terminals to the circuit:
The circuit is similar in operation to my original design but without the now-obsolete CA3094 op-amp. This design is every bit as sensitive and uses ordinary discrete components. The output will drive any variety of buzzers, relays or other devices but my favorite is a Radio Shack wireless doorbell:
schematic
This transmitter for this "wireless remote door chime" has a couple of connections on the back that are in parallel with the pushbutton and shorting these contacts causes the bell to ring. One of the contacts has a positive voltage near 9 volts and this contact should go to the drain of the FET. The other should go to the negative battery connection. I used a couple of brass standoffs to space the circuit board above these terminals to mount the board. If you want a different load (like a 9 volt buzzer), connect the negative end to the drain and the positive end to the positive battery terminal. You can use a different battery to power the load, if desired. I decided to power my circuit from the battery in the transmitter so I drilled a small hole in the back and ran two wires out from the PCB where the battery wires attached. If you use a separate battery, the transmitter can be left "unmolested".
I wedged the transmitter into the jar with a piece of styrofoam then screwed on the lid. It will take a minute or so before the circuit reaches full sensitivity. But then the slightest jarring of the jar will ring the bell! The circuit will take a while to recover after a trigger, too. To protect a large area with the sensor set it over a pipe buried just beneath the surface. A footstep anywhere along the pipe will shake the sensor.
Parts Considerations:
*
Just about any modern bipolar transistors will work including the 2N3904 (NPN) and 2N3906 (PNP).
*
The output mosfet transistor may be just about any N-channel enhancement type including the common 2N7000 or older VN10KM.
*
The resistors are not critical at all and any type or size should work fine and reasonably close values are OK.
*
The 1uF capacitors may be just about any type, with a 16 volt aluminum electrolytic being the most likely choice. The amplifier stages will work with smaller values. Increase the value of the 1uF on the gate of the mosfet for a longer on-time, if desired.
*
The battery is an ordinary 9 volt rectangular type and no power switch is needed because the current drain is extremely low. You may share the battery in the transmitter.
*
The doorbell transmitter is a Radio Shack wireless doorbell but others should be adaptable. If you find one without the rear contacts, connect two wires to the back of the pushbutton and use a voltmeter to determine which wire has the positive voltage.
*
The speaker is an ordinary 2", 8 ohm speaker but others will work, too.
*
The jar is chosen to be tall enough to accommodate the long doorbell transmitter.
Theory of Operation:
The first transistor is a very low current, high gain audio amplifier that boosts the tiny signal from the speaker. When the signal is large enough, due to vibration, it pulls down the PNP base enough to turn it on slightly. Once that happens, regenerative action cause it and the third NPN to turn on fully. The voltage rises on the gate of the mosfet, turning it on. The capacitor in the emitter of the PNP discharges and the transistor turns off suddenly due to reverse regenerative action. It takes many seconds for the circuit to recover and be ready for another trigger.
Other Ideas:
The circuit is a very sensitive "changing voltage" detector and the input could be any number of sensors besides a speaker.
*
For sensing vibration, a piezo speaker could be used in place of the dynamic speaker.
*
An electret microphone could make a sensitive sound detector.
*
A photocell or pin photo diode could be used to sense a sudden change in light level.
*
A diode detector could alert you to the presence of a transmitter or cell phone.
The following circuit is my original design an is supplied for historical purposes. If you happen to have a few of the CA3094s, this is a great sensor!
Deer Repellent/ Seismic Sensor
Here is a simple sensor which can detect the seismic vibrations caused by a person or large animal walking nearby. A representative application for the sensor is a deer repellent for the vegetable garden. When a deer steps near the sensor a loud buzzer or beeper sounds for a few seconds startling the would-be vegetable thief away. The sensor also makes an effective intruder detector to catch trespassers as soon as they step on the property!
The unit is designed to
temparature control circuits
Temperature Control Circuits
Here is a simple temperature controller suitable for a variety of purposes; it was designed to heat the copper float out of a toilet for a barometer! The values and components are not critical; the components shown were selected primarily for easy availability. The temperature setting resistor depends on the NTC type thermistor and should be selected to heat the oven to the desired operating temperature. Temporarily connect a current meter in series with the power supply to monitor the oven's performance. The circuit should draw about 500 ma until the set point is reached and then the current should cut back to about 200 ma (depending on the set temperature, oven structure, and insulation quality). A little cycling before settling is fine but if the current cuts on and off repeatedly, then move the thermistor closer to the heater resistor or decrease the 330k resistor. Higher heating current may be achieved by lowering the value of the heater resistors but at some point the power transistor dissipation may become too high.
Higher resistance negative temp-co thermistors may be used by increasing the resistor connecting from the thermistor to the zener by a proportional amount. The thermistor and 270 ohm resistors are mounted on the float (or other metal oven structure) and the other components are mounted in a separate metal box. To simplify construction, connect the oven structure to the negative terminal of the supply (negative ground) and connect the metal circuit box to the positive terminal (positive ground). This wiring allows the resistors and thermistors to be directly soldered to the oven structure and it allows the 2N3055 to be directly connected to the metal case without insulating hardware. Obviously, the case must not touch the oven structure.
baro4.gif (6206 bytes)
Below is a temperature controller capable of extremely precise control. See http://www.techlib.com/electronics/barometer.html for the oven structure details to make an oven as good as any commercial products with parts from the local plumbing supply house!
ovencontrol.gif (8284 bytes)
The oven controller below is about as simple at they get! It uses a TL431 shunt regulator as the error amplifier and power controller and only three other components. The temperature set by the 18k resistor and the 220 ohm, 1 watt resistor provides the heat, along with the TL431 itself. The oven structure is made by completely flattening one end a 1.25" long piece of 1/2 inch copper tubing with a vise, turning it 90 degrees and flattening the other end, leaving enough of an opening to accommodate the parts to be ovenized. The components are soldered and glued directly to the copper as seen in the photo. The thermistor is a bit difficult to see; it is the orange component that looks similar to a diode buried in the epoxy at the bottom of the photo. The thermal gain of this oven will be about 20 or 30 when it is inside a 2" x 2" x 2" Styrofoam insulator which is sufficient to greatly improve the performance of voltage references, crystal oscillators, etc. To achieve best performance use long lead wires for the circuits in the oven and tuck several inches of wire inside the oven after the circuit to prevent heat loss out the wires.
Warm-up current is about 50 mA and operating current is only about 25 mA with the foam described above. The operating temperature is about 50° C with a 10k NTC thermistor with a 3%/°C slope. Other NTC thermistors will work but the 18k resistor must be changed to set the proper operating point.
schematic
Here is a simple temperature controller suitable for a variety of purposes; it was designed to heat the copper float out of a toilet for a barometer! The values and components are not critical; the components shown were selected primarily for easy availability. The temperature setting resistor depends on the NTC type thermistor and should be selected to heat the oven to the desired operating temperature. Temporarily connect a current meter in series with the power supply to monitor the oven's performance. The circuit should draw about 500 ma until the set point is reached and then the current should cut back to about 200 ma (depending on the set temperature, oven structure, and insulation quality). A little cycling before settling is fine but if the current cuts on and off repeatedly, then move the thermistor closer to the heater resistor or decrease the 330k resistor. Higher heating current may be achieved by lowering the value of the heater resistors but at some point the power transistor dissipation may become too high.
Higher resistance negative temp-co thermistors may be used by increasing the resistor connecting from the thermistor to the zener by a proportional amount. The thermistor and 270 ohm resistors are mounted on the float (or other metal oven structure) and the other components are mounted in a separate metal box. To simplify construction, connect the oven structure to the negative terminal of the supply (negative ground) and connect the metal circuit box to the positive terminal (positive ground). This wiring allows the resistors and thermistors to be directly soldered to the oven structure and it allows the 2N3055 to be directly connected to the metal case without insulating hardware. Obviously, the case must not touch the oven structure.
baro4.gif (6206 bytes)
Below is a temperature controller capable of extremely precise control. See http://www.techlib.com/electronics/barometer.html for the oven structure details to make an oven as good as any commercial products with parts from the local plumbing supply house!
ovencontrol.gif (8284 bytes)
The oven controller below is about as simple at they get! It uses a TL431 shunt regulator as the error amplifier and power controller and only three other components. The temperature set by the 18k resistor and the 220 ohm, 1 watt resistor provides the heat, along with the TL431 itself. The oven structure is made by completely flattening one end a 1.25" long piece of 1/2 inch copper tubing with a vise, turning it 90 degrees and flattening the other end, leaving enough of an opening to accommodate the parts to be ovenized. The components are soldered and glued directly to the copper as seen in the photo. The thermistor is a bit difficult to see; it is the orange component that looks similar to a diode buried in the epoxy at the bottom of the photo. The thermal gain of this oven will be about 20 or 30 when it is inside a 2" x 2" x 2" Styrofoam insulator which is sufficient to greatly improve the performance of voltage references, crystal oscillators, etc. To achieve best performance use long lead wires for the circuits in the oven and tuck several inches of wire inside the oven after the circuit to prevent heat loss out the wires.
Warm-up current is about 50 mA and operating current is only about 25 mA with the foam described above. The operating temperature is about 50° C with a 10k NTC thermistor with a 3%/°C slope. Other NTC thermistors will work but the 18k resistor must be changed to set the proper operating point.
schematic
noise makers
Noise Makers
Emergency Siren Simulator
This siren circuit simulates police, fire or other emergency sirens that produce an up and down wail.
schematic
The heart of the circuit is the two transistor flasher with frequency modulation applied to the base of the first transistor. When the pushbutton is depressed, the frequency of oscillation climbs to a peak and when the button is released, the frequency descends due to the rising and falling voltage on the 22 uF capacitor. The rate of change is determined by the capacitor value and the 100k resistor from the pushbutton. The oscillation eventually stops if the button is not depressed and the current consumption drops to a tiny level so no power switch is needed.
The 0.1 uF determines the pitch of the siren: A 0.047uF will give a higher pitch siren and a 0.001 uF will give an ultrasonic (at least for me, anyway) siren from 15 to 30 kHz which might have an interesting effect on the neighborhood dogs! The 33k resistor from the collector of the PNP to the base of the NPN widens the pulse to the speaker giving greater volume.
The flasher circuit drives a PNP transistor which powers the speaker. This transistor may be a small-signal transistor like the 2N4403 in most applications since it will not dissipate much power thanks to the rapid on-and-off switching. The 100 ohm and 100uF capacitor in series with the speaker limit the current to about 60 mA and they may be replaced with a short circuit for a louder siren as long as the transistor can take the increased current. The prototype drew about 120 mA when shorted which is fine for the 2N4403.
Transistor substitutions should be fine - try just about any small-signal transistors but avoid high frequency types so that you do not end up with unwanted RF oscillations.
Electronic Chime
This circuit simulates a chime similar to the sound many cars make when the keys are left in the ignition. The bottom two gates form a squarewave audio oscillator that drives the base of the 2N4401, turning it on and off at an audio rate. The top two gates produce a short low-going pulse about once per second that discharges the 10 uF capacitor through the diode. The voltage then jumps up and slowly decays through the 15 k collector resistor when the 2N4401 is conducting. The result is a squarewave on the collector of the 2N4401 that jumps up quickly then decays slowly. The darlington emitter-follower buffers the squarewave and drives a small speaker.
schematic
The tone frequency is set by the 1000 pF capacitor and the cadence of the chime is set by the 0.1 uF capacitor. The 10 uF capacitor determines how quickly the chime dies out and the 3.3 k/3.3 uF soften the attack time of the leading edge of the chime. The volume is set by the 22 ohm resistor and 100 uF bypass capacitor. These values may be experimentally varied to produce the desired sound.
Emergency Siren Simulator
This siren circuit simulates police, fire or other emergency sirens that produce an up and down wail.
schematic
The heart of the circuit is the two transistor flasher with frequency modulation applied to the base of the first transistor. When the pushbutton is depressed, the frequency of oscillation climbs to a peak and when the button is released, the frequency descends due to the rising and falling voltage on the 22 uF capacitor. The rate of change is determined by the capacitor value and the 100k resistor from the pushbutton. The oscillation eventually stops if the button is not depressed and the current consumption drops to a tiny level so no power switch is needed.
The 0.1 uF determines the pitch of the siren: A 0.047uF will give a higher pitch siren and a 0.001 uF will give an ultrasonic (at least for me, anyway) siren from 15 to 30 kHz which might have an interesting effect on the neighborhood dogs! The 33k resistor from the collector of the PNP to the base of the NPN widens the pulse to the speaker giving greater volume.
The flasher circuit drives a PNP transistor which powers the speaker. This transistor may be a small-signal transistor like the 2N4403 in most applications since it will not dissipate much power thanks to the rapid on-and-off switching. The 100 ohm and 100uF capacitor in series with the speaker limit the current to about 60 mA and they may be replaced with a short circuit for a louder siren as long as the transistor can take the increased current. The prototype drew about 120 mA when shorted which is fine for the 2N4403.
Transistor substitutions should be fine - try just about any small-signal transistors but avoid high frequency types so that you do not end up with unwanted RF oscillations.
Electronic Chime
This circuit simulates a chime similar to the sound many cars make when the keys are left in the ignition. The bottom two gates form a squarewave audio oscillator that drives the base of the 2N4401, turning it on and off at an audio rate. The top two gates produce a short low-going pulse about once per second that discharges the 10 uF capacitor through the diode. The voltage then jumps up and slowly decays through the 15 k collector resistor when the 2N4401 is conducting. The result is a squarewave on the collector of the 2N4401 that jumps up quickly then decays slowly. The darlington emitter-follower buffers the squarewave and drives a small speaker.
schematic
The tone frequency is set by the 1000 pF capacitor and the cadence of the chime is set by the 0.1 uF capacitor. The 10 uF capacitor determines how quickly the chime dies out and the 3.3 k/3.3 uF soften the attack time of the leading edge of the chime. The volume is set by the 22 ohm resistor and 100 uF bypass capacitor. These values may be experimentally varied to produce the desired sound.
telephone circuits
Telephone Circuits
Telephone In-Use Indicator
Phoneuse.gif (8699 bytes)
When a new computer modem enters the household, the demands on the home phone line skyrockets. The Internet surfer can use phone time on a par with the most talkative teenager. And the computer modem user can be quite sensitive about his privacy: simply lifting another receiver can knock him off-line causing emotional stress. The phone wiring may be modified so that the modem is always in control by connecting the phone line directly to the modem and connecting the rest of the phones to the modem's "phone" jack. But this solution gives the computer user too much power over the phone line and it doesn't solve the problem if two computers share a single line. Here is a simple blinking LED circuit which will alert users when the line is in use before the receiver is lifted. The circuit loads the phone line so lightly that it meets the on-hook telephone equipment leakage specification and the short lamp flashes draw very little current from the nine-volt battery. One of these devices may be placed at each extension without significantly loading the phone line. The circuit is connected to the red and green wires for a single-line system or the yellow and black wires for the second line in a two-line system. Polarity doesn't matter, thanks to the full-wave rectifier. In order to preserve your phone line balance, do not power this device from a line-powered power supply. Only use a battery as shown and insulate the battery and circuitry by building the device into a plastic case. Do not ground the circuitry. The circuit will work with other batteries and battery voltage. Four AA, C, or even D cells (6 volts) will last considerably longer if you have teenagers burning up your batteries. A small 9-volt rectangular battery will be fine for most users.
Notes:
bullet The diode bridge eliminates polarity concerns. It may be left out but the wires to the phone line may need to be reversed if the circuit doesn't work properly.
bullet The 22 megohm resistors are sufficiently high to meet phone circuit leakage specifications.
bullet A 2N4401 will usually work in place of the MPSA-18 but if the transistor gain is too low the flashing will not stop.
Telephone Ringer
Caution: The circuit generates a high voltage which can shock.
The Phone Ringer circuit will work with any ordinary phone including older bell ringer types. The circuit rings the phone in a completely realistic manner until someone answers. When the receiver is lifted the user hears the audio of your choice. It might be another telephone, a tape recording, a favorite talk radio show, a fake busy-signal, a scanner tuned to weather or police, cues for the actor who forgot his next line, or whatever audio source strikes your fancy. DC current is passed through the phone to activate the phone’s electronics.
Ringer.gif (13955 bytes)
Provisions for experimenters include a ring inhibit control and an additional transistor will activate devices when the phone is answered. The ring inhibit control is used to start the ringing when a signal goes low and the activate-on-answer control can start a tape recording or other device when the phone is answered. For example, the ringer could be triggered by an alarm clock to make an artificial but realistic wake-up call. When you answer, your own voice instructs you about the importance of getting up. This wake-up caller is quite persistent, calling back the instant you hang up!
Do not connect this circuit or the phones used with this circuit to the phone lines.
The phone cable will have red and green wires which are simply connected to the points indicated by the schematic. Polarity should not matter. Other devices may be connected as described but no connection to a "real" phone line is intended.
The circuitry is simple and not particularly critical. The first two inverters form a slow pulse generator which controls the ringing rate. Change the 0.22 uf capacitor to change the ringing rate and change the 22 Meg. resistor in series with the diode to change the length of the ring. The second two inverters generate the 20 Hz ringing signal. This frequency can be changed by changing the .033uf capacitor. Mechanical bell ringers have a resonant clapper and should be driven with a frequency near 20 Hz but a slight variation may give a better ring. The last two inverters buffer the ringing signal and drive the two output transistors. Practically any transistors can be used for the output including 2N4401 and 2N4403 but power transistors in a TO-220 package might be more desirable if a lot of ringing is anticipated. The transistors should be capable of handling several hundred milliamperes. Any low-leakage signal diodes will work for the 1N914s.
The power transformer must handle 20 Hz with at least some efficiency so it is best to use larger units. Molded transformers will work fine but of course they cannot be DC types which have built-in rectifiers. Choose a transformer with a low voltage winding rated for an output voltage well below the DC power supply used. The circuit as shown runs on 12 volts with a 9 volt transformer. Some transformers have 220 volt windings which can give a stronger ring if necessary. A 6 volt filament transformer powered by the circuit as shown will give a quite strong ring. Reduce the 10 ohm emitter resistors to 4.7 ohms to get more ring power if power transistors are used . (Don't leave them out entirely since they help prevent high frequency oscillations.)
Ringfig2.gif (3724 bytes)
Ringing is inhibited by applying a voltage near VCC to the 1N914 diode. A simple transistor inverter can change the sense and increase the sensitivity so that a couple of volts will start the ringing (fig. 2). If the phone is to ring when the squelch of a modern scanner breaks try looking in the scanner for an analog switch integrated circuit. One of its pins will jump between 0 and 5 volts when the squelch breaks and this signal is fine for driving the inverter circuit. Fig. 2 also shows how to connect a photocell so that the phone rings only when the lights are off. (Record a dial tone so the victim concludes that the caller keeps hanging up just as he turns on the light.) The ringer control can also be used in a variety of other ways to automate the ringing. For example, a 470k pull-up resistor combined with a large electrolytic capacitor connected to ground makes an interesting doorbell. Just connect the doorbell switch across the capacitor and the phone will ring for a few seconds when the switch is pushed. (The capacitor discharges quickly but charges slowly.)
Ringfig3.gif (2798 bytes)
Fig. 3 shows how to add an answer activated control. The 1k resistor may be replaced with a relay for controlling a tape recorder. Put a diode across the winding to protect the transistor from inductive kick-back.
The phone ringer may be used to construct a pretend cellular phone system for the kids using an ordinary cordless phone and a regular phone wired in series. Keep the wiring neat and insulated so that the ring voltage doesn't "bite" anyone. Connect the ringer, ordinary phone and the base unit of the cordless phone in series. Wire a switch which enables the ringer and shorts the ordinary phone (two-pole switch). When the cordless phone is answered, flip the switch to talk. Shorting the ordinary phone is probably not necessary but be prepaired for a rather loud buzzing in the earpiece when the other phone rings! The advanced experimenter may wish to build an artificial phone system by adding a on-hook high voltage supply, dial tone oscillators, and appropriate switching circuitry. Quite a challange!
The Surfer’s Preserver
The Surfer’s Preserver is a simple device that prevents other phones in the house from disrupting your critical Internet session by disconnecting them from the line while you surf! The circuit is also useful in preventing eavesdropping from other extensions since other phones are "dead" until you hang up. The circuit wires in series with either of the offending phone’s wires (red or green) and it is small enough to tuck behind the wall cover plate.
Circuit Description: Due to the resistor divider, the SCR will not fire unless there is at least 17 volts across the bridge. When the receiver is lifted, the full line voltage appears across the circuit and the SCR triggers. The SCR will remain triggered since the DC phone current is about 25 mA and the SCR holding current is only about 5 mA. If the phone line is in use when the receiver is lifted, the line voltage is insufficient to trigger the SCR and the phone remains disconnected. When the phone rings, the 17 volt threshold is quickly passed and the SCR triggers early in the ring voltage cycle, supplying a nearly full amplitude ring voltage to the phone.
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A momentary push-button switch may be added across the 33k resistor to manually trigger the SCR so that the phone can connect when another phone is off-hook. This push-button could be mounted in the wall plate if the plate is in a convenient location or the circuit could be built into the telephone itself with a small switch added on the side. This push-button is handy if more than one telephone is on the line.
Other SCRs may be substituted as long as their working voltage is above 150 volts and their holding current is well below your phone’s current. Connect a current meter in series with your phone to determine your current - expect about 25 to 30 mA.
The circuit requires that the modem or other phones pull the line voltage below 17 volts when off-hook. A simple voltage check will determine if the voltage is dropping low enough. It will typically drop to 5 volts. If you must raise the trigger voltage, increase the 33k resistor. A very high resistor value may reduce the ringing volume on older phones or prevent normal phone use.
A separate circuit may be constructed for each phone or one circuit may be used to disconnect several phones. To use one circuit for several phones, make sure that they share a common wire not shared with the modem. Place the circuit in series with the common wire. The advantage of this connection is that the push-button is not needed to transfer a call from one phone to another but some custom wiring may be necessary.
Telephone In-Use Indicator
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When a new computer modem enters the household, the demands on the home phone line skyrockets. The Internet surfer can use phone time on a par with the most talkative teenager. And the computer modem user can be quite sensitive about his privacy: simply lifting another receiver can knock him off-line causing emotional stress. The phone wiring may be modified so that the modem is always in control by connecting the phone line directly to the modem and connecting the rest of the phones to the modem's "phone" jack. But this solution gives the computer user too much power over the phone line and it doesn't solve the problem if two computers share a single line. Here is a simple blinking LED circuit which will alert users when the line is in use before the receiver is lifted. The circuit loads the phone line so lightly that it meets the on-hook telephone equipment leakage specification and the short lamp flashes draw very little current from the nine-volt battery. One of these devices may be placed at each extension without significantly loading the phone line. The circuit is connected to the red and green wires for a single-line system or the yellow and black wires for the second line in a two-line system. Polarity doesn't matter, thanks to the full-wave rectifier. In order to preserve your phone line balance, do not power this device from a line-powered power supply. Only use a battery as shown and insulate the battery and circuitry by building the device into a plastic case. Do not ground the circuitry. The circuit will work with other batteries and battery voltage. Four AA, C, or even D cells (6 volts) will last considerably longer if you have teenagers burning up your batteries. A small 9-volt rectangular battery will be fine for most users.
Notes:
bullet The diode bridge eliminates polarity concerns. It may be left out but the wires to the phone line may need to be reversed if the circuit doesn't work properly.
bullet The 22 megohm resistors are sufficiently high to meet phone circuit leakage specifications.
bullet A 2N4401 will usually work in place of the MPSA-18 but if the transistor gain is too low the flashing will not stop.
Telephone Ringer
Caution: The circuit generates a high voltage which can shock.
The Phone Ringer circuit will work with any ordinary phone including older bell ringer types. The circuit rings the phone in a completely realistic manner until someone answers. When the receiver is lifted the user hears the audio of your choice. It might be another telephone, a tape recording, a favorite talk radio show, a fake busy-signal, a scanner tuned to weather or police, cues for the actor who forgot his next line, or whatever audio source strikes your fancy. DC current is passed through the phone to activate the phone’s electronics.
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Provisions for experimenters include a ring inhibit control and an additional transistor will activate devices when the phone is answered. The ring inhibit control is used to start the ringing when a signal goes low and the activate-on-answer control can start a tape recording or other device when the phone is answered. For example, the ringer could be triggered by an alarm clock to make an artificial but realistic wake-up call. When you answer, your own voice instructs you about the importance of getting up. This wake-up caller is quite persistent, calling back the instant you hang up!
Do not connect this circuit or the phones used with this circuit to the phone lines.
The phone cable will have red and green wires which are simply connected to the points indicated by the schematic. Polarity should not matter. Other devices may be connected as described but no connection to a "real" phone line is intended.
The circuitry is simple and not particularly critical. The first two inverters form a slow pulse generator which controls the ringing rate. Change the 0.22 uf capacitor to change the ringing rate and change the 22 Meg. resistor in series with the diode to change the length of the ring. The second two inverters generate the 20 Hz ringing signal. This frequency can be changed by changing the .033uf capacitor. Mechanical bell ringers have a resonant clapper and should be driven with a frequency near 20 Hz but a slight variation may give a better ring. The last two inverters buffer the ringing signal and drive the two output transistors. Practically any transistors can be used for the output including 2N4401 and 2N4403 but power transistors in a TO-220 package might be more desirable if a lot of ringing is anticipated. The transistors should be capable of handling several hundred milliamperes. Any low-leakage signal diodes will work for the 1N914s.
The power transformer must handle 20 Hz with at least some efficiency so it is best to use larger units. Molded transformers will work fine but of course they cannot be DC types which have built-in rectifiers. Choose a transformer with a low voltage winding rated for an output voltage well below the DC power supply used. The circuit as shown runs on 12 volts with a 9 volt transformer. Some transformers have 220 volt windings which can give a stronger ring if necessary. A 6 volt filament transformer powered by the circuit as shown will give a quite strong ring. Reduce the 10 ohm emitter resistors to 4.7 ohms to get more ring power if power transistors are used . (Don't leave them out entirely since they help prevent high frequency oscillations.)
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Ringing is inhibited by applying a voltage near VCC to the 1N914 diode. A simple transistor inverter can change the sense and increase the sensitivity so that a couple of volts will start the ringing (fig. 2). If the phone is to ring when the squelch of a modern scanner breaks try looking in the scanner for an analog switch integrated circuit. One of its pins will jump between 0 and 5 volts when the squelch breaks and this signal is fine for driving the inverter circuit. Fig. 2 also shows how to connect a photocell so that the phone rings only when the lights are off. (Record a dial tone so the victim concludes that the caller keeps hanging up just as he turns on the light.) The ringer control can also be used in a variety of other ways to automate the ringing. For example, a 470k pull-up resistor combined with a large electrolytic capacitor connected to ground makes an interesting doorbell. Just connect the doorbell switch across the capacitor and the phone will ring for a few seconds when the switch is pushed. (The capacitor discharges quickly but charges slowly.)
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Fig. 3 shows how to add an answer activated control. The 1k resistor may be replaced with a relay for controlling a tape recorder. Put a diode across the winding to protect the transistor from inductive kick-back.
The phone ringer may be used to construct a pretend cellular phone system for the kids using an ordinary cordless phone and a regular phone wired in series. Keep the wiring neat and insulated so that the ring voltage doesn't "bite" anyone. Connect the ringer, ordinary phone and the base unit of the cordless phone in series. Wire a switch which enables the ringer and shorts the ordinary phone (two-pole switch). When the cordless phone is answered, flip the switch to talk. Shorting the ordinary phone is probably not necessary but be prepaired for a rather loud buzzing in the earpiece when the other phone rings! The advanced experimenter may wish to build an artificial phone system by adding a on-hook high voltage supply, dial tone oscillators, and appropriate switching circuitry. Quite a challange!
The Surfer’s Preserver
The Surfer’s Preserver is a simple device that prevents other phones in the house from disrupting your critical Internet session by disconnecting them from the line while you surf! The circuit is also useful in preventing eavesdropping from other extensions since other phones are "dead" until you hang up. The circuit wires in series with either of the offending phone’s wires (red or green) and it is small enough to tuck behind the wall cover plate.
Circuit Description: Due to the resistor divider, the SCR will not fire unless there is at least 17 volts across the bridge. When the receiver is lifted, the full line voltage appears across the circuit and the SCR triggers. The SCR will remain triggered since the DC phone current is about 25 mA and the SCR holding current is only about 5 mA. If the phone line is in use when the receiver is lifted, the line voltage is insufficient to trigger the SCR and the phone remains disconnected. When the phone rings, the 17 volt threshold is quickly passed and the SCR triggers early in the ring voltage cycle, supplying a nearly full amplitude ring voltage to the phone.
Surfer.gif (5793 bytes)
A momentary push-button switch may be added across the 33k resistor to manually trigger the SCR so that the phone can connect when another phone is off-hook. This push-button could be mounted in the wall plate if the plate is in a convenient location or the circuit could be built into the telephone itself with a small switch added on the side. This push-button is handy if more than one telephone is on the line.
Other SCRs may be substituted as long as their working voltage is above 150 volts and their holding current is well below your phone’s current. Connect a current meter in series with your phone to determine your current - expect about 25 to 30 mA.
The circuit requires that the modem or other phones pull the line voltage below 17 volts when off-hook. A simple voltage check will determine if the voltage is dropping low enough. It will typically drop to 5 volts. If you must raise the trigger voltage, increase the 33k resistor. A very high resistor value may reduce the ringing volume on older phones or prevent normal phone use.
A separate circuit may be constructed for each phone or one circuit may be used to disconnect several phones. To use one circuit for several phones, make sure that they share a common wire not shared with the modem. Place the circuit in series with the common wire. The advantage of this connection is that the push-button is not needed to transfer a call from one phone to another but some custom wiring may be necessary.
noise generator
Pink Noise Generator (Flicker Noise)
The circuit shown in fig. 1 is an implementation of a flicker noise generator described in NBS technical note #604, "Efficient Numerical and Analog Modeling of Flicker Noise Processes" by J.A. Barnes and Stephen Jarvis, Jr. With the values shown the circuit will give a 1/f noise slope from below one hertz to over four kilohertz. The circuit employs a TLC2272 op-amp although other high impedance, low noise op-amps will work. The amplifier must have low noise current since a farily high value resistor is used to generate the 50 nV white noise; choose an op-amp with noise voltage less than 15 nV/root-Hz and noise current less than 0.1 pA/root-Hz, both easily obtained with several modern op-amps. The capacitor values vary slightly from the calculated values in the referenced paper to simplify construction and the circuit includes bias to allow the use of polarized electrolytic capacitors. The electrolytic capacitors should be selected carefully since many aluminum electrolytics have poor tolerances.
schematic
Unlike circuits employing zeners, reverse-biased transistors, and other noisy devices, this circuit gives a predictable and repeatable output level. Bringing the first op-amp's output out through a 100uF capacitor in a similar manner to the second will yield a handy, precise 5 uV/root-Hz white noise covering the audio band that will serve as an excellent source for calibrating audio noise measurements. In order to bias this capacitor at about +2.5 volts you may wish to add about 30 megohms from +5 VDC to the positive input of the first op-amp. If you use a tantalum capacitor, bias is not necessary since tantalums can withstand zero bias. (F.Y.I.: Even a small reverse bias voltage up to about 10% of the rating is OK with tantalum capacitors.) A summing amplifier could be added to combine the white noise at the output of the first op-amp to the flicker noise at the output to simulate various noisy devices and systems. The two inputs to the summing amplifier would allow for independent adjustment of the white and flicker components.
The circuit shown in fig. 1 is an implementation of a flicker noise generator described in NBS technical note #604, "Efficient Numerical and Analog Modeling of Flicker Noise Processes" by J.A. Barnes and Stephen Jarvis, Jr. With the values shown the circuit will give a 1/f noise slope from below one hertz to over four kilohertz. The circuit employs a TLC2272 op-amp although other high impedance, low noise op-amps will work. The amplifier must have low noise current since a farily high value resistor is used to generate the 50 nV white noise; choose an op-amp with noise voltage less than 15 nV/root-Hz and noise current less than 0.1 pA/root-Hz, both easily obtained with several modern op-amps. The capacitor values vary slightly from the calculated values in the referenced paper to simplify construction and the circuit includes bias to allow the use of polarized electrolytic capacitors. The electrolytic capacitors should be selected carefully since many aluminum electrolytics have poor tolerances.
schematic
Unlike circuits employing zeners, reverse-biased transistors, and other noisy devices, this circuit gives a predictable and repeatable output level. Bringing the first op-amp's output out through a 100uF capacitor in a similar manner to the second will yield a handy, precise 5 uV/root-Hz white noise covering the audio band that will serve as an excellent source for calibrating audio noise measurements. In order to bias this capacitor at about +2.5 volts you may wish to add about 30 megohms from +5 VDC to the positive input of the first op-amp. If you use a tantalum capacitor, bias is not necessary since tantalums can withstand zero bias. (F.Y.I.: Even a small reverse bias voltage up to about 10% of the rating is OK with tantalum capacitors.) A summing amplifier could be added to combine the white noise at the output of the first op-amp to the flicker noise at the output to simulate various noisy devices and systems. The two inputs to the summing amplifier would allow for independent adjustment of the white and flicker components.
receiver
Induction Receiver
The induction receiver shown below is very sensitive and can serve a variety of purposes. It is excellent for tracing wiring behind walls, receiving audio from an induction transmitter, hearing lightning and other electric discharges, and monitoring a telephone or other device that produces an audio magnetic field ("telephone pickup coil").
The receiving coil could be a "telephone pickup coil" if available or a suitable coil from some other device. The coil in the prototype was salvaged from a surplus 24 volt relay. Actually, two relays were needed since the first was destroyed in the attempt to remove the surrounding metal so that a single solenoid remained. Epoxy putty was used to secure the thin wires and the whole operation was a bit of a challenge. A reed relay coil will give reduced sensitivity but would be much easier to use. The experimentally inclined might try increasing the inductance of a reed relay by replacing the reed switch with soft iron. Avoid shielded inductors or inductors with iron pole pieces designed to concentrate the magnetic field in a small area or confine it completely (as in a relay or transformer) unless you can remove the iron. The resulting coil should be a simple solenoid like wire wrapped around a nail. Don't try to wind your own - it takes too many turns. Evaluate several coils simply by listening. Coils with too little inductance will sound "tinny" with poor low frequency response and other coils will sound muffled, especially larger iron core coils. This prototype was tested with a large 100 mH air core coil with superb results but the 2 inch diameter was just too big for this application.
The other components are not particularly critical. The 2N4401 can be just about any NPN general purpose small-signal transistor. The TL431 is a shunt voltage regulator but it is being used as an audio amplifier in this circuit. In fact, the whole device is nothing more than a low noise, high gain audio amplifier with a pickup coil connected to the input and other amplifiers will work equally well.
schematic
The circuit is built into a 8 mm cassette box with the power switch and earphone jack in the back. The circuit board is a piece of pink countertop laminate which looks good against the violet hue of the cassette box. The battery fits nicely into the box and a piece of foam fills in the remaining space. These video cassette boxes make nice project boxes, unlike audio cassette boxes which are too flimsy.
Induct.jpg (22981 bytes)
When you first turn on the unit you will probably hear a lot of buzzing from the wiring in the room. Rotate the receiver in a horizontal plane to find a "null" where the hum is minimal. If you can get a reasonable null, you should be able to hear distant lightning crackles or other magnetic noises. If you cannot get a null then go outside away from the building. Try holding the coil near electronic devices like your computer monitor, telephone (when in use), cell phone readout, etc. You can trace power wires behind a wall or ceiling by listening for a sharp increase in hum as the coil passes near the wire. Make sure that current is flowing in the wires to be traced by turning on a lamp or other appliance. (Here is an experiment to try: Build a line voltage lamp flasher that can be connected to the circuit to be traced. The desired wire will now have an on and off buzz - buzz sound that will be easy to distinguish. I wonder if you could even identify a specific breaker or fuse?)
Other wires can be traced if they are carrying alternating current in the audio range or a signal generator can be connected to produce the current. Connect the generator to the wire to be traced and connect the generator's "ground" to the house wiring ground. Also ground the far end of the wire you are tracing so that current flows in the wire. This ground connection can also just be a temporary wire laying on the floor running from the generator ground to the far end of the wire you wish to trace.
For the ambitious: try wrapping one or two turns of wire around the whole house and connect the loop to the output of an audio power amplifier (one channel of a stereo should work). Add a 4 ohm, high wattage resistor in series to protect the amplifier. You should be able to pick up the magnetic field fairly easily anywhere within the loop with the power amplifier supplying just a few watts of power.
The induction receiver shown below is very sensitive and can serve a variety of purposes. It is excellent for tracing wiring behind walls, receiving audio from an induction transmitter, hearing lightning and other electric discharges, and monitoring a telephone or other device that produces an audio magnetic field ("telephone pickup coil").
The receiving coil could be a "telephone pickup coil" if available or a suitable coil from some other device. The coil in the prototype was salvaged from a surplus 24 volt relay. Actually, two relays were needed since the first was destroyed in the attempt to remove the surrounding metal so that a single solenoid remained. Epoxy putty was used to secure the thin wires and the whole operation was a bit of a challenge. A reed relay coil will give reduced sensitivity but would be much easier to use. The experimentally inclined might try increasing the inductance of a reed relay by replacing the reed switch with soft iron. Avoid shielded inductors or inductors with iron pole pieces designed to concentrate the magnetic field in a small area or confine it completely (as in a relay or transformer) unless you can remove the iron. The resulting coil should be a simple solenoid like wire wrapped around a nail. Don't try to wind your own - it takes too many turns. Evaluate several coils simply by listening. Coils with too little inductance will sound "tinny" with poor low frequency response and other coils will sound muffled, especially larger iron core coils. This prototype was tested with a large 100 mH air core coil with superb results but the 2 inch diameter was just too big for this application.
The other components are not particularly critical. The 2N4401 can be just about any NPN general purpose small-signal transistor. The TL431 is a shunt voltage regulator but it is being used as an audio amplifier in this circuit. In fact, the whole device is nothing more than a low noise, high gain audio amplifier with a pickup coil connected to the input and other amplifiers will work equally well.
schematic
The circuit is built into a 8 mm cassette box with the power switch and earphone jack in the back. The circuit board is a piece of pink countertop laminate which looks good against the violet hue of the cassette box. The battery fits nicely into the box and a piece of foam fills in the remaining space. These video cassette boxes make nice project boxes, unlike audio cassette boxes which are too flimsy.
Induct.jpg (22981 bytes)
When you first turn on the unit you will probably hear a lot of buzzing from the wiring in the room. Rotate the receiver in a horizontal plane to find a "null" where the hum is minimal. If you can get a reasonable null, you should be able to hear distant lightning crackles or other magnetic noises. If you cannot get a null then go outside away from the building. Try holding the coil near electronic devices like your computer monitor, telephone (when in use), cell phone readout, etc. You can trace power wires behind a wall or ceiling by listening for a sharp increase in hum as the coil passes near the wire. Make sure that current is flowing in the wires to be traced by turning on a lamp or other appliance. (Here is an experiment to try: Build a line voltage lamp flasher that can be connected to the circuit to be traced. The desired wire will now have an on and off buzz - buzz sound that will be easy to distinguish. I wonder if you could even identify a specific breaker or fuse?)
Other wires can be traced if they are carrying alternating current in the audio range or a signal generator can be connected to produce the current. Connect the generator to the wire to be traced and connect the generator's "ground" to the house wiring ground. Also ground the far end of the wire you are tracing so that current flows in the wire. This ground connection can also just be a temporary wire laying on the floor running from the generator ground to the far end of the wire you wish to trace.
For the ambitious: try wrapping one or two turns of wire around the whole house and connect the loop to the output of an audio power amplifier (one channel of a stereo should work). Add a 4 ohm, high wattage resistor in series to protect the amplifier. You should be able to pick up the magnetic field fairly easily anywhere within the loop with the power amplifier supplying just a few watts of power.
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