Monday, March 30, 2009

Engineering Projects - VI


  • Fully Isolated 50/60Hz Sync Generator
    This circuit will produce a single pulse at the zero voltage cross points of the power line voltage. An opto-coupler provides a very safe 5KV isolation.

TIME TO DUST INDICATOR
I thought about this circuit when I heard that a lot cleaning personal in hotels were either dusting rooms more often than necessary or not enough. I have not yet built and tested this circuit completely but in concept it should work. The circuit draws very low current from a +3v battery and could be housed in a package similar to a small ashtray. The assembly might be placed in a suitable out of the way area to collect dust. It would alert a maid when it was time to dust the room. The circuit detects dust with an infrared LED that is pulsed so its light shines onto a smooth flat plate. Any dust settling onto the surface is detected by a phototransistor, mounted at a 90-degree angle from the LED. When the dust reaches a particular level, sufficient light is reflected into the phototransistor to change the logic state of the circuit to an alarm condition. The alarm output could be connected to a beeper, a flashing LED or to one of the LED flashing circuits in this hobby circuit section.

· OPTICAL INTERRUPTER DRAWS MICROAMPS
This circuit is great for battery powered systems that use slotted type optical interrupters. It draws only 10uA from a 3v battery which should allow up to 5 years of operation from a lithium battery.

  • AIR TRANSPARENCY MONITOR, XENON FLASH RECEIVER
    I designed this circuit many years ago to monitor the quality of a mile long column of air for future optical communications experiments. The transmitter system (circuit 72 below) uses a powerful xenon flash in conjunction with a large 12 inch Fresnel lens at the transmitter end and a matching 12-inch lens with a PIN photo diode at the receiver. The receiver system was connected to a weather station and a computer to collect the changes in intensity of the light flashes under different weather conditions. It has the potential for a 30+-mile range. I have also used this system to conduct cloud bounce experiments.
  • Light to Frequency Converter
    This circuit uses a CMOS version of the classic 555 timer, to form a light intensity to frequency converter. A small PIN photo diode is used as the light detector. The pulses produced are short, so in some applications you may want to stretch them or feed them through a flip/flop to produce a square wave signal. Although the circuit shown is designed for a 5v supply, it could operate from almost any voltage from 3v to 15v.
  • LINE POWERED XENON FLASH TRANSMITTER
    This line powered xenon flash circuit drives a small camera type flash tube. It has an optical isolator to allow the flash to be safely triggered from some remote device. A flash rate of 2Hz is possible with the circuit.
  • MICROPOWER C-MOS OSCILLATOR, DRAWS ONLY 0.5uA
    If truly low power oscillators interest you, this circuit draws a mere 2 microwatts (500nA) from a 6v battery. It uses a very inexpensive C-MOS IC to produce a frequency of 2Hz. However, by changing the component values you can push it to 300Hz. The circuit draws much less current than traditional C-MOS oscillators using a Schmitt trigger inverter. It also produces fast leading edges. However, if you need more drive current, a C-MOS buffer should be included.
    Published in EDN, September 2, 1993.
  • Precision 40KHz Triangle Generator
    This circuit generates a precision 40KHz triangle waveform.
  • ULTRA PURE 125KHz SINE WAVE SIGNAL SOURCE
    For some RFID systems operating at 125KHz, a very low distortion signal source reference is needed. The circuit shown on this page produces a 10-volt peak-to-peak signal into a 50-ohm load, with a distortion of only 0.01%.
  • 40KHz Voltage to Frequency Converter
    This circuit was designed to frequency modulate a 40KHz carrier, using human voice frequencies. A common flip/flop is used at the core of the circuit.
  • 200MHz - 400MHz VOLTAGE controlled OSCILLATOR
    If you need a clean emitter coupled logic (ECL) type signal between 200MHz and 400MHz this circuit works fine. It uses four voltage controlled capacitors to change the frequency.
  • Air Transparency Monitor, Xenon Flash Receiver, Page 2
    This is Page 2 of the receiver circuit Air Transparency Monitor, Xenon Flash Receive
  • CMOS SCHMITT TRIGGER IC MAKES VCO
    By changing the supply voltage fed to a classic 4584 Schmitt trigger type oscillator, the oscillator frequency can be changed over a range of 50:1. A 74HCU04 inverter is used at the output of the 4584 to maintain a constant TTL logic level signal.

· Light to Frequency Converter
This circuit uses a CMOS version of the classic 555 timer, to form a light intensity to frequency converter. A small PIN photo diode is used as the light detector. The pulses produced are short, so in some applications you may want to stretch them or feed them through a flip/flop to produce a square wave signal. Although the circuit shown is designed for a 5v supply, it could operate from almost any voltage from 3v to 15v.

  • Wide Band Zero Cross Detector
    This circuit was designed to convert a low amplitude 40KHz signal into a clean square wave signal. It will work with inputs as small as 5mv peak-to-peak or as large as 3 volts peak to peak. The input frequency can range from a few kilohertz to about 150KHz.
  • MICRO POWER OVER-TEMPERATURE ALARM
    The circuit is powered for years by a single 3 volt lithium battery. It sounds an alarm when the temperature exceeds a certain point. With some minor changes the circuit could also be configured for an under temperature (freeze) alarm. The circuit uses a cheap but accurate thermostat as the temperature sensor.

  • Reduced Power Relay Driver Aug 3, 2008
    Relays can handle a lot of power. However, for certain power sensitive designs you would like to reduce the power needed to hold a relay closed. The circuit below performs such a task. It uses a single CD4093 quad NAND gate. When the “on” logic input signal is detected, the relay is first pulsed on for about 500ms. This is sufficient time to insure the relay is fully closed. After that initial pulse the relay is then driven with a square wave signal, whose duty cycle can be adjusted. The signal duty cycle can be adjusted from about 10% to 90%. In most cases a 50% duty cycle will hold the relay closed. This reduces the average DC current required by the same factor, which means a 4:1 reduction in power. The circuit can operate over a wide 3v to 15v range.
  • 555 Timer Forms Simple PWM Motor Controller
    Using a CMOS version of the 555 timer, this circuit can be used to control the speed of a motor by adjusting the duty cycle of the pulses sent to the motor. (added 7/06)
  • PWM Circuit for Motor Speed Control
    Sometimes you want to slow down a brush type DC motor. The most efficient way to do this is with a pulse width modulation (PWM) technique. The hobby circuit below can operate from about 3 volts to 15 volts. The frequency is fixed at about 2KHz but the pulse width can be varied from nearly 100% to 0%. ...
  • Audio Signal Detector Switch
    This circuit will activate a transistor switch when it detects at least 50mv peak to peak of an audio signal. It could be used to turn on a relay, routing the signal to be it is needed. (added 7/06)
  • AIR TRANSPARENCY MONITOR, XENON FLASH RECEIVER
    I designed this circuit many years ago to monitor the quality of a mile long column of air for future optical communications experiments. The transmitter system (circuit 72 below) uses a powerful xenon flash in conjunction with a large 12 inch Fresnel lens at the transmitter end and a matching 12-inch lens with a PIN photo diode at the receiver. The receiver system was connected to a weather station and a computer to collect the changes in intensity of the light flashes under different weather conditions. It has the potential for a 30+-mile range. I have also used this system to conduct cloud bounce experiments.
  • Poor Man’s Digital Counter Using Pedometer
    There are many occasions when you may want to count something electronically. Perhaps it is car traffic on a street or items moving down an assembly line. It might be the number of times a machine is activated or maybe you want to count the number of people entering doorway. Commercial counting modules do exist but if you want to use something cheaper, you can modify a pedometer to do much the same thing....

ANOTHER VERY LOUD PIEZO ALARM BEEPER
This is yet another beeper circuit that really draws attention. It sweeps the drive frequency slightly to produce a very annoying sound. It uses a transformer to increase the drive voltage across the piezoelectric device to more than 200 volts peak to peak. It cranks out an ear splitting 120db when measured at 12 inches.

HIGH SOUND OUTPUT BEEPER CIRCUIT
I you need a real loud beeper, this circuit delivers about 110db (12 inches away) from a 9v battery using a single inexpensive C-MOS IC. An off-the-shelf piezoelectric beeping device is driven at resonance to insure maximum efficiency. By changing the control IC to a 74AC14, the same circuit can operate from 3v and 1.5v batteries.
Published in Electronic Design, Aug 5, 1993

MACHINE VIBRATION TRIGGERS HOUR METER
This circuit allows an off-the-shelf battery powered digital hour meter to be turned on and off, according to a machine's operation, without requiring a direct electrical connection to the machine. Machine vibrations are detected by an off-the-shelf piezoelectric wafer. The wafer is normally used as a sound generator. The 3v power could be tapped from the hour meter's own lithium battery or from a separate 3v lithium battery. The circuit only draws about 1.5uA, so a 250mA-hr battery would power the circuit for about 10 years.
Published in EDN, Jan 16, 1997

  • Motion Alarm
    Using a piezoelectric device, this circuit will activate a beeper whenever the circuit is moved. It could be used as an earthquake alarm.
  • Motion Alarm Using Piezoelectric Device
    An inexpensive piezoelectric device is used as a motion sensing device for this motion alarm. This circuit will activate a beeper whenever the circuit is moved. It could be used as an earthquake alarm. (added 7/06).
  • Piezoelectric Triggered Switch
    Two different switch circuits are shown. One sources current and the second sinks current. Both switches are connected to a piezoelectric wafer. When the wafer is tapped, the switches are activated. (added 7/06)
  • Piezoelectric Vibration Sensor Activates Switch
    An inexpensive piezoelectric wafer is used to detect vibration and when the vibration is sufficient a switch is activated. (added 7/06)

Ultra Low Power 32KHz Crystal Oscillator
I have used this circuit many times when I needed a low frequency reference, which did not draw much power. With the components show, the current from a 3v battery is less than 1.2 micro amps.

Very Loud Piezoelectric Beeper Circuit
This circuit is similar to circuit number 7, but generates a continuous tone instead of a pulsing one. The circuit delivers about 110db (12 inches away) from a 9v battery using a single inexpensive C-MOS IC. An off-the-shelf piezoelectric beeping device is driven at resonance to insure maximum efficiency.

  • DARKROOM CAMERA SHUTTER TIMER
    This circuit was designed to control a film exposure shutter for a darkroom. It has 8 time steps ranging from 0.35 seconds to 4 seconds. It is activated by a foot switch and draws power from an external 12-volt DC supply.
  • 12V TOUCH SWITCH EXCITER CIRCUIT
    This circuit is designed to generate a 20KHz pseudo sine wave signal that can power about 50 remote touch activated switch circuits. It can support a cable length of about 2500 feet. A typical remote switch circuit is also shown as well as a receiver circuit for those switches.
  • 3V CAPACITANCE PROXIMITY SWITCH
    This circuit was designed to provide a touch activated switch function without an external power supply. It draws so little power that a single 3v battery will operate the circuit for many years. It is discussed in more detail in the section on Capacitance Proximity Switch Technology.
  • 5 VOLT MOMENTARY OPERATION TOUCH SWITCH
    This simple circuit uses a single IC to form a nice touch switch circuit. A single transistor forms the remote active switch sensor. Multiple switches can be wired in parallel. The switch circuit can be located about 500 feet from the control circuit.
  • 5V CAPACITANCE TOUCH ACTIVATED MOMENTARY SWITCH
    This circuit is discussed in more detail in the section on Capacitance Proximity Switch Technology. The circuit is powered from a standard +5v supply. It has both a source and sink output that change state whenever a metal button connected to the circuit is touched. An earth ground reference is required.
  • 9v POWERED XENON PHOTOFLASH CONTROLLER
    This 9v battery powered circuit is designed for remote control flash needs. A charge control circuit turns off the high voltage generator when the photoflash capacitor is fully charged. A neon lamp is included to indicate when the system is ready to flash.
  • HIGH POWER TOUCH SWITCH EXCITER CIRCUIT
    If you have hundreds of touch switches that need an excitation signal, then this circuit is what you need. Its 20KHz 20v peak to peak output signal can supply up to 3 watts of touch switch excitation power.
  • OPTICAL INTERRUPTER DRAWS MICROAMPS
    This circuit is great for battery-powered systems that use slotted type optical interrupters. It draws only 10uA from a 3v battery that should allow up to 5 years of operation from a lithium battery.
  • Ten Turn Pot Forms Frequency Readout
    Let’s say you are designing a signal generator for a test fixture. To adjust the frequency you would like to use a ten turn pot with a nice dial mechanism attached as a frequency display. However, a display range of 0 to 9.99 would be impractical since there is no such frequency as 0. But since many pot dials have a display....
  • CHARGE COUPLED BI-DIRECTIONAL POWER MOSFET RELAY
    The circuit uses an inexpensive C-MOS inverter package and a few small capacitors to drive two power MOS transistors from a 12v to 15v supply. Since the coupling capacitor values used to drive the FETs are small, the leakage current from the power line into the control circuit is a tiny 4uA. Only about 1.5mA of DC is needed to turn on and off 400 watts of AC or DC power to a load.
  • SOLID STATE RELAY REQUIRES ONLY 50uA DRIVE CURRENT
    This circuit demands a control current that is 100 times smaller than that needed by a typical optically isolated solid state relays. It is ideal for battery powered systems. Using a combination of a high current triac and a very sensitive low current SCR, the circuit can control about 600 watts of power to load while providing full isolation and transient protection.

CIRCUIT PROTECTS FROM BATTERY POLARITY REVERSAL
This simple circuit can protect a sensitive electronic circuit from an accidental connection of a battery with a reversed polarity. The N-channel FET connects the electronic device to the battery only when the polarity is correct. The circuit shown was designed for a device powered from a single 1.5 volts button cell battery. However, the circuit will operate with higher voltages as well.

  • MACHINE POWER LOSS BEEPER
    For some medical equipment it is important for an operator when power is lost to the machine. The beeper is powered from a 9v battery and requires the machine to have a power switch with a third set of contacts.
  • ISOLATED AC CURRENT MONITOR
    This circuit uses a small AC current transformer from Magnetek to produce an isolated voltage proportional to the AC current in the primary winding. The transformer contains a single turn primary with a low 0.001-ohm resistance. It can easily handle 30 amps of AC current and provides at least 500vac of isolation. With the components shown, the output AC voltage is scaled so 1 amp of current produces 100mv of AC voltage.
  • UNPLUGGED POWER CORD ALARM
    Many medical devices, such a portable X-ray and ultrasound equipment, carry their own batteries. However, the frantic pace of some hospital emergency rooms might cause the device to be shoved off into a corner without being plugged back into an AC power source to recharge their batteries. So, I designed this circuit to emit an audible alarm whenever the device’s power cord was unplugged from the wall for a period of time. The device was designed to snap onto the outside of a power cord, where it senses the AC electric field emitted. No direct connection to the internal wires of the power cord is needed. I also included a low battery voltage monitor.
  • VARIABLE ISOLATED AC VOLTAGE SPANS 0VAC TO 280VAC
    I designed and built this circuit about 25 years ago. It came in handy for many projects that were powered from 120 VAC, 240 VAC and 277 VAC. It provides complete isolation from the power line. It uses two 1:1 isolation transformers that are wired in parallel for the140vac range and in series for the 280vac range. The selector switch also diverts power to the appropriate output socket to avoid mishaps in sending the wrong voltage to the load. My home-built unit also included an AC volt and amp meter to monitor the output. However, this circuit only indicates the AC power connections.
  • LIGHT RECEIVER WORKS FROM 1KHz TO OVER 70MHz
    This circuit uses one tiny C-MOS inverter IC to form a modulated light receiver with a very fast response. It is designed around a PIN photo diode that is packaged for use with plastic optical fibers. It can be used as an optical fiber receiver. By using the open end of the optical fiber it can "sniff" out any modulated light signals.

CIRCUIT PROTECTS FROM BATTERY POLARITY REVERSAL
This simple circuit can protect a sensitive electronic circuit from an accidental connection of a battery with a reversed polarity. The N-channel FET connects the electronic device to the battery only when the polarity is correct. The circuit shown was designed for a device powered from a single 1.5 volts button cell battery. However, the circuit will operate with higher voltages as well.

  • CHEAP LOW POWER 3V 2Hz OSCILLATOR
    This is yet another method to generate low frequency output pulses. The circuit uses an inexpensive under-voltage monitor IC and a couple of cheap MOSFETs. It could easily be modified to produce pulse frequencies from about one cycle per minute to a maximum of about 10KHz. It draws only about 1uA from a 3v battery. This circuit should not be used if frequency stability is important, since it will change as a function of supply voltage.
  • FLASHING LED POWER INDICATOR DRAWS LOW CURRENT
    This circuit flashes a power indicator LED to keep the average current low.
  • MICROPOWER PULSE GENERATOR
    it is hard to design a pulse generator that produces clean logic pulses while drawing very low current. This circuit is designed to produce short 2mS pulses at a rate of one per second while drawing only 1 micro amp from a 9 volt battery.
  • Ultra Low Power Astable Multivibrator
    Taking advantage of some voltage comparators, this circuit can produce a nice square wave signal while drawing only 1.6 micro amps. With the inclusion of a diode, the circuit can also produce short pulses instead of a square wave signal.

300V PEAK TO PEAK SIGNAL GENERATOR
This circuit converts a square wave signal to a +-150 volt output signal with fast 100nS rise and fall times.

  • N-CH AND P-CH TRANSISTORS FORM PUSH-PULL DRIVER
    This circuit can produce high speed output signals with fast rise and full times. The unique change pump action allows the voltage of the upper P-ch device to range from mill volts to hundreds of volts. The output current is only limited by the rating of the transistors. I have used this circuit beyond 2MHz.

175KHz INDUCTIVE PULSE TRANSMITTER
This circuit is discussed in more detain in the
Experimenters Journal. The transmitter’s six-inch diameter coil launches powerful magnetic 175KHz ring pulses that can be detected by the circuit below.

SQUARE WAVE DRIVER HAS FLEXIBLE OUTPUTS
This circuit can produce an output signal ranging from DC to 100KHz. It can source a voltage ranging from 1v to 30v. It can sink a voltage ranging from zero volts to –30v. It can drive up to 200ma of current and can even be switched to a floating tristate output.


CMOS INVERTER 125KHz LC OSCILLATOR
This circuit uses a single CMOS inverter to form a series resonant LD oscillator. The values shown set the oscillation at about 125KHz but other frequencies are possible by changing the main LC values.

  • CMOS INVERTERS FORM 125KHZ OSCILLATOR
    This circuit is similar to schematic
    CMOS INVERTER 125KHz LC OSCILLATOR but inverts the LC components so the inductor is grounded. Two inverters are needed to produce the needed oscillation. Again, the values shown set the frequency at 125KHz but can be changed to produce other frequencies.
  • CMOS NAND GATE FORMS GATED 125KHZ OSCILLATOR
    This circuit is similar to schematic
    CMOS INVERTER 125KHz LC OSCILLATOR but uses a NAND gate as an inverter. The gate allows the oscillator to be gated on and off. Again, the values shown set the frequency at 125KHz but can be changed to produce other frequencies.
  • CMOS INVERTER 125KHz LC OSCILLATOR
    This circuit uses a single CMOS inverter to form a series resonant LD oscillator. The values shown set the oscillation at about 125KHz but other frequencies are possible by changing the main LC values.
  • MEDIUM POWER 125KHZ OSCILLATOR-2
    This circuit is similar to
    MEDIUM POWER 125KHZ OSCILLATOR but adds even more inverters in parallel to deliver yet more power. The values shown are for 125KHz.
  • OPTICAL RFID TEST CIRCUIT
    I designed this test the concept of using light techniques to send identification data instead of RF. A more detailed discussion on this scheme can be found in the Imagineered new products section.
  • Wireless RFID Smart Key Detector: Many cars now use a key with an imbedded RFID microchip. The microchip is read when the key is inserted into the car’s ignition. The keys containing a microchip often look the same as most other keys. People duplicating car keys will often not know of the microchip. This circuit will light a LED if it detects a key containing a 125KHz RFID chip.
  • CMOS Logic Inverter Amplifier Characteristics
    This chart shows some of the measured characteristics of three different unbuffered CMOS logic gates, used as voltage amplifiers.

2.45GHz RF Signal Detector
This passive RF indicator hobby circuit is made from a few simple parts. A bent piece of wire forms a 2.5GHz antenna. A 100uA moving coil meter is used to display the relative intensity of the RF signal. Two microwave rated detector diodes form a simple voltage doubler circuit, which feeds a DC voltage to the meter movement, proportional to the RF field strength. This circuit can be used with some cell phones and many cordless telephones. It can also detect RF leaking from a microwave oven.

175KHz INDUCTIVE PULSE RECEIVER
This circuit is discussed in more detain in the
Experimenters Journal. The receiver’s six inch diameter coil detects the ring signal from the above transmitter and use a single NPN transistor to provide enough amplification for the signal to be easily viewed on an oscilloscope.

Plastic Pipe Finder
I used this circuit once to track down the location of a plastic pipe, buried underground. It transmits a weak RF signal, which can be picked up by a portable AM radio from about 6 feet away. (added 12/04)

175KHz INDUCTIVE PULSE TRANSMITTER
This circuit is discussed in more detain in the
Experimenters Journal. The transmitter’s six inch diameter coil launches powerful magnetic 175KHz ring pulses that can be detected by the circuit below.

  • Low Power 40KHz Light Receiver New
    Drawing only 100uA, this circuit provides high sensitivity with excellent ambient light immunity.
  • 175KHz INDUCTIVE PULSE RECEIVER
    This circuit is discussed in more detain in the
    Experimenters Journal. The receiver’s six inch diameter coil detects the ring signal from the above transmitter and use a single NPN transistor to provide enough amplification for the signal to be easily viewed on an oscilloscope.
  • Full Wave Rectifier
    This is a classic circuit that can accurately convert an AC signal to DC. At 40KHz the input signal can be as low as 0.05 volts peak to peak. (added 12/04)

PRECISION FULL WAVE RECTIFIER
I have used this handy circuit many times. It accurately converts an AC signal into pulsing DC, which can be filtered to provide an average of the input voltage. It works from mill volts to volts. The circuit shown requires a stable +5v reference if a single power supply is used.

  • Reduced Power Relay Driver Aug 3, 2008
    Relays can handle a lot of power. However, for certain power sensitive designs you would like to reduce the power needed to hold a relay closed. The circuit below performs such a task. It uses a single CD4093 quad NAND gate. When the “on” logic input signal is detected, the relay is first pulsed on for about 500ms. This is sufficient time to insure the relay is fully closed. After that initial pulse the relay is then driven with a square wave signal, whose duty cycle can be adjusted. The signal duty cycle can be adjusted from about 10% to 90%. In most cases a 50% duty cycle will hold the relay closed. This reduces the average DC current required by the same factor, which means a 4:1 reduction in power. The circuit can operate over a wide 3v to 15v range.
  • Ultra Low Power Latching Relay Circuit
    The circuit below takes advantage of some inexpensive small super capacitors. The circuit pumps 6 volt pulses into the separate 5 volt latching and unlatching relay coils. A short 25ms pulse is all that is needed to flip the relay states. A third super capacitor is used to supply a higher peak current than a small lithium cell might otherwise be able to supply.
  • Universal Flasher using Latching Relay: This circuit is powered by a 9v battery and controls any AC to DC load through a set of relay contacts, rated at 10 amps. To conserve power, the circuit uses a latching relay. A variable frequency oscillator controls the flashing speed from 0.2Hz to 2Hz. A pair of pulse generators first latch the contacts closed then unlatches them

1uS LIGHT PULSE RECEIVER PLUS POST AMP
This circuit is designed to detect very weak light pulses lasting 1uS. It uses a tuned LC feedback network to provide high sensitivity while giving high ambient light immunity. A post voltage amplifier is included with a gain of about X20. The circuit is described in more detail in the receiver section of my
Handbook of Optical Through the Air Communications. Note: The LF357 op amp is no longer available, this circuit is for reference only.

30KHZ LIGHT RECEIVER AMP
This circuit uses NPN Darlington transistor to amplify the signal produced from short light flashes, as detected by a PIN photo diode. The circuit draws only about 330uA from a 6v battery.

  • 40KHz LASER BURST DETECTOR
    This circuit was originally designed to detect weak flashed of laser light bounced off of a fabric video projection screen. It was used as part of a firearm training system. It generates a 100mS output pulse whenever it detects a 3ms to 5ms-laser burst, modulated at 40KHz. It is very sensitive and could be modified for long-range laser communications.

40KHZ LED TEST SIGNAL GENERATOR
This 40KHz crystal controlled oscillator circuit drives an infrared LED with powerful 40ma pulses. The circuit can be used to test optical communications circuits, designed to receive 40KHz modulated light signals.

40KHz TV-VCR LIGHT SOURCE REPEATER
This circuit is designed to be placed directly in front of a standard TV or VCR remote. The exiting light pulses produced by the circuit match the pulses from the remote but are about 10 times more powerful. Using the device, the remote can operate a TV or VCR over three times the normal distance.

  • Infrared Remote Extender
    This electronic hobby circuit is designed to be placed directly in front of a standard TV or VCR remote. The exiting light pulses produced by the electronic circuit match the pulses from the remote but are about 10 times more powerful. Using the device, the remote can operate a TV or VCR over three times the normal distance....
  • TV/VCR Infrared Remote Booster
    This circuit will boost the signal from any infrared TV or VCR remote, extending the range by a factor of 3X.

SINGLE IC FORMS SENSITIVE MODULATED LIGHT RECEIVER
The circuit uses a very inexpensive C-MOS IC that is connected to a small photodiode. Using a unique inductive feedback network, the circuit provides high sensitivity under high ambient light conditions. It is a great circuit when you want to extend the range of an optical remote control transmitter.

  • UL GROUND RESISTANCE TESTER
    UL requires that an electrical system using a metal chassis earth ground connection have a resistance of less than 0.1 ohms. This resistance must be tested with actual line current in excess or 10 amps. The circuit shown uses a couple of LEDs to indicate if in fact the resistance is less than 0.1 ohms. It uses a modified transformer to produce a 2.5vac output with at least 10 amps of supply current

  • Fully Isolated 50/60Hz Sync Generator
    This circuit will produce a single pulse at the zero voltage cross points of the power line voltage. An opto-coupler provides a very safe 5KV isolation.

TIME TO DUST INDICATOR
I thought about this circuit when I heard that a lot cleaning personal in hotels were either dusting rooms more often than necessary or not enough. I have not yet built and tested this circuit completely but in concept it should work. The circuit draws very low current from a +3v battery and could be housed in a package similar to a small ashtray. The assembly might be placed in a suitable out of the way area to collect dust. It would alert a maid when it was time to dust the room. The circuit detects dust with an infrared LED that is pulsed so its light shines onto a smooth flat plate. Any dust settling onto the surface is detected by a phototransistor, mounted at a 90-degree angle from the LED. When the dust reaches a particular level, sufficient light is reflected into the phototransistor to change the logic state of the circuit to an alarm condition. The alarm output could be connected to a beeper, a flashing LED or to one of the LED flashing circuits in this hobby circuit section.

· OPTICAL INTERRUPTER DRAWS MICROAMPS
This circuit is great for battery powered systems that use slotted type optical interrupters. It draws only 10uA from a 3v battery which should allow up to 5 years of operation from a lithium battery.

  • AIR TRANSPARENCY MONITOR, XENON FLASH RECEIVER
    I designed this circuit many years ago to monitor the quality of a mile long column of air for future optical communications experiments. The transmitter system (circuit 72 below) uses a powerful xenon flash in conjunction with a large 12 inch Fresnel lens at the transmitter end and a matching 12-inch lens with a PIN photo diode at the receiver. The receiver system was connected to a weather station and a computer to collect the changes in intensity of the light flashes under different weather conditions. It has the potential for a 30+-mile range. I have also used this system to conduct cloud bounce experiments.
  • Light to Frequency Converter
    This circuit uses a CMOS version of the classic 555 timer, to form a light intensity to frequency converter. A small PIN photo diode is used as the light detector. The pulses produced are short, so in some applications you may want to stretch them or feed them through a flip/flop to produce a square wave signal. Although the circuit shown is designed for a 5v supply, it could operate from almost any voltage from 3v to 15v.
  • LINE POWERED XENON FLASH TRANSMITTER
    This line powered xenon flash circuit drives a small camera type flash tube. It has an optical isolator to allow the flash to be safely triggered from some remote device. A flash rate of 2Hz is possible with the circuit.
  • MICROPOWER C-MOS OSCILLATOR, DRAWS ONLY 0.5uA
    If truly low power oscillators interest you, this circuit draws a mere 2 microwatts (500nA) from a 6v battery. It uses a very inexpensive C-MOS IC to produce a frequency of 2Hz. However, by changing the component values you can push it to 300Hz. The circuit draws much less current than traditional C-MOS oscillators using a Schmitt trigger inverter. It also produces fast leading edges. However, if you need more drive current, a C-MOS buffer should be included.
    Published in EDN, September 2, 1993.
  • Precision 40KHz Triangle Generator
    This circuit generates a precision 40KHz triangle waveform.
  • ULTRA PURE 125KHz SINE WAVE SIGNAL SOURCE
    For some RFID systems operating at 125KHz, a very low distortion signal source reference is needed. The circuit shown on this page produces a 10-volt peak-to-peak signal into a 50-ohm load, with a distortion of only 0.01%.
  • 40KHz Voltage to Frequency Converter
    This circuit was designed to frequency modulate a 40KHz carrier, using human voice frequencies. A common flip/flop is used at the core of the circuit.
  • 200MHz - 400MHz VOLTAGE controlled OSCILLATOR
    If you need a clean emitter coupled logic (ECL) type signal between 200MHz and 400MHz this circuit works fine. It uses four voltage controlled capacitors to change the frequency.
  • Air Transparency Monitor, Xenon Flash Receiver, Page 2
    This is Page 2 of the receiver circuit Air Transparency Monitor, Xenon Flash Receive
  • CMOS SCHMITT TRIGGER IC MAKES VCO
    By changing the supply voltage fed to a classic 4584 Schmitt trigger type oscillator, the oscillator frequency can be changed over a range of 50:1. A 74HCU04 inverter is used at the output of the 4584 to maintain a constant TTL logic level signal.

Light to Frequency Converter
This circuit uses a CMOS version of the classic 555 timer, to form a light intensity to frequency converter. A small PIN photo diode is used as the light detector. The pulses produced are short, so in some applications you may want to stretch them or feed them through a flip/flop to produce a square wave signal. Although the circuit shown is designed for a 5v supply, it could operate from almost any voltage from 3v to 15v.

  • Wide Band Zero Cross Detector
    This circuit was designed to convert a low amplitude 40KHz signal into a clean square wave signal. It will work with inputs as small as 5mv peak-to-peak or as large as 3 volts peak to peak. The input frequency can range from a few kilohertz to about 150KHz.
  • MICRO POWER OVER-TEMPERATURE ALARM
    The circuit is powered for years by a single 3 volt lithium battery. It sounds an alarm when the temperature exceeds a certain point. With some minor changes the circuit could also be configured for an under temperature (freeze) alarm. The circuit uses a cheap but accurate thermostat as the temperature sensor.

  • Reduced Power Relay Driver Aug 3, 2008
    Relays can handle a lot of power. However, for certain power sensitive designs you would like to reduce the power needed to hold a relay closed. The circuit below performs such a task. It uses a single CD4093 quad NAND gate. When the “on” logic input signal is detected, the relay is first pulsed on for about 500ms. This is sufficient time to insure the relay is fully closed. After that initial pulse the relay is then driven with a square wave signal, whose duty cycle can be adjusted. The signal duty cycle can be adjusted from about 10% to 90%. In most cases a 50% duty cycle will hold the relay closed. This reduces the average DC current required by the same factor, which means a 4:1 reduction in power. The circuit can operate over a wide 3v to 15v range.
  • 555 Timer Forms Simple PWM Motor Controller
    Using a CMOS version of the 555 timer, this circuit can be used to control the speed of a motor by adjusting the duty cycle of the pulses sent to the motor. (added 7/06)
  • PWM Circuit for Motor Speed Control
    Sometimes you want to slow down a brush type DC motor. The most efficient way to do this is with a pulse width modulation (PWM) technique. The hobby circuit below can operate from about 3 volts to 15 volts. The frequency is fixed at about 2KHz but the pulse width can be varied from nearly 100% to 0%. ...
  • Audio Signal Detector Switch
    This circuit will activate a transistor switch when it detects at least 50mv peak to peak of an audio signal. It could be used to turn on a relay, routing the signal to be it is needed. (added 7/06)
  • AIR TRANSPARENCY MONITOR, XENON FLASH RECEIVER
    I designed this circuit many years ago to monitor the quality of a mile long column of air for future optical communications experiments. The transmitter system (circuit 72 below) uses a powerful xenon flash in conjunction with a large 12 inch Fresnel lens at the transmitter end and a matching 12-inch lens with a PIN photo diode at the receiver. The receiver system was connected to a weather station and a computer to collect the changes in intensity of the light flashes under different weather conditions. It has the potential for a 30+-mile range. I have also used this system to conduct cloud bounce experiments.
  • Poor Man’s Digital Counter Using Pedometer
    There are many occasions when you may want to count something electronically. Perhaps it is car traffic on a street or items moving down an assembly line. It might be the number of times a machine is activated or maybe you want to count the number of people entering doorway. Commercial counting modules do exist but if you want to use something cheaper, you can modify a pedometer to do much the same thing....

ANOTHER VERY LOUD PIEZO ALARM BEEPER
This is yet another beeper circuit that really draws attention. It sweeps the drive frequency slightly to produce a very annoying sound. It uses a transformer to increase the drive voltage across the piezoelectric device to more than 200 volts peak to peak. It cranks out an ear splitting 120db when measured at 12 inches.

HIGH SOUND OUTPUT BEEPER CIRCUIT
I you need a real loud beeper, this circuit delivers about 110db (12 inches away) from a 9v battery using a single inexpensive C-MOS IC. An off-the-shelf piezoelectric beeping device is driven at resonance to insure maximum efficiency. By changing the control IC to a 74AC14, the same circuit can operate from 3v and 1.5v batteries.
Published in Electronic Design, Aug 5, 1993

MACHINE VIBRATION TRIGGERS HOUR METER
This circuit allows an off-the-shelf battery powered digital hour meter to be turned on and off, according to a machine's operation, without requiring a direct electrical connection to the machine. Machine vibrations are detected by an off-the-shelf piezoelectric wafer. The wafer is normally used as a sound generator. The 3v power could be tapped from the hour meter's own lithium battery or from a separate 3v lithium battery. The circuit only draws about 1.5uA, so a 250mA-hr battery would power the circuit for about 10 years.
Published in EDN, Jan 16, 1997

  • Motion Alarm
    Using a piezoelectric device, this circuit will activate a beeper whenever the circuit is moved. It could be used as an earthquake alarm.
  • Motion Alarm Using Piezoelectric Device
    An inexpensive piezoelectric device is used as a motion sensing device for this motion alarm. This circuit will activate a beeper whenever the circuit is moved. It could be used as an earthquake alarm. (added 7/06).
  • Piezoelectric Triggered Switch
    Two different switch circuits are shown. One sources current and the second sinks current. Both switches are connected to a piezoelectric wafer. When the wafer is tapped, the switches are activated. (added 7/06)
  • Piezoelectric Vibration Sensor Activates Switch
    An inexpensive piezoelectric wafer is used to detect vibration and when the vibration is sufficient a switch is activated. (added 7/06)

Ultra Low Power 32KHz Crystal Oscillator
I have used this circuit many times when I needed a low frequency reference, which did not draw much power. With the components show, the current from a 3v battery is less than 1.2 micro amps.

Very Loud Piezoelectric Beeper Circuit
This circuit is similar to circuit number 7, but generates a continuous tone instead of a pulsing one. The circuit delivers about 110db (12 inches away) from a 9v battery using a single inexpensive C-MOS IC. An off-the-shelf piezoelectric beeping device is driven at resonance to insure maximum efficiency.

  • DARKROOM CAMERA SHUTTER TIMER
    This circuit was designed to control a film exposure shutter for a darkroom. It has 8 time steps ranging from 0.35 seconds to 4 seconds. It is activated by a foot switch and draws power from an external 12-volt DC supply.
  • 12V TOUCH SWITCH EXCITER CIRCUIT
    This circuit is designed to generate a 20KHz pseudo sine wave signal that can power about 50 remote touch activated switch circuits. It can support a cable length of about 2500 feet. A typical remote switch circuit is also shown as well as a receiver circuit for those switches.
  • 3V CAPACITANCE PROXIMITY SWITCH
    This circuit was designed to provide a touch activated switch function without an external power supply. It draws so little power that a single 3v battery will operate the circuit for many years. It is discussed in more detail in the section on Capacitance Proximity Switch Technology.
  • 5 VOLT MOMENTARY OPERATION TOUCH SWITCH
    This simple circuit uses a single IC to form a nice touch switch circuit. A single transistor forms the remote active switch sensor. Multiple switches can be wired in parallel. The switch circuit can be located about 500 feet from the control circuit.
  • 5V CAPACITANCE TOUCH ACTIVATED MOMENTARY SWITCH
    This circuit is discussed in more detail in the section on Capacitance Proximity Switch Technology. The circuit is powered from a standard +5v supply. It has both a source and sink output that change state whenever a metal button connected to the circuit is touched. An earth ground reference is required.
  • 9v POWERED XENON PHOTOFLASH CONTROLLER
    This 9v battery powered circuit is designed for remote control flash needs. A charge control circuit turns off the high voltage generator when the photoflash capacitor is fully charged. A neon lamp is included to indicate when the system is ready to flash.
  • HIGH POWER TOUCH SWITCH EXCITER CIRCUIT
    If you have hundreds of touch switches that need an excitation signal, then this circuit is what you need. Its 20KHz 20v peak to peak output signal can supply up to 3 watts of touch switch excitation power.
  • OPTICAL INTERRUPTER DRAWS MICROAMPS
    This circuit is great for battery-powered systems that use slotted type optical interrupters. It draws only 10uA from a 3v battery that should allow up to 5 years of operation from a lithium battery.
  • Ten Turn Pot Forms Frequency Readout
    Let’s say you are designing a signal generator for a test fixture. To adjust the frequency you would like to use a ten turn pot with a nice dial mechanism attached as a frequency display. However, a display range of 0 to 9.99 would be impractical since there is no such frequency as 0. But since many pot dials have a display....
  • CHARGE COUPLED BI-DIRECTIONAL POWER MOSFET RELAY
    The circuit uses an inexpensive C-MOS inverter package and a few small capacitors to drive two power MOS transistors from a 12v to 15v supply. Since the coupling capacitor values used to drive the FETs are small, the leakage current from the power line into the control circuit is a tiny 4uA. Only about 1.5mA of DC is needed to turn on and off 400 watts of AC or DC power to a load.
  • SOLID STATE RELAY REQUIRES ONLY 50uA DRIVE CURRENT
    This circuit demands a control current that is 100 times smaller than that needed by a typical optically isolated solid state relays. It is ideal for battery powered systems. Using a combination of a high current triac and a very sensitive low current SCR, the circuit can control about 600 watts of power to load while providing full isolation and transient protection.

CIRCUIT PROTECTS FROM BATTERY POLARITY REVERSAL
This simple circuit can protect a sensitive electronic circuit from an accidental connection of a battery with a reversed polarity. The N-channel FET connects the electronic device to the battery only when the polarity is correct. The circuit shown was designed for a device powered from a single 1.5 volts button cell battery. However, the circuit will operate with higher voltages as well.

  • MACHINE POWER LOSS BEEPER
    For some medical equipment it is important for an operator when power is lost to the machine. The beeper is powered from a 9v battery and requires the machine to have a power switch with a third set of contacts.
  • ISOLATED AC CURRENT MONITOR
    This circuit uses a small AC current transformer from Magnetek to produce an isolated voltage proportional to the AC current in the primary winding. The transformer contains a single turn primary with a low 0.001-ohm resistance. It can easily handle 30 amps of AC current and provides at least 500vac of isolation. With the components shown, the output AC voltage is scaled so 1 amp of current produces 100mv of AC voltage.
  • UNPLUGGED POWER CORD ALARM
    Many medical devices, such a portable X-ray and ultrasound equipment, carry their own batteries. However, the frantic pace of some hospital emergency rooms might cause the device to be shoved off into a corner without being plugged back into an AC power source to recharge their batteries. So, I designed this circuit to emit an audible alarm whenever the device’s power cord was unplugged from the wall for a period of time. The device was designed to snap onto the outside of a power cord, where it senses the AC electric field emitted. No direct connection to the internal wires of the power cord is needed. I also included a low battery voltage monitor.
  • VARIABLE ISOLATED AC VOLTAGE SPANS 0VAC TO 280VAC
    I designed and built this circuit about 25 years ago. It came in handy for many projects that were powered from 120 VAC, 240 VAC and 277 VAC. It provides complete isolation from the power line. It uses two 1:1 isolation transformers that are wired in parallel for the140vac range and in series for the 280vac range. The selector switch also diverts power to the appropriate output socket to avoid mishaps in sending the wrong voltage to the load. My home-built unit also included an AC volt and amp meter to monitor the output. However, this circuit only indicates the AC power connections.
  • LIGHT RECEIVER WORKS FROM 1KHz TO OVER 70MHz
    This circuit uses one tiny C-MOS inverter IC to form a modulated light receiver with a very fast response. It is designed around a PIN photo diode that is packaged for use with plastic optical fibers. It can be used as an optical fiber receiver. By using the open end of the optical fiber it can "sniff" out any modulated light signals.

CIRCUIT PROTECTS FROM BATTERY POLARITY REVERSAL
This simple circuit can protect a sensitive electronic circuit from an accidental connection of a battery with a reversed polarity. The N-channel FET connects the electronic device to the battery only when the polarity is correct. The circuit shown was designed for a device powered from a single 1.5 volts button cell battery. However, the circuit will operate with higher voltages as well.

  • CHEAP LOW POWER 3V 2Hz OSCILLATOR
    This is yet another method to generate low frequency output pulses. The circuit uses an inexpensive under-voltage monitor IC and a couple of cheap MOSFETs. It could easily be modified to produce pulse frequencies from about one cycle per minute to a maximum of about 10KHz. It draws only about 1uA from a 3v battery. This circuit should not be used if frequency stability is important, since it will change as a function of supply voltage.
  • FLASHING LED POWER INDICATOR DRAWS LOW CURRENT
    This circuit flashes a power indicator LED to keep the average current low.
  • MICROPOWER PULSE GENERATOR
    it is hard to design a pulse generator that produces clean logic pulses while drawing very low current. This circuit is designed to produce short 2mS pulses at a rate of one per second while drawing only 1 micro amp from a 9 volt battery.
  • Ultra Low Power Astable Multivibrator
    Taking advantage of some voltage comparators, this circuit can produce a nice square wave signal while drawing only 1.6 micro amps. With the inclusion of a diode, the circuit can also produce short pulses instead of a square wave signal.

300V PEAK TO PEAK SIGNAL GENERATOR
This circuit converts a square wave signal to a +-150 volt output signal with fast 100nS rise and fall times.

  • N-CH AND P-CH TRANSISTORS FORM PUSH-PULL DRIVER
    This circuit can produce high speed output signals with fast rise and full times. The unique change pump action allows the voltage of the upper P-ch device to range from mill volts to hundreds of volts. The output current is only limited by the rating of the transistors. I have used this circuit beyond 2MHz.

175KHz INDUCTIVE PULSE TRANSMITTER
This circuit is discussed in more detain in the
Experimenters Journal. The transmitter’s six-inch diameter coil launches powerful magnetic 175KHz ring pulses that can be detected by the circuit below.

SQUARE WAVE DRIVER HAS FLEXIBLE OUTPUTS
This circuit can produce an output signal ranging from DC to 100KHz. It can source a voltage ranging from 1v to 30v. It can sink a voltage ranging from zero volts to –30v. It can drive up to 200ma of current and can even be switched to a floating tristate output.


CMOS INVERTER 125KHz LC OSCILLATOR
This circuit uses a single CMOS inverter to form a series resonant LD oscillator. The values shown set the oscillation at about 125KHz but other frequencies are possible by changing the main LC values.

  • CMOS INVERTERS FORM 125KHZ OSCILLATOR
    This circuit is similar to schematic
    CMOS INVERTER 125KHz LC OSCILLATOR but inverts the LC components so the inductor is grounded. Two inverters are needed to produce the needed oscillation. Again, the values shown set the frequency at 125KHz but can be changed to produce other frequencies.
  • CMOS NAND GATE FORMS GATED 125KHZ OSCILLATOR
    This circuit is similar to schematic
    CMOS INVERTER 125KHz LC OSCILLATOR but uses a NAND gate as an inverter. The gate allows the oscillator to be gated on and off. Again, the values shown set the frequency at 125KHz but can be changed to produce other frequencies.
  • CMOS INVERTER 125KHz LC OSCILLATOR
    This circuit uses a single CMOS inverter to form a series resonant LD oscillator. The values shown set the oscillation at about 125KHz but other frequencies are possible by changing the main LC values.
  • MEDIUM POWER 125KHZ OSCILLATOR-2
    This circuit is similar to
    MEDIUM POWER 125KHZ OSCILLATOR but adds even more inverters in parallel to deliver yet more power. The values shown are for 125KHz.
  • OPTICAL RFID TEST CIRCUIT
    I designed this test the concept of using light techniques to send identification data instead of RF. A more detailed discussion on this scheme can be found in the Imagineered new products section.
  • Wireless RFID Smart Key Detector: Many cars now use a key with an imbedded RFID microchip. The microchip is read when the key is inserted into the car’s ignition. The keys containing a microchip often look the same as most other keys. People duplicating car keys will often not know of the microchip. This circuit will light a LED if it detects a key containing a 125KHz RFID chip.
  • CMOS Logic Inverter Amplifier Characteristics
    This chart shows some of the measured characteristics of three different unbuffered CMOS logic gates, used as voltage amplifiers.

2.45GHz RF Signal Detector
This passive RF indicator hobby circuit is made from a few simple parts. A bent piece of wire forms a 2.5GHz antenna. A 100uA moving coil meter is used to display the relative intensity of the RF signal. Two microwave rated detector diodes form a simple voltage doubler circuit, which feeds a DC voltage to the meter movement, proportional to the RF field strength. This circuit can be used with some cell phones and many cordless telephones. It can also detect RF leaking from a microwave oven.

175KHz INDUCTIVE PULSE RECEIVER
This circuit is discussed in more detain in the
Experimenters Journal. The receiver’s six inch diameter coil detects the ring signal from the above transmitter and use a single NPN transistor to provide enough amplification for the signal to be easily viewed on an oscilloscope.

· Plastic Pipe Finder
I used this circuit once to track down the location of a plastic pipe, buried underground. It transmits a weak RF signal, which can be picked up by a portable AM radio from about 6 feet away. (added 12/04)

175KHz INDUCTIVE PULSE TRANSMITTER
This circuit is discussed in more detain in the
Experimenters Journal. The transmitter’s six inch diameter coil launches powerful magnetic 175KHz ring pulses that can be detected by the circuit below.

  • Low Power 40KHz Light Receiver New
    Drawing only 100uA, this circuit provides high sensitivity with excellent ambient light immunity.
  • 175KHz INDUCTIVE PULSE RECEIVER
    This circuit is discussed in more detain in the
    Experimenters Journal. The receiver’s six inch diameter coil detects the ring signal from the above transmitter and use a single NPN transistor to provide enough amplification for the signal to be easily viewed on an oscilloscope.
  • Full Wave Rectifier
    This is a classic circuit that can accurately convert an AC signal to DC. At 40KHz the input signal can be as low as 0.05 volts peak to peak. (added 12/04)

PRECISION FULL WAVE RECTIFIER
I have used this handy circuit many times. It accurately converts an AC signal into pulsing DC, which can be filtered to provide an average of the input voltage. It works from mill volts to volts. The circuit shown requires a stable +5v reference if a single power supply is used.

  • Reduced Power Relay Driver Aug 3, 2008
    Relays can handle a lot of power. However, for certain power sensitive designs you would like to reduce the power needed to hold a relay closed. The circuit below performs such a task. It uses a single CD4093 quad NAND gate. When the “on” logic input signal is detected, the relay is first pulsed on for about 500ms. This is sufficient time to insure the relay is fully closed. After that initial pulse the relay is then driven with a square wave signal, whose duty cycle can be adjusted. The signal duty cycle can be adjusted from about 10% to 90%. In most cases a 50% duty cycle will hold the relay closed. This reduces the average DC current required by the same factor, which means a 4:1 reduction in power. The circuit can operate over a wide 3v to 15v range.
  • Ultra Low Power Latching Relay Circuit
    The circuit below takes advantage of some inexpensive small super capacitors. The circuit pumps 6 volt pulses into the separate 5 volt latching and unlatching relay coils. A short 25ms pulse is all that is needed to flip the relay states. A third super capacitor is used to supply a higher peak current than a small lithium cell might otherwise be able to supply.
  • Universal Flasher using Latching Relay: This circuit is powered by a 9v battery and controls any AC to DC load through a set of relay contacts, rated at 10 amps. To conserve power, the circuit uses a latching relay. A variable frequency oscillator controls the flashing speed from 0.2Hz to 2Hz. A pair of pulse generators first latch the contacts closed then unlatches them open. The power consumption is so low that a 9v battery will last for several months.

1uS LIGHT PULSE RECEIVER PLUS POST AMP
This circuit is designed to detect very weak light pulses lasting 1uS. It uses a tuned LC feedback network to provide high sensitivity while giving high ambient light immunity. A post voltage amplifier is included with a gain of about X20. The circuit is described in more detail in the receiver section of my
Handbook of Optical Through the Air Communications. Note: The LF357 op amp is no longer available, this circuit is for reference only.

30KHZ LIGHT RECEIVER AMP
This circuit uses NPN Darlington transistor to amplify the signal produced from short light flashes, as detected by a PIN photo diode. The circuit draws only about 330uA from a 6v battery.

  • 40KHz LASER BURST DETECTOR
    This circuit was originally designed to detect weak flashed of laser light bounced off of a fabric video projection screen. It was used as part of a firearm training system. It generates a 100mS output pulse whenever it detects a 3ms to 5ms-laser burst, modulated at 40KHz. It is very sensitive and could be modified for long-range laser communications.

40KHZ LED TEST SIGNAL GENERATOR
This 40KHz crystal controlled oscillator circuit drives an infrared LED with powerful 40ma pulses. The circuit can be used to test optical communications circuits, designed to receive 40KHz modulated light signals.

40KHz TV-VCR LIGHT SOURCE REPEATER
This circuit is designed to be placed directly in front of a standard TV or VCR remote. The exiting light pulses produced by the circuit match the pulses from the remote but are about 10 times more powerful. Using the device, the remote can operate a TV or VCR over three times the normal distance.

  • Infrared Remote Extender
    This electronic hobby circuit is designed to be placed directly in front of a standard TV or VCR remote. The exiting light pulses produced by the electronic circuit match the pulses from the remote but are about 10 times more powerful. Using the device, the remote can operate a TV or VCR over three times the normal distance....
  • TV/VCR Infrared Remote Booster
    This circuit will boost the signal from any infrared TV or VCR remote, extending the range by a factor of 3X.

SINGLE IC FORMS SENSITIVE MODULATED LIGHT RECEIVER
The circuit uses a very inexpensive C-MOS IC that is connected to a small photodiode. Using a unique inductive feedback network, the circuit provides high sensitivity under high ambient light conditions. It is a great circuit when you want to extend the range of an optical remote control transmitter.

  • UL GROUND RESISTANCE TESTER
    UL requires that an electrical system using a metal chassis earth ground connection have a resistance of less than 0.1 ohms. This resistance must be tested with actual line current in excess or 10 amps. The circuit shown uses a couple of LEDs to indicate if in fact the resistance is less than 0.1 ohms. It uses a modified transformer to produce a 2.5vac output with at least 10 amps of supply current

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