Engineering Projects - Part 1

• 1.5V TOUCH ACTIVATED SWITCH)
  A single 1.5v silver oxide button cell powers this complete touch activated switch circuit for 5 years. It features both a normally open and a normally closed set of solid state switch thermals. It also has an adjustable sensitivity, which can be set for a touch capacitance change as small as 1 picofarad.
• 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 sinks output that change state whenever a metal button connected to the circuit is touched. An earth ground reference is required.
• 12V TOUCH SWITCH EXCITER CIRCUIT
  This circuit is designed to generate a 20 KHz 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.
• 120VAC Touch Switch
  This is a capacitance based touch on/touch off switch, which can control power to a 200 watt 120vac load.
• 240VAC Touch Switch
  This is another capacitance based touch on/touch off switch, which can control power to a 200 watt 240vac load.
• 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
• 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. (Note link is off site)
• 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.
• DOORKNOB ALARM
  Many companies offer simple alarm devices for personal use in bedrooms or hotel rooms. A metal chain attached to a box holding the electronics is placed around the inside doorknob of a wood door. Anyone grabbing the knob from the outside is detected by the electrical capacitance change that occurs from the human hand contact between the knob and the box. Almost all of the commercial devices sold use a more expensive and power consuming radio frequency circuit approach to detect the capacitance change. But, a very inexpensive and micro power technique can also work. This circuit schematic should dramatically reduce the cost of the device and allows it to operate for many years from one set of batteries.
• 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.

• FINGER TOUCH ACTIVATED SWITCH
  It does not get any easier if you want a solid state switch that is activated by the touch of a finger. Two small metal pins route voltage through the finger skin to a MOSFET switch. The circuit is great for situations where a membrane type mechanical switch is not desired.
• 5V Capacitance Touch Activated Monetary 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.
• 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
• Low Value Capacitance Meter
  This circuit was originally designed to measure the volume of fluid inside a medical syringe. As designed, it produces a zero to 5 volt output, corresponding to a capacitance change of about 10 microfarads. With a digital voltmeter, at its output, it can resolve a capacitance change of 0.002 picofarads or 2 fermifarads. (added 12/04)
• Capacitance Type Liquid Level Monitor
  This circuit was originally designed to monitor the level of liquid natural gas in a tank but it can be used to also measure almost any liquid. Two custom insulated metal tubes form a capacitor plate. The capacitance between the two tubes increases as the level of the liquid rises. The circuit converts an increase of capacitance into a positive voltage change. The circuit could be changed to handle almost any scale factor needed.
• Precision Low Capacitance Meter NEW
  This circuit was originally designed to measure the volume of the fluid inside a 10cc syringe. It used two copper foil strips attached to the outside barrel of the syringe. The fluid between the two copper strips increases the capacitance. As the fluid volume decreased, the capacitance also decreased. With the circuit shown below, it is possible to calibrate the circuit, so the voltage produced is proportional to the fluid volume inside the syringe. With the values shown, the circuit produces a 5v DC output,
• Short Range Reflective Object Sensor
  Drawing only 30uA from a 3v supply, this circuit will detect a human finger with a range of about 1 inch. The sensor uses an inexpensive infrared LED and a matching photo diode
• VERY LOW POWER GATED CRYSTAL OSCILLATOR
  The circuit gates the output of a continuously operating 32KHz crystal oscillator to the input of a C-MOS buffer when clock pulses are needed. The technique gets around the problem of a slow starting crystal oscillator by keeping the oscillator going and switching on a transistor power stage only as needed. The method keeps the standby power consumption to a very low 1uA when used with a 3v supply.
• VERY LOW POWER ASTABLE MULTIVIBRATOR
  This classic circuit draws only 200 nanoamps from a 1.5v supply.
• ULTRA PURE 125 KHz SINE WAVE SIGNAL SOURCE
  For some RFID systems operating at 125 KHz, 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%.
• SQUARE WAVE DRIVER HAS FLEXIBLE OUTPUTS
  This circuit can produce an output signal ranging from DC to 100 KHz. 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.
• MEDIUM POWER 125KHZ OSCILLATOR
  This circuit is similar to CMOS INVERTERS FORM 125KHZ OSCILLATOR but adds more invertors in parallel to deliver more power. The values shown are for 125 KHz.
• 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 125 KHz.
• 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 20 KHz 20v peak to peak output signal can supply up to 3 watts of touch switch excitation power.
• 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 very safe 5KV isolation.
• 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 125 KHz but can be changed to produce other frequencies.
• CMOS INVERTERS FORM 125KHZ OSCILLATOR
  This circuit is similar to schematic CMOS INVERTER 125 KHz 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 125 KHz but can be changed to produce other frequencies.
• 12V TOUCH SWITCH EXCITER CIRCUIT
  This circuit is designed to generate a 20 KHz 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.
• 200MHz400MHz 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
• 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.
• 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 40 KHz modulated light signals.
• AUDIO FREQUENCY DIGITAL NOISE GENERATOR
  when you need to test an audio circuit with broadband noise, this circuit works great. It uses just three inexpensive C-MOS ICs that generate a series of output pulses whose widths vary randomly. I included a level control pot.

• CAPS PROVIDE VOLTAGE BOOST TO SERIES REGULATOR
  This circuit adds some capacitors and diodes to a traditional transformer type series regulator circuit to extend the normal operating range. It can insure regulation during low line voltage conditions or it can squeeze a few more watts out of a plug-in-the-wall power adapter power supply.
• CHARGE PUMP EFFICIENTLY DIVIDES SUPPLY VOLTAGE
  This circuit uses a few FETs and logic devices to down convert a DC voltage to about one half its value.
• CMOS INVERTER 125 KHz LC OSCILLATOR
  This circuit uses a single CMOS inverter to form a series resonant LD oscillator. The values shown set the oscillation at about 125 KHz but other frequencies are possible by changing the main LC values.

• LINE POWERED 60Hz CLOCK GENERATOR
This circuit is connected to the 120vac power line and transfers 60Hz clock pulses to a logic circuit. The optoisolator used provides 5000 volts of isolation between the power line and the logic side of the circuit.

• ACCEPTABLE VOLTAGE INDICTOR
  I have used this circuit many times in custom test fixtures where a simple go-no go indication was needed. The circuit can also be used to adjust a particular voltage be within specific high or low limits. The three LEDs will indicate if the voltage is high, low or OK. When connected to other converters, such as a frequency to voltage converter, a current to voltage converter or a power to voltage meter, it could provide a quick indication of a proper level.
• Battery Charge Current Indicator
  This circuit turns on a LED whenever it detects at least 25ma of battery charge current.
• DC CURRENT INDICATOR #4 December 15, 2008
  The circuit below uses some common components to turn on an LED whenever DC current above a certain level is detected. The circuit uses a very popular LM393 dual voltage comparator from National Semiconductor and a common 1N4148 signal diode. The diode acts as a crude 0.7v voltage reference. Only one of the comparators inside the 8 pin package is used. A pair of resistors across the diode forms a voltage divider, which produce a reference voltage of about 0.015v. ....

• PRECISION AC PEAK DETECTOR
This unique circuit uses a very inexpensive voltage comparator to form a peak detector. The DC voltage produced tracks the positive peak of the input signal. It works from about ten mill volts to about 10 volts peak to peak. The maximum frequency is about 150KHz.

• XENON LAMP FLASH DETECTOR
  This circuit uses a small 2.5mm square photo diode in conjunction with a 100mH coil to detect the short light flashes from a xenon lamp. The coil makes the circuit immune to normal room lights. Its 10mv sensitivity can detect light flashes from a range of over 100 feet. Reflections from a room’s walls and ceiling is usually enough to trigger the circuit. The entire circuit draws only 3 Micro amps from a 6 to 9 volt battery.

• 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.
• 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.
• 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 LOW POWER GATED CRYSTAL OSCILLATOR
  The circuit gates the output of a continuously operating 32KHz crystal oscillator to the input of a C-MOS buffer when clock pulses are needed. The technique gets around the problem of a slow starting crystal oscillator by keeping the oscillator going and switching on a transistor power stage only as needed. The method keeps the standby power consumption to a very low 1uA when used with a 3v supply.
• Low Voltage H-bridge
  TTL type Q and inverted Q inputs control a classic H-bridge circuit, rated at 50 volts and about 10 amps. The circuit can control power and direction of a DC motor. (added 7/06)

• Motor Brake
This circuit will bring a fast moving DC motor to a stop in seconds. The braking action is automatically applied whenever DC power is removed.

• 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%. ...
• Two Button Motor Controller
  Two small pushbutton switches, a few diodes and two relays form a method to control on/off power to a brush motor as well as the motor direction. The circuit was originally designed for a motorized lifting platform.
• 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.
• 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)

• Automatic DC Motor Brake
  In many DC motor powered systems you would like the motor to quickly come to a full stop. This circuit provides an automatic electronic braking action to any DC motor ranging from 6v to 24v up to 1 amp of current. Some of the component values could be changed for larger motors....
• DC Motor Brake
  When the power switch is closed, the gate voltage of Q1 turns Q1 on. The drain of Q1 swings low, turning off Q2. Current flows freely into the DC motor with only one diode drop from D2. When the power switch is open. The gate voltage of Q1 swings low, causing the drain of Q1 to swing high. This turns on the transistor Q2. With Q2 on, the resistor R3 is switched in parallel with the motor, producing a braking action. The value of R3 can be selected for the desired braking action....
• 1 AMP CURRENT INJECTOR
  When you need to measure resistance down to a few micro ohms, this circuit works great. It is powered from two "C" cell batteries and is designed to inject a well regulated one amp of current into the unknown resistance. By measuring the voltage drop across the resistor with a digital voltmeter, the resistance value can be accurately measured. The circuit also is equipped with a low battery monitor. Published in Popular Electronics, November 1992
• 4 – 20ma Current Loop Tester
  This circuit injects an adjustable current through a wire loop. Using a digital current meter, the current can be adjusted from near zero to over 24 milliamps.
• Constant Current LED Driver
  Bob Pease from National Semiconductor came up with this circuit and I have used it many times. It maintains a constant current through one or more LEDs. A single resistor controls the desired current
• NEW Computer Controlled 100ma Current Source (July 11, 2008)
  Often in industrial control systems a constant current source is needed, which is controlled by a computer and referenced to circuit ground. The circuit below converts a zero to 5v signal from a computer’s analog output into a current, with a full scale of 100ma. The circuit shown requires a 9v DC supply but any voltage from 9v to 12v will work.
• 6v NMH/NiCd Battery Tester
  I designed this circuit to test rechargeable six volt battery packs under constant current conditions. As designed, the circuit applies a 10 amp load to the battery pack. A heat sink must be used on the main power transistor.
• 10MHz TO 20MHz LASER LIGHT DETECTOR
  This circuit was originally designed to detect laser light pulses for an optical Ethernet communications system. It has good ambient light immunity.
• 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.

• 40KHz MODULATED LIGHT DETECTOR
  This circuit uses a unique cascade amplifier circuit to convert the current from a PIN photo diode to a current without any feedback network. It is very stable and very sensitive. The circuit shown has the potential for a conversion factor of 10 volts per microwatt at 900nm. I included a simple JFET post-amplifier with a gain of about 20.
• 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 LIGHT RECEIVER IS IMMUNE TO AMBIENT LIGHT
  If you want even more sensitivity than the above circuit, try this design. When used with a one-centimeter square photodiode, you can achieve a range of several hundred feet with a standard TV or VCR remote control module.
• 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.
• BROAD BAND 2MHz OPTICAL FIBER RECEIVER
  If you need more sensitivity than the above circuit this circuit provides about ten times more gain. It too is designed around an inexpensive plastic optical fiber detector.
• BROAD BAND 5MHz OPTICAL FIBER RECEIVER
  This circuit is a simple broad band light detector that uses a very inexpensive IC and a PIN photodiode that is packaged for use with plastic optical fibers. It has a bandwidth from 1KHz to over 5MHz. It is great for experimenting with various modulated light sources
• CASCODE LIGHT RECEIVER CIRCUIT
  This page provides a detailed explanation of how the modified cascade light receiver circuit operates. The cascade technique in conjunction with an inductive load provides very high current to voltage conversion as well as very high speed.
• FET INPUT HIGH SPEED LIGHT DETECTOR
  This circuit is yet another design that converts current from a PIN photo diode to a voltage. It has a bandwidth that extends beyond 50MHz.
• 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.
• Photo Transistor Interface Circuits
  This page shows different ways to improve the response time of a photo transistor circuit.
• 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.
• Xenon Lamp Flash Detector
  This circuit has a very low standby current requirement yet has very high sensitivity toward the light flashes from a xenon lamp. When connected to a flip/flop it can serve as an on on/off Controller
• LASER/LED LIGHT OUTPUT INTENSITY METER
  This circuit uses a large 1cm X 1cm silicon PIN photo diode and a transimpedance amplifier to measure the light power output of infrared and visible LEDs and laser diodes. It can be modified to produce almost any milliwatts to volts scale factor. It can be connected to either a multi-meter or an oscilloscope.
• 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.
• 0.5V Negative Supply
  Although not very efficient, this simple circuit, consisting of two LEDs and a photo diode, generates a negative voltage with a current level of a couple milliamps. It is ideal for supplying a negative rail to low power “rail to rail” op amp circuits, which need to have a true zero volts output.
• Battery Powered Plus-Minus 15v Supply
  Many classic operational amplifier circuits call for a split positive and negative 15 volt supply. The circuit below makes the job of generating such a supply much easier by using an off-the-shelf DC to DC converter, powered from a cheap 6v battery made from four AA cells. The converter, part number VESD1-S5-D15-SIP, is available from Digikey, their catalog number 102-1410, for about $6.00. To insure clean voltages, I follow the output of the converter with two voltage regulators, also available from Digikey. The result is a very nice supply rated at 30ma from each supply voltage. This should be plenty of current for most circuits.

• Run Switching Type AC Power Adapters on DC NEW
I have received a couple emails from people looking for inexpensive DC to DC converters, which can convert 40v to 60v DC into say +5 or +12v. Such input voltages are often found in new automotive and industrial applications with a typical DC voltage of 48 volts. It turns out that many, not all, but many, standard AC line operated power adapters, which use switch mode techniques, will indeed work great when supplied with DC instead of AC. The unit below works down to about 30v DC and delivers 500ma at 5v. If you draw less current, it will operate at even lower DC input voltages....

• Charge Pump Efficiently Divides Supply Voltage
  This circuit uses a few FETs and logic devices to down convert a DC voltage to about one half its value.
• 1uS Light Pulse Discriminator Plus F TO V Converter
  This circuit is designed to detect the narrow 1uS pulses produced by the above amplifier circuit. The clean logic type pulses produced by the discriminator are then sent to a frequency to voltage converter. The circuit is designed to process a pulse frequency of 10KHz that is frequency modulated by voice audio signals. The circuit is described in more detail in the receiver circuit section of my Handbook of Optical Through the Air Communications.

• Pulse Period to Voltage Converter
This is a test circuit converts a square wave input signal into a voltage. But, the voltage produced is proportional to the time between edges (period) of the signal, not the frequency. The range is from 100uS to 10mS, which produces a voltage from 100mV to 10 volts. Other scale factors are also possible. The circuit is powered from single 15v supply and uses inexpensive parts. It is great when a signal's period instead of its frequency needs to be monitored.

• Line Powered 60Hz Clock Generator
  This circuit is connected to the 120vac power line and transfers 60Hz clock pulses to a logic circuit. The optoisolator used provides 5000 volts of isolation between the power line and the logic side of the circuit
  
• 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
• 0.5V Negative Supply
  Although not very efficient, this simple circuit, consisting of two LEDs and a photo diode, generates a negative voltage with a current level of a couple milliamps. It is ideal for supplying a negative rail to low power “rail to rail” op amp circuits, which need to have a true zero volts output.
• 0.5v to 6v Voltage Converter
  Conventional silicon transistors just can't operate at voltages less than about 0.7v. Old germanium transistors could be used, but those are hard to find these days and most are rather large in size. Some new n-channel MOSFET devices with very low gate-source threshold voltage can operate at quite low voltages. I've been experimenting with various devices and came up with one circuit, which demonstrates how to boost the low voltage from a single solar cell to a higher voltage.

• Charge Pump Efficiently Divides Supply Voltage
This circuit uses a few FETs and logic devices to down convert a DC voltage to about one half its value.

• 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
• 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.
• 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.
• Air Transparency Monitor, Xenon Flash Receiver, Page 2
  This is Page 2 of the receiver circuit Air Transparency Monitor, Xenon Flash Receiver

• 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.

• VOLTAGE TO FREQUENCY CONVERTER + 1uS LED PULSE DRIVER
  This circuit receives the signal from the above amplifier and launches powerful 1uS infrared light pulses from a low cost LED that are frequency modulated by the audio information. The 10KHz center frequency of the pulse stream is low enough so a standard infrared LED can emit ten times more light than conventional long pulse techniques. The circuit is described in more detail in the transmitter section of my Handbook of Optical Through the Air Communications.
• 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.
• 12KV High Voltage Generator The hobby circuit below uses an unusual method to generate about 12,000 volts with about 5uA of current. Two SCRs form two pulse generator circuits. The two SCRs discharge a 0.047uF a 400v capacitor through a xenon lamp trigger coil at 120 times a second. The high voltage pulses produced at the secondary of the trigger coil are rectified using two 6KV damper diodes. The voltage doubler circuit at the secondary of the trigger coil charges up two high voltage disc capacitors up to about 12KV. Although this circuit can’t produce a lot of current be very careful with it. A 12KV spark can jump about 0.75 of an inch so the electronic circuit needs to be carefully wired with lots of space between components.
• LOW POWER 12,000 VOLT POWER SUPPLY
  If you need about 12,000 volts DC for an ion generator this circuit might be the ticket. It draws power from the 120vac power line but it uses a small 6KV camera flash trigger coil. The output signal is isolated from the power line. Although the circuit can only deliver about 5uA of current it can produce dangerous shocks, so be careful
• SIMPLE NITROGEN SPARK GENERATOR
  Nitrogen or air sparks are very powerful light sources that produce flashes that last only a few nanoseconds. This line powered circuit generates a continuous series of very small sparks across electrodes with a 0.05 inch gap.
• 0.5V Negative Supply
  Although not very efficient, this simple circuit, consisting of two LEDs and a photo diode, generates a negative voltage with a current level of a couple milliamps. It is ideal for supplying a negative rail to low power “rail to rail” op amp circuits, which need to have a true zero volts output.
• CHARGE PUMP EFFICIENTLY DIVIDES SUPPLY VOLTAGE
  This circuit uses a few FETs and logic devices to down convert a DC voltage to about one half its value
• 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
  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.
• Air Transparency Monitor, Xenon Flash Receiver, Page 2
  This is Page 2 of the receiver circuit Air Transparency Monitor, Xenon Flash Receiver
• 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.

• 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
• CIRCUIT FORMS DIVIDE BY 1.5 COUNTER
  Two inexpensive ICs divide a TTL clock signal by 1.5. By following the circuit with another flip/flop, you could
• DARKROOM CAMERA SHUTTER TIMER
  This circuit was designed to control a film exposure shutter for a darkroom. It has 8 time steps ranging from
• HONEYBEE COUNTER
  I designed a circuit similar to this one a long time ago to help a beekeeper count the number of bees going
• LONG PERIOD COMPUTER WATCH DOG TIMER
  This circuit uses a simple 4060 IC oscillator/timer which is reset periodically by a computer. Should the compo
• On/Off Flip/flop Circuit with Automatic Timeout
  This circuit is ideal when a device needs to be turned on and off with a single pushbutton switch, but also needs
• 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
• Poor Man's Timer
  Often during testing of certain equipment and components, you would like to keep track of the elapsed time in
• 10MHz TO 20MHz LASER LIGHT DETECTOR
  This circuit was originally designed to detect laser light pulses for an optical Ethernet communications system. It has good ambient light immunity.
• 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.
• 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 LIGHT RECEIVER IS IMMUNE TO AMBIENT LIGHT
  If you want even more sensitivity than the above circuit, try this design. When used with a one-centimeter square photodiode, you can achieve a range of several hundred feet with a standard TV or VCR remote control module.
• 40KHz MODULATED LIGHT DETECTOR
  This circuit uses a unique cascade amplifier circuit to convert the current from a PIN photo diode to a current without any feedback network. It is very stable and very sensitive. The circuit shown has the potential for a conversion factor of 10 volts per microwatt at 900nm. I included a simple JFET post-amplifier with a gain of about 20.
• 10MHz TO 20MHz LASER LIGHT DETECTOR
  This circuit was originally designed to detect laser light pulses for an optical Ethernet communications system. It has good ambient light immunity.
• 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.
• 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 MODULATED LIGHT DETECTOR
  This circuit uses a unique cascade amplifier circuit to convert the current from a PIN photo diode to a current without any feedback network. It is very stable and very sensitive. The circuit shown has the potential for a conversion factor of 10 volts per microwatt at 900nm. I included a simple JFET post-amplifier with a gain of about 20.
• 40KHz LIGHT RECEIVER IS IMMUNE TO AMBIENT LIGHT
  If you want even more sensitivity than the above circuit, try this design. When used with a one-centimeter square photodiode, you can achieve a range of several hundred feet with a standard TV or VCR remote control module.
• LASER/LED LIGHT OUTPUT INTENSITY METER
  This circuit uses a large 1cm X 1cm silicon PIN photo diode and a transimpedance amplifier to measure the light power output of infrared and visible LEDs and laser diodes. It can be modified to produce almost any milliwatts to volts scale factor. It can be connected to either a multi-meter or an oscilloscope.
• 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.
• Xenon Lamp Flash Detector
  This circuit has a very low standby current requirement yet has very high sensitivity toward the light flashes from a xenon lamp. When connected to a flip/flop it can serve as an on on/off Controller
• Photo Transistor Interface Circuits
  This page shows different ways to improve the response time of a photo transistor circuit.
• 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.
• 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.
• 4-20ma Current Loop Tester: Using just a few components, this circuit can simulate signals from various instruments, which source a constant current.
• Pump Motor Monitor
  This circuit can turn on a light or sound a beeper, whenever an AC line powered pump motor turns on. It is designed to detect the motor current using a small inductor placed on the outside of the motor’s power cable. No direct wire contact is needed.
• TEMPERATURE SENSOR WITH 4 TO 20mA CURRENT LOOP
  I designed a circuit similar to this one years ago to accurately measure the air temperature inside a building 1000s of feet from a control room. The circuit uses a very robust current loop method. It uses a highly accurate semiconductor temperature sensor and an equally accurate voltage reference. The circuit includes a diode bridge, so it is polarity independent. By using the component values indicated, the circuit should not require calibration. It has a range from –40F to +120F and an accuracy of +/- one degree F.
• AC Current Monitor
  The hobby circuit below shows how a small current transformer from Magnetek can be used to monitor AC current to a load. The AC voltage developed at the secondary of the transformer can be scaled so it can be read by any digital multimeter. The transformer is rated at 30 amps.
• Battery Charge Current Indicator
  This circuit turns on a LED whenever it detects at least 25ma of battery charge current.
• DC CURRENT INDICATOR #4 December 15, 2008
  The circuit below uses some common components to turn on an LED whenever DC current above a certain level is detected. The circuit uses a very popular LM393 dual voltage comparator from National Semiconductor and a common 1N4148 signal diode. The diode acts as a crude 0.7v voltage reference. Only one of the comparators inside the 8 pin package is used. A pair of resistors across the diode forms a voltage divider, which produce a reference voltage of about 0.015v. ....
• Electrical Current Indicator
  I designed this circuit as a simple current indicator for any load ranging from 40 watts to 250 watts. The circuit turns on a small LED, whenever it detects current flowing to a remote load. (added 12/04)
• 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.
• Low Value Capacitance Meter
  This circuit was originally designed to measure the volume of fluid inside a medical syringe. As designed, it produces a zero to 5 volt output, corresponding to a capacitance change of about 10 picofarads. With a digital voltmeter, at its output, it can resolve a capacitance change of 0.002 picofarads or 2 fermifarads.
• Pump Motor Monitor
  This circuit can turn on a light or sound a beeper, whenever an AC line powered pump motor turns on. It is designed to detect the motor current using a small inductor placed on the outside of the motor’s power cable. No direct wire contact is needed.
• 1 AMP CURRENT INJECTOR
  When you need to measure resistance down to a few micro ohms, this circuit works great. It is powered from two "C" cell batteries and is designed to inject a well regulated one amp of current into the unknown resistance. By measuring the voltage drop across the resistor with a digital voltmeter, the resistance value can be accurately measured. The circuit also is equipped with a low battery monitor. Published in Popular Electronics, November 1992
• 4 – 20ma Current Loop Tester
  This circuit injects an adjustable current through a wire loop. Using a digital current meter, the current can be adjusted from near zero to over 24 milliamps.
• 6v NMH/NiCd Battery Tester
  I designed this circuit to test rechargeable six volt battery packs under constant current conditions. As designed, the circuit applies a 10 amp load to the battery pack. A heat sink must be used on the main power transistor.

• NEW Computer Controlled 100ma Current Source (July 11, 2008)
  Often in industrial control systems a constant current source is needed, which is controlled by a computer and referenced to circuit ground. The circuit below converts a zero to 5v signal from a computer’s analog output into a current, with a full scale of 100ma. The circuit shown requires a 9v DC supply but any voltage from 9v to 12v will work.
• Constant Current LED Driver
  Bob Pease from National Semiconductor came up with this circuit and I have used it many times. It maintains a constant current through one or more LEDs. A single resistor controls the desired current.