Monday, March 30, 2009

Engineering Projects - II


  • C-MOS INVERTER MAKES LOW POWER AMP
    With the addition of one resistor and a capacitor, some common logic ICs can be transformed from digital to analog duties. This circuit outlines some the features to expect from different inverter ICs.
  • 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 also generate a divide by three functions.
  • DVD Recorder + TV Camera Make Cheap Data Logger
    Sometimes recording the results of a test requires as much ingenuity as designing the product. Consider a situation which requires the collection of many different variables over a long period of time. Perhaps you are conducting temperature cycle tests on an electronic circuit and you want to know what changes occur in circuit performance over a 24 hour period. You could stay up all night and collect the data yourself....
  • 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.
  • 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.

· 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....

  • LONG PERIOD COMPUTER WATCH DOG TIMER
    This circuit uses a simple 4060 IC oscillator/timer that is reset periodically by a computer. Should the computer fail to send a pulse, the output changes state. The time can easily be set from seconds to hours.
  • 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.
  • C-MOS INVERTER MAKES LOW POWER AMP
    With the addition of one resistor and a capacitor, some common logic ICs can be transformed from digital to analog duties. This circuit outlines some the features to expect from different inverter ICs.
  • 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 also generate a divide by three function.
  • Isolated Power Supply for Digital Panel Meters -- September 13, 2008
    Many inexpensive digital panel meters, such as the one shown below, are designed to operate from a 9v battery. However, the nature of the panel meter internal design often requires that the DC supply used to power the device be isolated from the ground terminal of the panel meter’s input. This often makes it difficult to use with a common connection between a DC power supply ground and a sensor’s circuit ground. But, you can generate this floating voltage fairly easily using the simple circuit below.
  • 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....
  • 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.
  • Low Voltage Latching Relay Driver
    Using some small super capacitors, this circuit can latch and unlatch a mechanical relay with 10 amp contacts, from a small 3 volt power source. By using a latching relay, power can be controlled to a load with a tiny battery. (added 7/06)
  • Medium Power 40KHz Ultrasound Transducer Driver
    This crystal controlled circuit drives a 40KHz piezoelectric transducer with a 30v peak to peak signal. (added 7/06).
  • 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.
  • Ramping LED Driver
    The circuit below was designed to drive a LED with an intensity ramping mode. Two 555 timers generate two different triangle waveforms. The upper device generates a 10KHz signal while the lower unit produces a 1Hz signal. The two signals are fed to a voltage comparator. The result is a pulse width modulation (PWM) signal, which with the aid of the FET, drives the LED in such a way that its average light output slowly ramps from about zero light to maximum and then slowly dims back down. The circuit should operate over a supply voltage ranging from 3v to 12v. You can easily vary the ramping time by changing the value of the 1M resistor. For an interesting effect, you can place a 1N4148 diode in parallel with the 1M resistor, with the cathode (banded end) side connected to pin 3.

  • 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.
  • 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.
  • 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. Note: Since the MF6-50 switched capacitor filter IC is no longer available; this circuit is for reference only. Use circuit number Discriminator in place of the IC filter.
  • Isolated Power Supply for Digital Panel Meters -- September 13, 2008
    Many inexpensive digital panel meters, such as the one shown below, are designed to operate from a 9v battery. However, the nature of the panel meter internal design often requires that the DC supply used to power the device be isolated from the ground terminal of the panel meter’s input. This often makes it difficult to use with a common connection between a DC power supply ground and a sensor’s circuit ground. But, you can generate this floating voltage fairly easily using the simple circuit below.
  • 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....
  • Basement Doorbell Beeper
    If you can't hear your doorbell when you are in your basement try this circuit. This circuit takes advantage of the 24vac power source located near the furnace. Using a simple current transformer technique, the circuit sounds a beeper whenever the main door chime is activated.
  • Basement Doorbell
    This circuit will activate a beeper in the basement, whenever the front doorbell is pressed.

  • 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.
  • 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.
  • 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.
  • Charge Coupled MOSFET Relay
    This circuit acts as an AC/DC relay with a rating up to 50 volts peak and up to 10 amps of current. The differential oscillator supplies voltage to the gates of the two FETs with good isolation will draw only 1.5ma of current.
  • 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.
  • 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.

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


I have used these circuits many times. They are great when you need a low gain AC signal amplifier with a very high input impedance. It is good to beyond 50MHz. (added 12/04)

· VERY LOW POWER ASTABLE MULTIVIBRATOR
This classic circuit draws only 200 nanoamps from a 1.5v supply.

  • 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.
  • 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
  • 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.
  • 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 50MHz OPTICAL FIBER RECEIVER VERSION A
    If the above circuit it still too slow, you can try this circuit. What it lacks in sensitivity it makes up for in speed. The circuit attaches a plastic fiber optic PIN photodiode assembly to a small box containing a small 3v battery and a standard BNC plug. When the box is plugged into to the input of an oscilloscope with a 50-ohm termination resistor, it can detect light pulse frequencies beyond 100MHz.
  • 1.5V LED FLASHER VERSION A
    Many published circuits that flash LEDs need 3 volts or more. This circuit uses only a single inexpensive C-MOS IC and flashes the LED for a full year on a single 1.5 volt AA alkaline battery cell. The circuit uses a charge pump technique to provide the LED the needed voltage.
  • 3v Low Battery Voltage Flasher
    Many battery powered devices use two AA alkaline cells. Often you will not know when it is time to replace the batteries until the device powered by them actually stops operating. The hobby circuit below can be connected to a 3v battery, to give you some warning when the battery is nearing its end of life. It will flash a LED when the battery voltage drops to about 2.4 volts. The electronic circuit draws only 1ua of current in standby mode and jumps to only 20ua when flashing, so it can safely...
  • 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.
  • FLASHING LED ADVERTISING BADGE #1
    I have seen numerous flashing light badges at trade shows and conventions. They are often handed out as gifts to promote some business. The devices often use inefficient circuits, which cause the battery power source to be quickly depleted. My circuit is simple but efficient enough to provide months of continuous LED flashing. It also has a tiny push-button switch to turn on and off the light flashing, extending battery power.
  • Fake Car Alarm Light
    Whenever the car’s ignition is turned off, this circuit activates a flashing LED, which can be positioned to appear as an active alarm system. (added 7/06)
  • 1.5V LED FLASHER VERSION B
    To squeeze even more energy from a alkaline battery cell, this circuit adds two transistors to a circuit similar to the above design to boost the efficiency. A small 1.5 volt alkaline N cell should flash the LED for a full year. It too uses a "charge pump" technique to provide a LED the needed voltage.
  • FLASHING LED POWER INDICATOR DRAWS LOW CURRENT
    This circuit flashes a power indicator LED to keep the average current low.
  • FLASHING LED ADVERTISING BADGE #2
    This circuit is similar to
    FLASHING LED ADVERTISING BADGE. It uses a CD4013 dual D Flip/Flop IC. The 74HCT74 IC in
  • High Power LED Flasher August 3, 2008
    Power LEDs are becoming more popular these days. The DC input power to these devices ranges from 1 watt to 5 watts. Normal LEDs only receive about 0.05 watts. At this higher power, these devices can emit a lot of light. You can buy them in just about any color in the rainbow but white seems to be the most popular. One application for these devices is a flashing light. If the flash duration and flash rate is kept low, the average current can be low enough that even a small battery can last quite a while.....
  • Lighthouse LED Flasher
    This was originally designed for a model in a HO train set. It simulates the behavior of the light from a lighthouse. The LED intensity gradually increases, then flashes with a bright light and finally decreases slowly in intensity.
  • High Intensity Line Powered LED Flasher
    There are times when you would like to generate an intensely bright flash of light using electrical power from the AC line. The circuit below can drive a one to three watt power LED with about 350ma of peak current. Since the circuit drives the LED with a constant current, any LED color can be used. I have also shown how you could use an array of 7 LEDs, wired in parallel. This array may not produce as much light as the single power LED but the array would be cheaper to build. ....
  • 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.
  • LOW BATTERY VOLTAGE FLASHER
    This circuit is designed to monitor two alkaline cells (3v) that from the battery often used in portable electronic equipment. It use an inexpensive IC from Panasonic that is connected to an efficient LED flashing circuit. When the battery voltage drops below a certain point the circuit flashes the LED. In the off state the circuit draws only 1uA, while in the active flashing state it draws 20uA.
    Published in EDN, Jan 2, 1997

  • Reduced Power Sequential LED Flasher
    The 74HC4017 is a neat decade counter which has 10 decoded outputs. When supplied with a low frequency clock and with a LED attached to each of the outputs, an interesting ten step sequential flasher effect can be produced. The LEDs can be configured in a wide variety of patterns of loops, lines or arrows. But, these circuits can often be power hogs. To reduce the average current needed to produce the same effect, you can strobe the individual LEDs with a short 10mS pulse.
  • 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.
  • 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.

Ultra Low Power LED Flasher NEW
The efficiency of some newer LEDs is amazing. Some of the latest green LEDs can launch blinding light with just one milliamp of current. I take advantage of one of these newer devices in the circuit below. The flashing circuit uses a classic multivibrator oscillator, made from a tiny National Semiconductor’s LMC7215 low power voltage comparator. The circuit produces a short 10ms pulse every two seconds, drawing power from a 3v supply. I suggest using a surface mounted green LED from King bright.....

  • Wig/wag LED Flasher
    This simple circuit will flash two LEDs in an alternating fashion.
  • 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.
  • 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)
  • D-FLIP/FLOP ONE SHOT CIRCUITS
    Yes you can use cheap D flip/flop logic circuits as nice one-shot pulse generators. This schematic shows how the popular CD4013 and the CD74HC74 can be used to generate pulses ranging from nanoseconds to seconds.
  • D-FLIP/FLOP OSCILLATORS
    Yes you can turn flip/flop ICs into low current oscillators. This schematic shows you how.

  • FLASHING LED ADVERTISING BADGE #1
    I have seen numerous flashing light badges at trade shows and conventions. They are often handed out as gifts to promote some business. The devices often use inefficient circuits, which cause the battery power source to be quickly depleted. My circuit is simple but efficient enough to provide months of continuous LED flashing. It also has a tiny push-button switch to turn on and off the light flashing, extending battery power
  • FLASHING LED ADVERTISING BADGE #2
    This circuit is similar to Flashing LED Advertising Badge #1. It uses a CD4013 dual D Flip/Flop IC. The 74HCT74 IC in Flashing LED Advertising Badge #1 does not always work. As in #81, a single lithium battery will provide months of continuous LED flashing. It also has a tiny push-button switch to turn on and off the light flashing.
  • HONEYBEE COUNTER
    I designed a circuit similar to this one a long time ago to help a beekeeper count the number of bees going into or out of a hive. The low power circuit uses a slotted opto-sensor to detect the passing bees. The circuit advances an electronic counting module whenever a honeybee passes through the sensor. The device only counts the number of bees going through the sensor. A different circuit would be needed to count the number of bees only going out or only coming into the hive.
  • Miniature Pushbutton On/Off Switch NEW
    In many battery powered devices, you would like to turn the circuit on and off by pressing a small pushbutton switch. One press turns the device on and second press turns it back off. The flip/flop circuit below uses a minimum number of parts to perform this function. It operates from DC supply voltage which can range from 2v to 6v. It draws very little current and takes up little space. Yet, it can control up to one amp of current. The circuit uses a single flip/flop device made by Texas Instruments. ...
  • 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 to turn itself off after some period of time. With the components shown, the output will stay on for only about 20 seconds. (added 12/04)
  • PUSHBUTTON ONESHOT AND LATCH
    This circuit uses a single IC to convert a noisy pushbutton switch signal into a clean pulse or a sustained push on-push off signal. It can operate from 3v to 18v.
  • 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.

· 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.

  • 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
  • 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,...
  • 1 WATT FLUORESCENT LAMP NIGHT LIGHT
    If you are tired of replacing small night light lamps, try this circuit. The line powered circuit uses a long life cold cathode fluorescent lamp. A simple charge pump technique runs the lamp without any transformer.
  • 5W FLUORESCENT LAMP INTENSITY MODULATOR
    The circuit was designed to experiment with using small fluorescent lamps as a broad pattern source of modulated light. The circuit hits the small lamp with narrow 1us pulses at a rate of 10KHz. Each pulse launches about 10 watts of visible light. The lamp starting method is a bit crude but the circuit does work.
  • 5W FLUORESCENT LAMP INTENSITY MODULATOR
    The circuit was designed to experiment with using small fluorescent lamps as a broad pattern source of modulated light. The circuit hits the small lamp with narrow 1us pulses at a rate of 10KHz. Each pulse launches about 10 watts of visible light. The lamp starting method is a bit crude but the circuit does work.
  • 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.
  • 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.
  • 20MHz VCSEL 3mW LASER TEST CIRCUIT
    This circuit takes advantage of some new vertical cavity surface emitting lasers (VCSEL) that don’t require light output control circuits. The circuit shows how to drive the device from a single high speed CMOS IC. The circuit can easily be modified to transmit signals from kilohertz to about 50MHz.
  • 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 Light Detector With High Ambient Light Immunity
    The circuit below was designed to turn on an external 12v relay, whenever it detects light from a nearby LED light source, modulated at 40KHz to 50KHz. This circuit was originally designed to operate from a fast moving vehicle. The light transmitter was positioned at a stationary position, while the matching receiver was mounted on the vehicle. The circuit has high ambient light immunity and in most cases, can operate in direct sunlight....

40KHz Light Detector with Sunlight Immunity NEW
The circuit below was designed to turn on an external 12v relay, whenever it detects light from a nearby LED light source, modulated at 40KHz to 50KHz. This circuit was originally designed to operate from a fast moving vehicle. The light transmitter was positioned at a stationary position, while the matching receiver was mounted on the vehicle. The circuit has high ambient light immunity and in most cases, can operate in direct sunlight.

  • 40KHZ LIGHT RECEIVER AMP
    This circuit is similar to
    30KHZ LIGHT RECEIVER AMP but provides more gain and operates up to 40KHz. However it draws more power supply current.
  • 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.
  • 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.
  • 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 Dave Johnson's Handbook of Optical Through the Air Communications. (this link is off-site)
  • 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 above.
  • AIR TRANSPARENCY MONITOR, XENON FLASH TRANSMITTER
    This is the matching transmitter for the above receiver. The transmitter launches powerful 1000-watt light pulses that last about 20 microseconds.
  • 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.
  • 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 DETECTORS WITH AMIBIENT LIGHT COMPENSATION
    These circuits were taken from a few application notes on infrared remote control devices. They use a current compensation method to separate the modulated light pulses from ambient light. They appear to have limited bandwidth and may only work at the 30KHz to 50KHz frequencies often used by TV and VCR remotes. I have not yet tested the circuits.
  • 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. t 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.
  • Low Power 40KHz Light Receiver
    Drawing only 100uA, this circuit provides high sensitivity with excellent ambient light immunity.
  • Low power 100KHz Light Receiver
    By starving a high speed logic inverter for current, this circuit can produce a sensitive 100KHz light receiver circuit, which is immune to ambient light, but only drawing 100 micro amps from a 3 volt supply.
  • MICRO POWER 40KHz BURST LASER DIODE DRIVER
    Some laser tag or simulated combat games can use this circuit to send short bursts of modulated laser light at the opponent's vest, equipped with a matching light receiver. The circuit operates from three 1.5v cells (4.5v) that should provide enough energy for about 200,000 shots.
  • 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.
  • PULSED LED TEST CIRCUIT
    This circuit is designed to test visible and infrared LEDs in pulsed mode operations. It can drive the LED with peak currents in excess of 10 amps. A light detector nearby can monitor the response time and intensity of the LED under test.
  • 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.
  • SINGLE IC FORMS SENSITIVE MODULATED LIGHT RECEIVER
    The circuit uses a very inexpensive C-MOS IC that is connected to a small photodiode. Using an 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.
  • 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 Dave Johnson's
    Handbook of Optical Through the Air Communications. (this link is off-site).
  • 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.
  • Freezing Temperature Alarm
    This circuit turns on a beeper whenever the outside air temperature is less than zero degrees Centigrade. The circuit draws only a few micro amps from a 9 volt battery. It uses a glass bead thermostat accurate to 1 degree C.
  • Freezer Over-Temperature Alarm
    This electronic hobby circuit below is designed to activate a beeper alarm, whenever the outside air temperature is below 0 degrees C (32F). A 9v battery powers the hobby circuit. The average 9ua current is so low that the battery should last for many years....
  • 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 also generate a divide by three function.
  • 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.

  • Cheap Close-up Camera Lens
    I often use my digital camera to take photos of some electronic projects. At times, I need to get in close to capture some details. But, without some additional optics, my camera just could not maintain focus, as I got closer to an object. Being a cheap person, I didn’t want to spend money on an expensive close-up lens...
  • Electrical Device Harvests Earthworms
    A 120vac or 240vac isolation transformer is used to force earthworms from the ground. (added 7/06)
  • Homemade Corner Cube Reflector Using Glass Mirror Tiles
    Corner cubes are optical devices which have a unique property. Light sent into the front face will be sent back to the source in a parallel path. As long as light can strike the reflective surfaces the it will always be sent back to the source. Tiny corner cubes can be molded into plastic for form an array of individual reflectors. These are often used on bicycles to make them stand out at night....
  • Pigeon Scarecrow
    I have an area on my roof where a large flock of pigeons like to roost. Their nesting materials and droppings really make a mess. Often there is enough junk from them that it clogs my roof gutters. Perhaps one way to scare them away is with a device mounted on the roof near their favorite roosting spot. The device....

Modified LED Night Light NEW
The circuit uses the classic series capacitor method to produce a current limiting LED driver, powered from the AC line. The series capacitor forms a constant current source and will route the same amount of current through the parts, even if 3 or 30 LEDs were used in series. This type of circuit made it much easier to insert 10 series wired LEDs instead of the three original devices. I also switched from a NPN Darlington driver circuit to a single n-channel FET. This made it easier to select the right resistor value to turn off the light in the day. I set the required CdS cell resistance at about 100K, so anything less than that in resistance should keep the LEDs turned off. I did have to change the resistor R2 to a 39v sneer diode, to limit the peak voltage across the filter capacitor C1, when the LEDs are turned off during daylight.

Shake to Charge Flashlight NEW
The complete schematic for this device is shown below. A few things should be pointed out. First, they use some very cheap 1N4001 diodes in the bridge rectifier circuit instead more efficient Schottky diodes. They also use a small 0.5 Farad cap with a 5.5 volt rating. I noticed that that this kind of super capacitor was originally designed for maintaining data in memory chips and has a rather high internal equivalent series resistance. This reduces the overall efficiency, since the device can’t be charged or discharge very quickly. Some of the power that should go to the LED will end up being dissipated inside the capacitor. Better super capacitors do exist.

  • Ocean Wave Power Generator Demo Unit
    A while back, a Discover Circuits visitor wanted a simply way to demonstrate how electricity could be generated from the up and down movement of ocean waves. The system was to be part of a science fair project, showing different renewable energy sources. I gave this some thought and came up with the system shown below. It uses a simple plastic foam float attached to a lever arm,...
  • Wave Power Generator Demo System
    An inexpensive stepper motor is used as a voltage generator in this wave generator demo. (added 7/06)
  • Worm Harvester
    When I was a kid my engineer uncle built a circuit similar to the one below. My cousin and I used it to force earth worms out of the ground to be used for fishing. It worked like a champ. Please be careful since there is high voltage at the probe tips.

Home Made Geophone Detects Foot Stomp
A home made geophone is made from a strong magnet, a coil of wire and a rubber band. The circuit is sensitive enough to detect the vibrations of a nearby foot stomp. It could be used as an earthquake detector. (added 7/06)

Human Traffic Footstep Detector
This circuit uses a surplus geophone to detect human footsteps from 100 feet away. It was designed for monitoring the Mexico/USA border. This preamp part of the system draws about 50 micro amps from a 12 volt supply. (added 12/04)

  • Human Traffic Footstep Alarm
    This circuit processes the amplified signals from the footstep detector circuit and activates a microprocessor when the circuit detects footsteps. At has an adjustable sensitivity. (added 12/04)

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