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Circuits designed by David Johnson, P.E.
Last Updated on: Thursday, December 07, 2017 02:06 PM


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David Johnson's Circuits
Circuits + Descriptions

from Dave's Circuits & Circuit Solutions


#'s          A         B - C          D - F        G - K          L          M - O          P - R         S - Z


Self-Powered AC Current Indicator  -  Often there is a need to monitor when AC current flows into a load.  The circuit below works well for this application.  It uses an off-the-shelf torrid type AC-1020 current transformer from Triad, available from Mouser.  A wire carrying the AC current is . . . Circuit by Dave Johnson P.E.-December 30, 2007

Serial Data Bus Status Monitor  -  I designed this circuit many years ago and have used it several times since.  It uses a single logic IC to indicate the TXD and RXD line activity of a typical RS-232 serial data communications link.  The thresholds are set at plus 3 volts and minus 3 volt . . . Circuit by Dave Johnson P.E.-December 15, 2004

Shadow activated (or hand wave) Motion Detector  -  The circuit below works quite well in typical indoor room lighting.   I would not recommend its use in direct sunlight.   Two small PIN photodiodes positioned about one inch apart form a shadow detector.   With no shadow cast on the devices, both devices produce nearly identical current levels.   The current is converted to a voltage with two load resistors in parallel with the photodiode.   One resistor is adjustable to the two voltages can be carefully balanced under uniform lighting. . . . Circuit Solution by David A. Johnson P.E.-August 21, 2005

Shooting Range Flasher  -  Problem:  Design a unique indicating device which would detect when a steel target on a shooting range was hit with a bullet by flashing a bright light.  A crowded range masked the pinging sounds.  Solution:  The circuit is shown below.  The solution is broken into two separate parts - (1) a small box containing a piezoelectric shock sensor and (2) a trigger circuit connected to the main control box, positioned some 15 feet away.  The two assemblies were linked through a coax cable.  The coax cable has a thick jacket and is buried a couple inches below the surface.  . . . Dave Johnson, Lubbock TX-DC Magazine - 1st Q 2012

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 . . . Circuit by David A. Johnson P.E.-July 09, 2006

Simple Circuit for White LED Night Light  -  This simple circuit is designed to plug into a standard AC electrical outlet.  It uses four super bright white light emitting diodes (LED) in conjunction with a capacitor coupled full wave rectifier circuit . . . Circuit by David A. Johnson P.E.-July 27, 2003

Simple Circuit 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 . . . Circuit by Dave Johnson P.E.-June 04, 2000

Simple Circuit Wire Security Loop Alarm  -  A wire loop is used to protect valuable objects in this alarm circuit.  The circuit is powered by a 9v battery.   . . . Circuit by David A. Johnson P.E.-July 09, 2006

Single IC forms Senesitive 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 . . . Circuit by Dave Johnson P.E.-June 04, 2000

Single Lead-Acid Cell Voltage Monitor  -  The circuit below uses a a low voltage reference and a voltage comparator from National Semiconductor to monitor the battery voltage.  The green LED is controlled by a tiny charge pump type voltage converter IC from Texas Instruments.  This device efficiently converts the 1.8v from the battery into 3.3 volts.  A dual Schmitt trigger inverter IC, also from Texas Instruments, was used to latch the normal and low battery states and a Honeywell Hall switch was used as reset switch. . . . Dave Johnson, Lubbock TX-DC Magazine - 3rd Q 2012

Sink to Source Universal DC Converter  -  As illustrated below, many control systems use an open collector transistor to turn something on.  Perhaps it is a LED light, a relay or a small motor.  However there are other systems which require a common ground connection instead of a common V+ supply connection . . . Circuit by David A. Johnson P.E.-March 20, 2011

Small Pushbutton Controls 10 Amps AC  -  The circuit below uses a pair of long lasting lithium coil cell batteries to power a latching relay circuit.  With each activation of the small pushbutton, the relay is either latched closed or unlatched open.  Since coin cells can’t provide the necessary peak current, a 500uF capacitor is used to store the needed energy.  The circuit standby current is kept very low by using a stack of five 100uF ceramic capacitors instead of the usual electrolytic cap . . . Circuit by Dave Johnson P.E.-January 16, 2012

Solar Fireflies In Jar Light  -  When I was a kid, growing up in Illinois, I remember catching fireflies and putting them into a jar.  I loved watching them flashing away at night.  Later, I would let them out to live another day. . . . Circuit by David Johnson P.E.-September 21, 2017

Solar Insolution Meter  -  You don’t need a fancy and expensive meter to measure the sun’s intensity.  With some inexpensive parts you can build your own meter.  I used a 1ma meter movement and a single solar cell I got on eBay with an area of about one square inch.  I calibrated the meter on a very clear bright summer day when the sun was directly overhead. . . . Circuit by David Johnson P.E.-July 14, 2017

Solar Lithium Ion Battery Charger using LT1129  -  I have been designing a number of solar powered devices lately.  Many of them use just a single 3.  6v lithium ion battery.  These batteries like to be charged from a fixed 4.  2v source with a charging current limited to about C/8, where C is the milliamp-hour rating of the part . . . Circuit by David Johnson P.E.-December 24, 2012

Solar Mason Jar Light  -  I have modified several commercial solar garden lights over the years.  I switch out the LED, solar panel, battery pack and control circuit.  The result is a much better overall light.  One of my disappointments in using store-bought garden light assemblies is their construction.  They are built so cheaply that over time, water and insects get inside, killing the light operation. . . . Circuit by David Johnson P.E.-September 21, 2017

Solar Path Light Improvement  -  Use the circuit below was designed to replace the guts of an existing path light assembly with one which has a much better performance.  I suggest using a quality 10 cell solar panel with at least a 4 square inch area.  Buy a panel with solar panels that . . . Circuit by David A. Johnson P.E.-March 13, 2011

Solar Powered Cell Phone Charger  -  The simple circuit below regulates the voltage from a 6v solar panel to a fixed +5v.  That voltage can be fed to any cell phone or USB connected portable device, to charge its battery.  The circuit uses a Reg113-5 voltage regulator from Texas instruments . . . Circuit by Dave Johnson P.E.-January 02, 2011

Solar Powered Flood Light using NiMH Cells  -  There are many solar powered garden lights available.  Most of the devices offered at the home improvement stores have very poor performance.  Their batteries are too small, the light launched is weak and the solar panels are undersized.  None of the devices I have tested would operate through a typical night . . . Circuit by Dave Johnson P.E.-December 20, 2013

Solar Powered Infrared 1W Security Camera Illuminator  -  Many security cameras are equipped with an array of infrared LEDs to illuminate an area in front of the camera with invisible light for night time surveillance.    However, there are times when a remote area, well away from the camera, needs additional infrared light.    The circuit below uses a modified solar powered path light to bathe a fairly large area with 850nm infrared light . . . Circuit by David Johnson P.E.-June 23, 2014

Solar Powered Lighthouse  -  While at a discount store one day, I spotted an interesting tall lighthouse.  It had a tiny solar panel and an equally tiny LED light.  The LED was pointing downward, at the center of a rotating light reflector.  I thought it might be turned into an interesting addition to my wife’s garden. . . . Circuit by David Johnson P.E.-September 21, 2017

Solar Powered Quartz Clock  -  Connect a solar panel to a quality AA size 1One of my homes in Colorado had a large battery powered quartz clock above the fireplace.  It was powered by an AA size 1.5v alkaline cell.  The battery would run the clock for about a year or so, then I would have to break out a tall step ladder and replace the battery for another year or so of operation. That gave me a great 10:1 energy margin. . . . Circuit by David Johnson P.E.-September 21, 2017

Solar Powered Wireless Sensor  -  A company in sunny Florida needed a DC power supply for a remote sensor. The sensor was connected to a low power microprocessor and a RF transmitter.  When switched on, it quickly transmitted collected data in a short burst.  The transmitter drew a peak current of 1 Amp for about 0.01 seconds from a 3v supply.  The transmitter was turned on only once every 10 seconds . . . Dave Johnson, Lubbock TX-DC Magazine - Issue  5 Jan 2010

Solar PV Cell Test Fixture  -  This is an illustration of a test fixture, which can be used to test individual solar cells for short circuit current.  Using a PIN photo diode and a control circuit, the solar cells can be tested under constant light level conditions . . . Circuit by David Johnson P.E.-July 09, 2006

Solar-Powered Pump Motor Controller  -  A Discover Circuits visitor had a problem.  He needed a simple on/off controller for his solar powered water pump.  His system used two 12v 50 watt solar panels wired in series.  The power from the solar panels drove a submerged water pump.  However, during . . . Circuit by David Johnson P.E.-October 03, 2010

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 . . . Circuit by David A. Johnson P.E.-June 03, 2000

Sound Triggered Night Light  -  A while back, I received a schematic from a guy by the name of Karl.  He described a situation he and his wife found themselves in aboard a cruise ship in the Caribbean.  They were staying in a room toward the inside of the ship, which did not have any port holes, windows or a balcony . . . Circuit by Dave Johnson P.E.-June 24, 2013

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 . . . Circuit by Dave Johnson P.E.-July 08, 2000

Star Trek Doorbell  -  in the Star Trek “Next Generation” TV series, the doorbell outside the private quarters of a crew member makes a particular “beep-boop” sound.  The 3v battery powered circuit below tries to simulate this sound.  The circuit uses one 74HCT74 dual D flip/flop . . . Circuit by Dave Johnson P.E.-October 12, 2001

Super Bright 3v Powered LED Flasher  -  I have posted several kinds of LED flashers over the years.   This is yet one more flasher, designed to flash any high power LED of any color.  It hits the LED with a 25ms 250ma pulse once every 2 seconds.   This works out to an average current about 3ma.  It . . . Circuit by David Johnson P.E.-April 22, 2009

Supercapacitor Voltage Limiting Circuit  -  Supercapacitors are working their way into more and more applications where electrical energy needs to be stored.  These robust devices can be charged and discharged 1000s of times and will typically outlast a battery . . . Circuit by David Johnson P.E.-March 27, 2010

Switch Detects Audio Signal  -  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 were it is needed.   . . . Circuit by David Johnson P.E.-July 09, 2006

Table Lamp with Auxiliary 12v Automotive Style LED Light  -  This circuit provides about one watt of non-isolated DC power for an automotive type 12v LED array lamp in addition to a standard incandescent table lamp . . . Circuit by David A. Johnson P.E.-July 09, 2006

Temperature Sensitive MOSFET  -  Recently I was asked by a company to discover why one of their products failed to operate properly in cold temperatures.  Two n-channel MOSFETs and a few resistors are wired to form a flip/flop latch circuit.  Each time the pushbutton switch is pressed, the circuit changes state and remains in that state until another activation of the switch.   . . . Dave Johnson, Lubbock TX (December 1, 2010)-DC Magazine - Issue 14 Dec 2010

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 . . . Circuit by David Johnson P.E.-July 04, 2006

Ten-LED Sequential Flasher using Low Powr  -  A while back I posted a flasher circuit which flashed 10 LEDs sequentially.  That circuit was powered by four AA alkaline cells.  The circuit below operates much the same way but has been modified to run from a single 3v CR2032 lithium coin cell . . . Circuit by David Johnson P.E.-April 17, 2011

Ten-Second Ultra Lower Power Oscillator  -  This circuit works much like the classic 555 timer, but draws only about 1.5 microamps from a 3 volt battery.  It is highly stable under varying temperature and supply voltages . . . Circuit by David Johnson P.E.-May 29, 2000

Test UL Ground Resistance  -  UL requires that an electrical system using a metal chassis earth ground connection have a resistance of less than 0.1ohms.   This resistance must be tested with actual line current in excess or 10 amps.    The circuit shown uses a couple of LEDs to indicate . . . Circuit by David Johnson P.E.-April 28, 2001

Tester for Brown Out Voltage  -  Using a cheap 24vac transformer, this circuit can test a product under low 95vac conditions.    With the components show, it has a rating of 250 watts max.   . . . Circuit by Dave Johnson P.E.-July 11, 2006

Thermocouple Temperature Activates Relay  -  Voltage from a high temperature thermocouple activates a relay when the sensor detects a temperature above 1000 degrees F . . . Circuit by Dave Johnson P.E.-July 04, 2006

Three-Minute Closet Light  -  I have a coat closet by my front door.  The thing is very dark at night and really needs some kind of illumination inside.  I could hire an electrician to install a light in the closet ceiling and then wire it to a toggle switch outside the door but that would cost me quite a bit of money . . . Circuit by David A. Johnson P.E.-December 25, 2011

Through Glass Push Button Switch  -  This simple technique allows a mechanical pushbutton switch assembly to activate a reed type relay through a pane of thick glass.    Pushing the button moves a magnet near the reed relay, closing the relay contacts.   . . . Circuit by David A. Johnson P.E.-February 10, 2002

Time Garage Door to Close  -  The circuit below is designed to automatically close a garage door, if it was left open for a long period of time.  A reed relay and magnet combination sense when the door is fully open.  An ultra low power voltage comparator oscillator and two 74HC4040 ICs  are used to generate the time delay . . . Dave Johnson, Lubbock TX (May 14, 2011)-DC Magazine - Issue 18 May 2011

Tiny 5v Line Powered Isolated Supply  -  Often a circuit requires a 5v DC supply to power some logic circuits.  The conventional method is to use an AC wall adapter.  But, many systems, which bring AC power onto a circuit board, need a small AC to DC power supply right on the circuit board.  The . . . Circuit by David A. Johnson P.E.-July 15, 2009

Tiny Solar Panels Using Photodiodes  -  There are times when you would like to harvest just a little bit of energy from the sun.  Unfortunately, there are not many miniature solar panels available.  Most panels are 10 to 50 times larger than needed.   You can make your own and wire in series.  Just as in the larger panels, be sure to insert a small signal diode between the photodiode array and any battery or super capacitor being charged.  The diode will prevent current from leaking back into the array at night. . . . Dave Johnson, Lubbock TX-DC Magazine - Issue 20 Nov 2011

Tool Shed LED Light  -  I have a lean-to 8ft by 12ft shed next to my house.  It is full of lawn chemicals and garden supplies.  There were times when I needed to get something from inside the shed during the dead of night.  Early on I had to use a flashlight.  But later, I installed a series of five 12v LED light strips on the ceiling.  Each strip contained 15 3v white LEDs.  The strip had 5 strings of three LEDs, wired in series, with a 150 ohm current limiting resistor. . . . Circuit by David Johnson P.E.-July 14, 2017

Touch Activated Battery Powered Latching Relay  -  A client needed a touch activated switch.  The unit needed to be fairly small and wired into the underside of some kitchen cabinets.  When touched, a string of fluorescent or LED lights, mounted under the cabinets, would turn on.  The total light power is less than 100 watts.  The system into two parts.  (1) A relay is mounted inside a plastic junction box and (2) a touch switch module.    Four wires leading from plastic junction box would connect to a second box containing the touch switch module.  The circuit is a simple relay driver powered by a9v battery. . . . Dave Johnson, Lubbock TX-DC Magazine - Issue 20 Nov 2011

Touch Activated Switch  -  This circuit provides about one watt of non-isolated DC power for an automotive type 12v LED array lamp in addition to a standard incandescent table lamp.   . . . Circuit by David A. Johnson P.E.-January 18, 2002

TV /VCR Remote Range Extender Circuit  -  I often get frustrated by how poor some infrared remotes in many new electronic devices operate.  I used to be able to turn on the TV from 30 feet away.  Now, it seems that I have to get within 15 feet.  To address this DC Magazine - Issue, I designed the circuit shown below.  The circuit taps into the TV remote’s existing infrared LED and drives 8 more efficient devices.  The result is a communications range some 4 to 5 times longer.   . . . Circuit by Dave Johnson P.E.-October 05, 2005

Two Pushbutton 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 . . . Circuit by David Johnson P.E.-February 11, 2002

Two Transistor Oscillators  -  This page has two unusual two-transistor oscillators.  I set the component values for a low frequency application.  Both circuits draw only about 1 micro amp of current . . . Circuit by David Johnson P.E.-December 18, 2004

UL Ground Resistance Tester  -  UL requires that an electrical system using a metal chassis earth ground connection have a resistance of less than 0.1ohms.   This resistance must be tested with actual line current in excess or 10 amps.    The circuit shown uses a couple of LEDs to indicate . . . Circuit by Dave Johnson P.E.-April 28, 2001

Ultra Low Control Current Isolated Relay  -  The circuit uses a classic capacitor “voltage dropper” to capture a bit of power from the AC power line and store some energy in a capacitor.  The energy is then used to activate a latching relay.  The relay only needs short 20ms pulses to open or close the relay contacts.  So, the average power needed to hold the relay state is very low. . . . Circuit by David Johnson P.E.-September 23, 2017

Ultra Low Current 3v LED Flasher #3  -  This circuit takes advantage of a tiny LMC7215 low current draw voltage comparator from National Semiconductor and a super efficient green LED from King Bright.  The 3v powered circuit forms a classic pulse generator circuit, which drives the LED.  The pulse width is set at about 20ms, the pulse rate is about 0.  5Hz and the peak LED current is only 100ua . . . Circuit by David A. Johnson P.E.-January 28, 2011

Ultra Low Current Oscillator  -  Here is a challenge.  Design an astable oscillator which draws only a few nanoamps of current from a +3v supply.  I gave this some thought and came up with the circuit below.  I used some pretty standard parts except for three surface mounted 1000M resistors I had on hand.  The oscillator frequency measured a low 1Hz frequency and the average current was a very low 3 nanoamps.  If I had some higher resistors values handy, I think I could have gotten the current down below one nanoamp . . . Circuit by Dave Johnson P.E.-March 08, 2009

Ultra Low Current Oscillator #4  -  The circuit below was inspired by a programmable unjunction transistor (PUJT) circuit sent to me by Karl isbrecht.  In Karl’s circuit, the unijunction circuit made click sounds in a speaker and consumed very little power.  His circuit reminded me that a PU . . . Circuit by David Johnson P.E.-January 28, 2010

Ultra Low Power Astable Multivibrator  -  Taking advantage of some new 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 . . . Circuit by David A. Johnson P.E.-December 18, 2004

Ultra Low Power Human Finger Detector  -  The circuit below seems to work well and averages a current drain of only 10 microamps from a lithium cell.  It uses a single CA4013 dual D flip/flop.  One section is wired as a pulse generator.  It applies 10us 10ma pulses to a LED inside a slotted optical sensor at a rate of about 50Hz.  The phototransistor in the second half of the sensor detects the pulses and sends a pulse to the data input of the second flip/flop in the 4013 package.  If there is nothing between the LED and the phototransistor, the inverting output of the 4013 remains low.   . . . Circuit Solution by David A. Johnson P.E.-May 01, 2006

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 . . . Circuit Solution by Dave Johnson P.E.-April 29, 2007

Ultra Lower Power Oscillator  -  This circuit works much like the classic 555 timer, but draws only about 1.5 microamps from a 3 volt battery.  It is highly stable under varying temperature and supply voltages . . . Circuit by David A. Johnson P.E.-May 29, 2000

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% . . . Circuit by David Johnson P.E.-February 11, 2002

Under/Over Voltage Alarm on AC Line  -  Power lines, which deviate much beyond normal voltages can damage expensive electronic equipment.  The circuit below sounds an alarm whenever the line voltage is higher or lower than normal.  I set the alarm limits at about +-15% from standard levels . . . Circuit by David Johnson P.E.-March 29, 2009

Underground Gate Opener  -  Provide power to mechanical gates located opposite ends of a looping branch tunnel.  The gate was required to open and close perhaps only 2 times each month. The gates were set to open and then close again at ten minutes intervals.  Solution:  Use a wind generator.  . . . Dave Johnson, Lubbock TX-DC Magazine - Issue 15 Jan 2011

Universal LED Flasher  -  This low power LED flasher circuit draws about 100ua from any DC power supply ranging from 3 volts to 24 volts.  The 1.  0Hz 10ms LED pulsed current is held to a constant 10ma . . . Circuit by David A. Johnson P.E.-July 11, 2006

Universal-3v-LED-Flasher  -  The circuit below will flash any LED color with fixed current pulses powered by a 3v battery.  It uses a charge pump approach, which routes a constant current pulse through the LED, regardless of the LED voltage requirement.  Using this approach, white and . . . Circuit by David A. Johnson P.E.-June 22, 2008

Unplugged Appliance Alarm  -  I received a call from an administrator at a large prison.  He told me that he was having problems with inmates who keep unplugging refrigerators, vending machines and microwave ovens in a commons area.  He wanted a battery-powered device, which could be mount . . . Circuit by Dave Johnson P.E.-April 25, 2010

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 . . . Circuit by Dave Johnson P.E.-July 08, 2000

Up/Down Ramping Light Pushbutton Control  -  There are times when you want a lamp to gradually change from dark to light and from light to dark, with each push of a pushbutton switch.  The circuit below performs this control function using a flip/flop and a classic pulse width modulation or PWM . . . Circuit by David A. Johnson P.E.-March 09, 2009

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 . . . Circuit by Dave Johnson P.E.-July 08, 2000

VCO from CMOS Schmitt Trigger IC  -  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 . . . Circuit by David Johnson P.E.-December 01, 2002

Version a of 1.5v LED Flasher  -  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 . . . Circuit by Dave Johnson P.E.-June 03, 2000

Vertically Lifting Security Gate Power Source  -  The circuit that I came up with is shown below.  It uses 5 identical circuits.  Each one is wired in parallel with it assigned 650F super capacitor.  A LM385 provides a 1.  2v voltage reference.  A resistor divider measures the voltage across the capacitor and feeds the voltage to a low power, low voltage op amp.  The output of the op amp drives an n-channel FET.  I designed the circuit for a maximum charge current of 600ma.  This would allow the use of a 10 watt solar panel, if needed, in the future.  Each of the five circuits forms a shunt type voltage regulator, limiting the voltage across each capacitor to about 2.  6v.   . . . Dave Johnson, Lubbock TX  (10/13/2009)-DC Magazine - Issue 3 Nov 2009

Very Loud 3v Powered Beeper  -  Getting a high sound intensity from a piezoelectric type beeper is not easy when the available DC supply is only 3v.    The circuit below is not only efficient but produces a very intense sound.    The circuit combines a voltage boost section with a resonant  . . . Circuit by Dave Johnson P.E.-February 18, 2009

Very Loud Pulsating Beeper  -  Many battery powered alarms draw excessive current when operating.  Some of the most attention-getting noise makers, such as the unit shown below, use piezo sound generators with a frequency sweeping tone.  When operating with a 9v battery, these units can often draw 100ma or more.  In many applications, a continuous alarm sound is not needed . . . Circuit by David A. Johnson P.E.-October 23, 2010

Very Low Power Astable Multivibrator  -  This classic circuit draws only 200 nanoamps from a 1.5v supply . . . Circuit by Dave Johnson P.E.-June 10, 2000

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 . . . Circuit by David Johnson P.E.-June 03, 2000

Video Signal Edge Enhancements  -  I designed this circuit many years ago, based on the claims that the technique would improve the quality of standard TV images.  The circuit adds information to the edges of the objects and was reported to bring out more detail.  After building and testing . . . Circuit by David A. Johnson P.E.-July 08, 2000

Voltage Converters-0.5 to 6 Volts  -  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 . . . Circuit by Dave Johnson P.E.-May 21, 2007

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 . . . Circuit by David Johnson P.E.-March 22, 1999

Water Cistern Level Alarm  -  I received an unusual request from a Discover Circuits visitor a while back.    The guy had a deep water cistern which has to be filled when nearly empty.  The cistern was described as a deep cylindrical tube, holding 1000s of gallons of drinking water.    Periodically, the cistern is filled from a tanker truck, when the water inside reached a critical level.    The guy wanted a simple circuit he could build which would flash an LED when the water level reached a certain depth.    That flasher would tell him it was time to order more water . . . Circuit by David Johnson P.E.-April 27, 2014

Water Flowing in Pipe Indicator  -  The vibrations associated with water flowing through a pipe are picked up by an inexpensive piezoelectric wafer.    The signal from the wafer is first boosted by a micropower transistor amplifier and then fed to an ultra low power voltage comparator.     When the vibration signal has sufficient amplitude, a FET transistor switch is activated…[Circuit designed by Dave Johnson P.  E.  , 6/06/2009] . . . Circuit by David Johnson P.E.-July 26, 2009

Water Heater Flame Detector  -  Design a device that detected whether the gas flame of the heater was off or on - whether it was heating water or not -  and convey that information to the bathroom.  With a readout in the bathroom, one would know if there were enough hot water to take a shower. A battery powered circuit would be best for his situation.  The flame detector circuit could be connected to a battery powered LED flasher circuit.  The actual LED could be mounted next to the thermostat, using the same wire pathway as the furnace thermostat wires, located nearby.  A remote flashing LED would change color from green to red, whenever the flame was on. . . . Dave Johnson, Lubbock TX-DC Magazine - 3rd Q 2012

Water Seepage Monitor  -  A while back I received a request from a company who needed a water seepage detector.  They wanted a device that ran on 24vac and detected water using a fully isolated circuit.  They wanted a circuit which switched off 24vac to a load when water was detected . . . Circuit by David A. Johnson P.E.-January 16, 2011

Wave Power Generator Demo System  -  An inexpensive stepper motor is used as a voltage generator in this wave generator demo . . . Circuit by Dave Johnson P.E.-July 27, 2003

Well Pump Flasher & Beeper  -  Several years ago I installed a box on my water well pump head.  At the well head, the solar powered box flashed a very bright LED light and chirped a very load siren, whenever the pump motor was in operation.  After many years of service, the box finally became corroded from rain water and died.  So, I thought it was time to redesign the circuit and build up a replacement. . . . Circuit by David Johnson P.E.-June 29, 2017

White LED is Powered By One Nimh Cell  -  There are a lot of white LED driver circuits floating around.  A popular circuit called the “Joule Thief” can drive one white LED from a single 1.  2v or 1.5v battery cell.  Most of these circuits use one or two transistors to form a voltage boost circuit . . . Circuit by Dave Johnson P.E.-September 26, 2009

White LED Lifetime Tests  -  All white LEDs fade over time.  I find that some LEDs fade in intensity very rapidly while others fade more slowly but every one I have tested would be worthless in a few months to a year.   To speed up the testing process I have modified my constant current LED driver circuit as shown below. 

 . . . Dave Johnson, Lubbock TX-DC Magazine - Issue 7 Mar 2010

White LEDs powered by 24V  -  24vac is a popular voltage.  It is often used in low voltage lighting systems and industrial controls.  It is much less dangerous than 120vac/240vac line voltages.  The circuit below is one method to efficiently power a cluster of 7 white LEDs from 24vac . . . Circuit by David A. Johnson P.E.-December 19, 2009

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 150 . . . Circuit by Dave Johnson P.E.-July 18, 2000

Wig/Wag LED Flasher  -  This simple circuit will flash two LEDs in an alternating fashion . . . Circuit by Dave Johnson P.E.-December 18, 2004

Wireless LED Flasher  -  There are times when you would like to flash a LED through a thick plastic case without wires.  The circuit below performs this feat.  It can be powered from a supply ranging from 3v to 6v.  It draws about 300ua of average current at 5v . . . Circuit by David A. Johnson P.E.-January 09, 2011

X100 Scope Probe  -  A 10M load a typical X10 oscilloscope probe will produce large measurement errors and will influence the circuit operation.  To reduce the effect such a probe has on the circuit, I reach for my X100 scope probe.  My probe, with its 100M impedance, is a commercially made device but if you want to make one, it is not hard to do. . . . Dave Johnson, Lubbock TX-DC Magazine - Issue 9 May 2010

Xenon Lamp Flash Detector Amplifier  -  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 . . . Circuit by Dave Johnson P.E.-December 14, 2004

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 . . . Circuit by Dave Johnson P.E.-June 03, 2000


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