Battery-Powered Freezer Alarm  -  A typical freezer can contain hundreds of dollars with of food.  If the device becomes unplugged or if the door is left open too long, the food inside can quickly become spoiled.  The hobby circuit below is a battery powered alarm system, which is attached . . . [Hobby Circuit designed by David A. Johnson P.E.,  07/06/06]

Battery-Powered LED Flashing Advertising #2  -  This circuit is similar to flashing LED advertising badge #1.  It uses a CD4013 dual D Flip/Flop IC.  The 74HCT74 IC in #81 does not always work.  As in #81, a single lithium battery will provide months of continuous LED flashing. . . . [Hobby Circuit designed by David Johnson P.E.,  03/26/02]

Bidirectional DC Current Indicator  -  This circuit is handy for battery powered systems.  The circuit is self-powered using a single 3v lithium coin cell.  The current draw is so low that the battery should last for years.  The hobby circuit flashes a red LED when DC current is drawn from the battery and flashes a green LED whenever current flows into the battery.  Two “probe” input wires connect across a section of cable, which acts as a shunt resistor.  A voltage of only 5mv in either direction will trigger a LED indication . . . [Hobby Circuit designed by David A. Johnson P.E.,  03/31/11]

Brownout Voltage Tester  -  I used this circuit years ago to test AC line powered devices under 95vac line voltage conditions.  It has a rating of 250 watts.  . . . [Hobby Circuit designed by David A. Johnson P.E.,  07/12/06]

Button Battery Cell Internal Resistance Measurement Circuit  -  The open cell voltage of a small silver oxide button cell battery remains very close to 1.5 volts over most of the life of the cell.  However, there is a relationship between battery health and the internal resistance of the cell. This circuit when used . . . [Circuit by David A. Johnson P.E.,  07/11/06]

Capacitance Proximity Switch-3 volts  -  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 . . . [Hobby Circuit designed by David A. Johnson P.E.,  02/12/02]

Capacitance Touch 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 . . . [Hobby Circuit designed by David Johnson P.E.,  01/18/02]

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 . . . [Hobby Circuit designed by David Johnson P.E.,  06/13/00]

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 . . . [Circuit by David A. Johnson P.E.,  01/01/98]

Characteristics of CMOS Logic Inverter Amplifiers  -  This chart shows some of the measured characteristics of three different unbuffered CMOS logic gates, used as voltage amplifiers . . . [Hobby Circuit designed by David A. Johnson P.E.,  02/11/02]

Cheap Light Power Meter  -  Some time ago I was experimenting with the use of some low leakage photovoltaic solar cells, to harvest some energy from the diffuse lighting inside a room.  The plan was to use a small array of solar cells (the type often used on solar powered calculators) . . . [Hobby Circuit designed by Dave Johnson P.E.,  10/18/06]

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 . . . [Circuit by David Johnson P.E.,  07/06/00]

Circuit Indicates Battery Charge Current  -  This circuit was inspired by a company needing a DC current indicator light which was small enough to be molded into a medium size 28v rechargeable battery assembly.  They specified that they wanted the circuit to turn on the LED indicator light with 500ma or more of battery charge current . . . [Hobby Circuit designed by David A. Johnson P.E.,  01/23/11]

Circuit indicates Battery Charging Current  -  Anyone who has ever had a shocking experience on an especially dry day is acutely aware of the kinds of high voltage that can be generated from a simple walk across a carpeted floor. Those same finger to metal sparks can destroy or disrupt sensitive elect . . . [Hobby Circuit designed by Dave Johnson P.E.,  01/23/11]

Circuit monitors 120 Power Line  -  This hobby electronic circuit produces a short beep once each minute. It is powered directly off the 120vac power line to remind you that a device connected to the circuit is turned on. It is simple enough to be packaged inside a small plastic box . . . [Hobby Circuit designed by Dave Johnson P.E.,  06/03/00]

Circuit Monitors 12v Lamp Current   -  This circuit monitors DC current and turns on a LED indictor light whenever DC current flows into a 12v lamp . . . [Hobby Circuit designed by Dave Johnson P.E.,  07/09/06]

Circuit Monitors Power Loss  -  For some medical equipment it is important for an operator to know when power is lost to the machine. The beeper is powered from a 9v battery and requires the machine to have a power switch with a third set of contacts . . . [Hobby Circuit designed by David Johnson P.E.,  03/26/99]

Circuit tests the self-resonance frequency  -  This circuit tests the self-resonance frequency is of an inductor. During the test, only a few picofarads of parallel capacitance are added to the inductor.  When connected to an oscilloscope and a signal generator, you can measure resonant frequencies . . . [Hobby Circuit designed by Dave Johnson P.E.,  06/09/00]

Clear Fluid Detector #2  -  Using an ultra low power technique, this circuit detects when a clear fluid is inside a piece of clear plastic tubing. It can be used for fluid level control and monitoring . . . [Circuit by David Johnson P.E.,  10/22/05]

Clear Fluid Monitor  -  An optical technique seemed like the best choice for this application.  The simple circuit below works well but only when the light detector can be shielded from bright ambient light.  A visible red LED launches light pulses through the side of the clear tubing and emerges at the opposite side.  A phototransistor forms a light detector.  The circuit takes advantage of the lens effect from any clear fluid inside the tubing.  When there is only air in the tubing, the light scatters and spreads out quickly before it reaches the phototransistor . . . [Circuit Solution by David A. Johnson P.E.,  05/01/05]

Clip-on AC Current Indicator Circuit  -  Several times over the last few years I have received calls from people looking for a simple circuit that would tell them when a sump pump or water well pump motor was running.  These people told me that the electrical circuits that turn on and off . . . [Hobby Circuit designed by David Johnson P.E.,  11/18/04]

Clip-on AC Current Indicator Circuit  -  Several times over the last few years I have received calls from people looking for a simple circuit that would tell them when a sump pump or water well pump motor was running.  These people told me that the electrical circuits that turn on and off . . . [Hobby Circuit designed by Dave Johnson P.E.,  11/18/04]

CMOS Logic Inverter Amplifier Characteristics  -  This chart shows some of the measured characteristics of three different unbuffered CMOS logic gates, used as voltage amplifiers . . . [Circuit by David A. Johnson P.E.,  02/11/02]

Control both Power & Direction of a DC Motor  -  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 . . . [Hobby Circuit designed by David Johnson P.E.,  02/04/07]

Control Variable LED Intensity  -  The circuit below uses power from four rechargeable AA NiMH cells to drive 3 white LEDs.  A potentiometer varies the duty cycle of a pulse width modulator circuit to vary the intensity of the LEDs from 0% to 100%.  The beauty of the circuit is that when the . . . [Hobby Circuit designed by David A. Johnson P.E.,  09/27/09]

Convert AC to DC signal with is Full Wave Rectifier  -  This is a classic circuit that can accurately convert an AC signal to DC.  Using a LF412 high speed dual op amp, this circuit will rectify any signal up to about 100KHz with a minimum amplitude of about 50mv peak to peak. At 40KHz the input signal can be . . . [Hobby Circuit designed by David Johnson P.E.,  07/11/06]

Cooling Fan Thermostat (12v)  -  This circuit will turn on a 12v DC powered cooling fan when the air temperature reaches a certain high temperature and will keep the fan turned on until the temperature drops below a second lower level.  Both the high and low temperatures are both adjustable . . . [Hobby Circuit designed by David A. Johnson P.E.,  02/06/11]

Corona Monitor  -  The circuit is powered by +5v and has an on-board voltage regulator as a reference voltage.  The current from a reversed biased photodiode is converted into a voltage using an op amp circuit.  That output voltage is fed to a voltage comparator circuit.  Included is an adjustment in the current to voltage converter, to calibrate the installed circuit . . . [Circuit by David Johnson P.E.,  02/01/10]

CT Hour Meter  -  Many systems require routine maintenance based on machine operation time.  The circuit below is a simple way to turn on a hour meter whenever AC power is supplied to the machine.  An inexpensive snap-on current transformer from Magnetics Inc, is used to . . . [Hobby Circuit designed by David A. Johnson P.E.,  02/13/10]

CT Hour Meter  -  Many systems require routine maintenance based on machine operation time.  The circuit below is a simple way to turn on a hour meter whenever AC power is supplied to the machine.  An inexpensive snap-on current transformer from Magnetics Inc, is used to . . . [Hobby Circuit designed by David Johnson P.E.,  02/13/10]

Current Indicator for Battery Chargers  -  This circuit was inspired by a company needing a DC current indicator light which was small enough to be molded into a medium size 28v rechargeable battery assembly.  They specified that they wanted the circuit to turn on the LED indicator light with 500ma or more of battery charge current . . . [Circuit by Dave Johnson P.E.,  01/23/11]

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 - modified on July 11, 2006  . . . [Circuit by David A. Johnson P.E.,  06/04/00]

CW & CCW Alternating Motor Driver  -  A while back a company needed a motor controller for a custom door latching system.  The system used a small brush motor with a speed reduction gear to both latch and unlatch the door.  When pulsed for about 250ms in the clockwise direction, the motor would unlatch the door . . . [Hobby Circuit designed by Dave Johnson P.E.,  09/05/10]

Darkroom Camera Shutter Timer  -  This circuit was designed to control a film exposure shutter for a darkroom. It has 8 time steps ranging from 0.35 seconds to 4 seconds. It is activated by a foot switch and draws power from an external 12-volt DC supply . . . [Circuit by David A. Johnson P.E.,  06/11/00]

DC Brushless Motor Monitor  -  The circuit below was requested by a person needing a way to keep track of the number of hours a 24v DC brushless motor was in operation.  The circuit taps into a bit of the power going into the motor using a bridge rectifier and filter capacitor.  The voltage produced feeds the input of a small solid state relay.  The output of the relay routes AC power to an hour meter, every time the brushless motor is turned on.  This same circuit could be used with other DC motors with some simple modifications . . . [Hobby Circuit designed by Dave Johnson P.E.,  04/01/05]

DC Current Bidirectional Indicator  -  This circuit is handy for battery powered systems.  The circuit is self-powered using a single 3v lithium coin cell.  The current draw is so low that the battery should last for years.  The hobby circuit flashes a red LED when DC current is drawn from the battery and flashes a green LED whenever current flows into the battery.  Two “probe” input wires connect across a section of cable, which acts as a shunt resistor.  A voltage of only 5mv in either direction will trigger a LED indication . . . [Hobby Circuit designed by David Johnson P.E.,  03/31/11]

Delayed Pulse Generator  -  This circuit generates a short 10ms pulse 15 minutes after a “start” pushbutton switch is activated . . . [Circuit by David A. Johnson P.E.,  10/16/05]

Detect Clear Fluid  -  This circuit detects when a clear fluid is inside a piece of clear plastic tubing. It can be used for fluid level control and monitoring . . . [Hobby Circuit designed by Dave Johnson P.E.,  05/01/12]

Detect Motion with a Piezo Device  -  Using a piezoelectric device, this circuit will activate a beeper whenever the circuit is moved.  It could be used as an earthquake alarm.  . . . [Hobby Circuit designed by Dave Johnson P.E.,  07/14/06]

Detector for Human Footsteps  -  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 . . . [Hobby Circuit designed by David Johnson P.E.,  07/11/06]

Detector/Switch for Audio Signal  -  For some audio systems it is desirable to have an automatic switch circuit turn on other devices, such as an amplifier, if an audio signal is detected on one line.  The hobby circuit below will activate a transistor switch when it detects at least 50mv peak to peak of an audio signal.   The switch could be used to turn on a relay, routing the signal to where it is needed.  A single 3v lithium cell will power the electronic circuit for about 5 years . . . [Hobby Circuit designed by David A. Johnson P.E.,  07/09/06]

Detects Low Voltage 120Vac Power Line   -  This hobby circuit will turn on a beeper when the line voltage drops below 100 volt AC . . . [Hobby Circuit designed by Dave Johnson P.E.,  07/09/06]

Detects Shadow Activated Motion  -  This circuit can sound an alarm when the shadow of a hand or arm moves over  two small photo diodes . . . [Circuit by Dave Johnson P.E.,  08/21/05]

D-Flip/Flop One Shot Circuit  -  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 . . . [Circuit by David A. Johnson P.E.,  06/06/00]

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 . . . [Hobby Circuit designed by Dave Johnson P.E.,  07/04/00]

Divide a TTL Clock Signal by 1.5  -  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 . . . [Hobby Circuit designed by David Johnson P.E.,  07/06/00]

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 . . . [Circuit by David A. Johnson P.E.,  07/06/00]

Does Power Line have 120 VAC or 240 VAC?  -  Two LEDs indicate if a power line has 120vac or 240vac voltage on it . . . [Circuit by Dave Johnson P.E.,  07/09/06]

Does Power Line have 120 VAC or 240 VAC?  -  Two LEDs indicate if a power line has 120vac or 240vac voltage on it . . . [Circuit by Dave Johnson P.E.,  07/09/06]

Doggie Water Dispenser Alarm  -  I have two Beagle dogs.  They drink a lot of water; they go through about 3 gallons a week.  They have two water dispensers.  One is on the inside and second unit on the outside of my house.  Often, I forget to fill one of their two jugs and discover them completely empty.  I do worry that my dogs might get dehydrated if I ever forget to fill both of their containers . . . [Circuit by David Johnson P.E.,  06/03/12]

Doggie Water Dispenser Alarm  -  I have two Beagle dogs.  They drink a lot of water; they go through about 3 gallons a week.  They have two water dispensers.  One is on the inside and second unit on the outside of my house.  Often, I forget to fill one of their two jugs and discover them completely empty.  I do worry that my dogs might get dehydrated if I ever forget to fill both of their containers . . . [Circuit by David A. Johnson P.E.,  06/04/12]

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 . . . [Hobby Circuit designed by David Johnson P.E.,  04/01/05]