120 VAC / 240 VAC Indicator  -  Two LEDs indicate if a power line has 120vac or 240vac voltage on it . . . [Hobby Circuit designed by Dave Johnson P.E.,  07/09/06]

120 VAC / 240 VAC Indicator  -  Two LEDs indicate if a power line has 120vac or 240vac voltage on it . . . [Hobby Circuit designed by David Johnson P.E.,  07/09/06]

120 VAC Low Voltage Alarm  -  This circuit will activate a beeper alarm whenever the AC line voltage is less than 100vac. . . . [Circuit by David Johnson P.E.,  07/11/06]

120VAC Under Voltage Tester  -  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 David Johnson P.E.,  07/12/06]

12v Battery Voltage Monitor-Expanded Scale   -  The circuit below expands the battery voltage monitoring scale for a 12v battery so a more accurate voltage reading can be made.  The numbers printed onto the face of a standard 0v to 5v meter are changed so the scale ranges from 10v to 15v . . . [Hobby Circuit designed by Dave Johnson P.E.,  10/16/05]

12v Cooling Fan Thermostat  -  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 . . . [Circuit by Dave Johnson P.E.,  02/06/11]

12v Lamp Current Indicator  -  This circuit turns on a LED indictor light whenever DC current flows into a 12v lamp . . . [Circuit by David A. Johnson P.E.,  07/09/06]

12v Touch Switch Exciter Circuit  -  This circuit is designed to generate a 20KHz pseudo sine wave signal that can power about 50 remote touch activated switch circuits.  It can support a cable length of about 2500 feet.  A typical remote switch circuit is also shown as well as a receiver circuit for those switches. . . . [Circuit by David Johnson P.E.,  02/10/02]

1Khz to Over 70Mhz Light Receiver  -  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 . . . [Hobby Circuit designed by David A. Johnson P.E.,  06/11/00]

1-Nimh Cell powers White LED  -  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 . . . [Hobby Circuit designed by David Johnson P.E.,  09/26/09]

20ma Current Loop Tester  -  A while back I needed to test a circuit at 240vac.  Since I needed to safely test the circuit using my oscilloscope, I needed the 240vac to be fully isolated from the power line.  So, to step up the voltage, I bought a small isolation transformer from Digikey . . . [Circuit by Dave Johnson P.E.,  05/29/11]

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 . . . [Circuit by Dave Johnson P.E.,  06/17/00]

3 Volt Low Battery Voltage Flasher  -  This circuit is designed to monitor two alkaline cells (3v) that form the battery often used in portable electronic equipment . . . [Circuit by Dave Johnson P.E.,  01/01/98]

36-Hour Watchdog Timer  -  A watchdog timer is often used in computer systems to detect when a computer is hung up and is no longer running properly. If it does become locked up and is not able to supply a reset pulse to the circuit within a defined time frame the timer times out a . . . [Circuit by Dave Johnson P.E.,  04/11/10]

3mw Laser Test-20Mhz VCSEL  -  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 . . . [Hobby Circuit designed by Dave Johnson P.E.,  06/17/00]

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 . . . [Circuit by David A. Johnson P.E.,  02/12/02]

4 to 20ma Current Loop Senses Temperature  -  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 . . . [Hobby Circuit designed by Dave Johnson P.E.,  07/04/00]

4 to 20ma Current Loop Senses Temperature  -  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 . . . [Hobby Circuit designed by David Johnson P.E.,  07/04/00]

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, . . . [Circuit by Dave Johnson P.E.,  06/13/00]

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 . . . [Circuit by David A. Johnson P.E.,  02/14/02]

40KHz Light Receiver Front-End Circuit  -  This circuit uses a unique cascode 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. This hobby 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. . . . [Circuit by David Johnson P.E.,  06/14/00]

6v NMH/NiCd Battery Tester  -  I designed this circuit to test rechargeable six volt battery packs under constant current conditions.  As designed, the circuit applies a 10 amp load to the battery pack.  A heat sink must be used on the main power transistor . . . [Circuit by David Johnson P.E.,  12/12/04]

9v Battery Voltage Monitor  -  This circuit turns on a LED whenever the voltage of a standard 9v battery connected to the circuit drops below 7.2 volts.  It uses a LTC1440 comparator, which also contains a 1.18v reference diode.  In standby mode, the circuit draws only 4uA . . . [Circuit by David A. Johnson P.E.,  07/09/06]

9v Battery Voltage Monitor  -  This circuit turns on a LED whenever the voltage of a standard 9v battery connected to the circuit drops below 7.2 volts.  It uses a LTC1440 comparator, which also contains a 1.18v reference diode.  In standby mode, the circuit draws only 4uA . . . [Circuit by David Johnson P.E.,  07/09/06]

A Current Transformer used to Harvest Energy  -  Energy harvesting is all the rage these days.  With some modern electronics, information from low power sensors can be sent to a distant data collection point using a low power RF transmitter.  To power these remote sensors, various energy sources can be . . . [Hobby Circuit designed by David A. Johnson P.E.,  01/17/10]

AC Current Controls 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 . . . [Circuit by David Johnson P.E.,  02/13/10]

AC Current Controls 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 . . . [Circuit by David A. Johnson P.E.,  02/13/10]

AC Current Indicator Light  -  This circuit could be wired into a 120vac power line, which feeds power to any load, ranging from 40 watts to 250 watts.  It will turn on a LED light whenever current is being drawn by a load.  It is especially useful for remote lights, where you may not . . . [Hobby Circuit designed by Dave Johnson P.E.,  10/16/05]

AC Current Monitor  -  This circuit uses a small AC current transformer from Magnetek to produce an isolated voltage proportional to the AC current in the primary winding.  The transformer contains a single turn primary with a low 0.001-ohm resistance.  It can easily handle 30 . . . [Hobby Circuit designed by David Johnson P.E.,  07/10/06]

AC Isolated Current Monitor  -  The hobby circuit below shows how a small current transformer from Magnetek can be used to monitor AC current to a load.  The AC voltage developed at the secondary of the transformer can be scaled so it can be read by any digital multimeter.  The transformer . . . [Hobby Circuit designed by Dave Johnson P.E.,  01/20/01]

AC Line Under/Over Voltage Alarm  -  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 . . . [Hobby Circuit designed by David Johnson P.E.,  03/29/09]

AC Peak Detector that is Precise  -  This unique circuit uses a very inexpensive voltage comparator to form a peak detector. The DC voltage produced tracks the positive peak of the input signal.  It works from about ten millivolts to about 10 volts peak to peak.  The maximum frequency is about . . . [Hobby Circuit designed by Dave Johnson P.E.,  02/11/02]

AC Power Line High/Low Voltage Tester  -  If you wish to test a line-powered device under both a 15% high and a 15% low voltage condition, you can use the circuit below.  The circuit uses an 18v transformer with a 3 amp rating.  A double pole, double throw toggle switch then switches in the . . . [Circuit by David A. Johnson P.E.,  03/14/10]

Acceptable Voltage Indictor  -  I have used this circuit many times in custom test fixtures where a simple go-no go indication was needed.  The circuit can also be used to adjust a particular voltage be within specific high or low limits.  The three LEDs will indicate if the voltage is . . . [Circuit by Dave Johnson P.E.,  02/09/02]

Accurate Low Capacitance Meter  -  The circuit measures the unknown capacitance by applying a clean 20KHz 5v peak to peak triangle signal to the capacitor.  The current routed through the capacitance is sent to a current to voltage converter. The voltage that emerges is therefore a 20KHz square wave signal, whose amplitude is proportional to the capacitance.  The signal’s peak amplitude is rectified and filtered to produce a DC voltage. . . . [Hobby Circuit designed by David A. Johnson P.E.,  12/12/04]

Accurate One Shot Pulse Generator  -  This timer circuit draws only 3ua of current, and can be powered by DC supplies ranging from 3v to 15v.  The circuit produces accurate pulses whose time can easily be adjusted, based on the R1 x C1 time constant.  The time should not change, . . . [Circuit by David A. Johnson P.E.,  09/02/10]

Advertising Badge with Flashing LED #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 . . . [Circuit by Dave Johnson P.E.,  03/26/02]

Advertising Badge with Flashing LED #2  -  This circuit uses a CD4013 dual D Flip/Flop IC.  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 . . . [Circuit by David Johnson P.E.,  03/26/02]

Alarm Discriminator + Battery Monitor of the Electric Field Disturbance Monitor  -  This schematic is the alarm discriminator plus the batterry monitor of my "Electric Field Disturbance Monitor".  The system can detect human and animal motion by the electric fields they disturb . . . [Hobby Circuit designed by David Johnson P.E.,  03/26/02]

Alarm Discriminator + Battery Monitor of the Electric Field Disturbance Monitor  -  This schematic is the alarm discriminator plus the batterry monitor of my "Electric Field Disturbance Monitor".  The system can detect human and animal motion by the electric fields they disturb . . . [Hobby Circuit designed by David A. Johnson P.E.,  03/26/02]

Alarm Sounds At Freezing Temperatures  -  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 . . . [Hobby Circuit designed by David Johnson P.E.,  07/11/06]

Alarm sounds when Power Cord is Unplugged  -  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 . . . [Hobby Circuit designed by David Johnson P.E.,  07/08/00]

Alarm sounds when Water is Detected  -  This simple circuit sounds a beeper when its electrodes detect water. It is powered by a single 1.5v N cell. A small 1.5v button battery will also work . . . [Circuit by David A. Johnson P.E.,  06/11/00]

Alarm-Piezo Device detects Motion  -  Using a piezo device, this circuit will activate a beeper whenever the circuit is moved.  It could be used as an earthquake alarm . . . [Circuit by Dave Johnson P.E.,  07/09/06]

Alkaline Low Battery Voltage Indicator  -  In many battery powered systems a user would like to know when it was time to replace the batteries  before they were completely discharged, causing the device being powered to fail completely.  Alkaline batteries have an open cell voltage of about 1.5 volts.  As they are discharged, the voltage slowly drops.  When the cell voltage reaches about 1.25v, they have delivered about 90% of their stored energy . . . [Circuit by David Johnson P.E.,  06/17/12]

Alternating CW & CCW 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 . . . [Circuit by David Johnson P.E.,  09/05/10]

Audio Activated Switch  -  This circuit will activate a transistor switch when it detects at least 50mv peak to peak of an audio signal.  It could be used to turn on a relay, routing the signal to were it is needed. . . . [Circuit by David A. Johnson P.E.,  07/09/06]

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 . . . [Circuit by David A. Johnson P.E.,  07/04/00]

Automatic Timeout included with this On/Off Flip/flop Circuit  -  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 . . . [Hobby Circuit designed by David A. Johnson P.E.,  05/01/12]