Electrical Current Indicator  -  I designed this circuit as a simple current indicator for any load ranging from 40 watts to 250 watts.  The circuit turns on a small LED, whenever it detects current flowing to a remote load . . . [Circuit by David A. Johnson P.E.,  10/16/05]

Electrodeless Water Level Monitor  -  This is an illustration how a laser could be used to count traffic and measure the speed of each car passing through the sensor area . . . [Circuit by Dave Johnson P.E.,  07/09/06]

Energy Harvesting Using a Current Transformer  -  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 . . . [Circuit by Dave Johnson P.E.,  01/17/10]

Expanded Scale 12v Battery Voltage Monitor  -  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. http://archil . . . [Circuit by David Johnson P.E.,  10/16/05]

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 . . . [Hobby Circuit designed by Dave Johnson P.E.,  03/26/02]

Fluid 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 . . . [Circuit by David A. Johnson P.E.,  06/13/00]

Footstep Alarm for Human Traffic  -  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 . . . [Hobby Circuit designed by David A. Johnson P.E.,  07/11/06]

Freeze Alarm  -  I chose an accurate thermistor from Keystone as the temperature sensor for this circuit.  This device has a very specific resistance at a particular temperature.  According to the data sheet for the thermistor, the device will have a resistance of 361K at zero degrees C, which is the freezing point of water.  The thermistor is wired into a simple bridge circuit, which uses 1% resistors. The bridge is connected to a very low power voltage comparator.  The circuit forms a switch circuit, which changes state, when the resistance of the thermistor reaches a certain value.  At zero degrees C the voltage at the non-inverting input of the comparator exceeds . . . [Circuit Solution by David Johnson P.E.,  05/07/06]

Freezer Over-Temperature Alarm  -  This circuit turns on a beeper whenever the inside temperature of a freezer is greater then zero degrees Centigrade.  The circuit draws only a few microamps from a 9 volt battery.  It uses a glass bead thermistor accurate to 1 degree C. . . . [Circuit by David A. Johnson P.E.,  11/05/14]

Freezing Temperature Alarm  -  The 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 circuit.  The average 9ua current is so low that the battery should last for many years. . . . [Circuit by Dave Johnson P.E.,  11/05/14]

Front End + Power Supply Circuits of the Electric Filed Disturbance Monitor  -  This schematic is the power supply and front-end sections of my field monitor, "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]

Full Wave Rectifier  -  This is a classic circuit that can accurately convert an AC signal to DC.  At 40KHz the input signal can be as low as 0.05 volts peak to peak. . . . [Circuit by David A. Johnson P.E.,  07/11/06]

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

Ground Resistance Tester  -  UL requires that an electrical system using a metal chassis earth ground connection have a resistance of less than 0.1 ohms.  This resistance must be tested with actual line current in excess or 10 amps.  The circuit shown uses a couple of LEDs to indicat . . . [Hobby Circuit designed by Dave Johnson P.E.,  04/28/01]

H Bridge Motor Controller uses Low Power  -  This circuit uses a few logic gates & a 74AC240 to control power to a small gear motor . . . [Hobby Circuit designed by Dave Johnson P.E.,  07/03/05]

High Speed Light Detector from FET Input  -  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 . . . [Hobby Circuit designed by David Johnson P.E.,  06/17/00]

High/Low Voltage Tester on AC Power Line   -  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 . . . [Hobby Circuit designed by David Johnson P.E.,  03/14/10]

High-speed 75MHz 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 . . . [Circuit by Dave Johnson P.E.,  06/11/00]

Home Made Geophone Detects Foot Steps  -  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. . . . [Hobby Circuit designed by David Johnson P.E.,  06/02/06]

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 . . . [Circuit by David A. Johnson P.E.,  07/08/00]

Hour Meter triggered by Machine Vibration  -  This hobby circuit allows an off-the-shelf battery powered digital hour meter to be turned on and off, according to a machine's operation, without requiring a direct electrical connection to the machine . . . [Hobby Circuit designed by Dave Johnson P.E.,  01/01/98]

Hour Meter triggered by Machine Vibration  -  This hobby circuit allows an off-the-shelf battery powered digital hour meter to be turned on and off, according to a machine's operation, without requiring a direct electrical connection to the machine . . . [Hobby Circuit designed by David Johnson P.E.,  01/01/98]

Improved Solar Path Light  -  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 fills . . . [Hobby Circuit designed by David Johnson P.E.,  03/13/11]

Indicate Electrical Current with this Circuit  -  I designed this circuit as a simple current indicator for any load ranging from 40 watts to 250 watts.  The circuit turns on a small LED, whenever it detects current flowing to a remote load . . . [Hobby Circuit designed by Dave Johnson P.E.,  10/16/05]

Indicates 120 VAC or 240 VAC  -  Two LEDs indicate if a power line has 120vac or 240vac voltage on it . . . [Hobby Circuit designed by David A. Johnson P.E.,  01/07/07]

Indicates 120 VAC or 240 VAC  -  Two LEDs indicate if a power line has 120vac or 240vac voltage on it . . . [Hobby Circuit designed by David A. Johnson P.E.,  01/07/07]

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

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

Indicates when it is Time to Dust  -  I have used the circuit below for many different applications, which require ultra low power drain.  It uses a single CA4013 flip/flop.  One section forms a pulse generator.  The generator produces 100uS pulses at a rate of about 1 per second . . . [Circuit Solution by David Johnson P.E.,  05/07/06]

Indicator for Acceptable Voltage  -  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 . . . [Hobby Circuit designed by Dave Johnson P.E.,  02/09/02]

Indicator for Bad Light Bulb  -  This request came from a Discover Circuits visitor.  He wanted to turn on a LED indicator light whenever a circuit detected a burned out tail/brake light on a car or truck.  The circuit shown below uses a transistor switch, to detect current flowing through a low value resistor . . . [Hobby Circuit designed by Dave Johnson P.E.,  12/17/06]

Inductor Self-Resonance Tester  -  If you want to know what the self-resonance frequency is of an inductor, this circuit will help. It adds only a few picofarads of parallel capacitance to the inductor under test. With the aid of an oscilloscope and a signal generator this circuit will allow you to measure resonant frequencies from kilohertz to over 10 megahertz . . . [Circuit by David Johnson P.E.,  06/09/00]

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

Isolation Transformer Circuits  -  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 David A. Johnson P.E.,  05/29/11]

It's 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 . . . [Hobby Circuit designed by David Johnson P.E.,  06/06/00]