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

Lead-Acid Battery Low Voltage Indicator  -  Lead-acid batteries should not be discharged below a certain voltage.  Discharging a cell below about 1.8v and can often damage it, which will result in a lower amp-hour capacity.  Large battery banks are critical in off-grid energy storage systems.  One . . . [Circuit by David A. Johnson P.E.,  07/30/06]

LED Flashing 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 . . . [Circuit by David Johnson P.E.,  03/26/02]

LED Flashing Badge #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 A. Johnson P.E.,  03/26/02]

LED Test Signal Generator-40Khz  -  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 . . . [Hobby Circuit designed by David Johnson P.E.,  02/14/02]

LED/Laser 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 b . . . [Hobby Circuit designed by David Johnson P.E.,  07/08/00]

Light Detector  -  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. . . . [Hobby Circuit designed by David Johnson P.E.,  06/14/00]

Light Intensity Monitor  -  For an on-going consulting project, I had to determine the light output decay time of different fluorescent lamps. I wanted to know how fast the light from a lamp dropped to a low level, after current was interrupted in the lamp. I used a high voltage . . . [Hobby Circuit designed by Dave Johnson P.E.,  04/01/05]

Light Monitor Circuit  -  The circuit below takes advantage of a low power op amp with a “rail to rail” feature.  The single resistor R1 defines the light sensitivity of the circuit.  The output voltage is directly proportional to the intensity of light striking the small photodiode . . . [Hobby Circuit designed by David Johnson P.E.,  08/29/10]

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 . . . [Circuit by David Johnson P.E.,  06/11/00]

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

Lithium Battery Tester  -  I bought some lithium ion rechargeable batteries a while back. I was planning on using them to modify some solar powered LED spot lights.  What I like about these new batteries is that they are the same size as a standard 1.5v alkaline AA cell . . . [Hobby Circuit designed by David A. Johnson P.E.,  02/05/12]

Locate a Plastic Pipe  -  I used this circuit once to track down the location of a plastic pipe, buried underground.  It transmits a weak RF signal, which can be picked up by a portable AM radio from about 6 feet away . . . [Hobby Circuit designed by David A. Johnson P.E.,  12/14/04]

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

Low Power Amp Monitors Light  -  The circuit below takes advantage of a low power op amp with a “rail to rail” feature.  The single resistor R1 defines the light sensitivity of the circuit.  The output voltage is directly proportional to the intensity of light striking the small photodiode . . . [Circuit by Dave Johnson P.E.,  08/29/10]

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

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 . . . [Circuit by David A. Johnson P.E.,  12/12/04]

Low Voltage 120vac Power Line Alarm  -  This circuit will turn on a beeper when the line voltage drops below 100 volt AC. . . . [Circuit by David Johnson P.E.,  07/09/06]

Low Voltage H-Bridge  -  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 . . . [Circuit by David A. Johnson P.E.,  02/04/07]

Low Voltage Indicator for Lead-Acid Battery  -  Lead-acid batteries should not be discharged below a certain voltage.  Discharging a cell below about 1.8v and can often damage it, which will result in a lower amp-hour capacity.  Large battery banks are critical in off-grid energy storage systems.  One . . . [Hobby Circuit designed by David A. Johnson P.E.,  07/30/06]

Machine Vibration Triggers Hour Meter  -  This 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. Machine vibrations are detected by an off-the-shelf piezo . . . [Circuit by David A. Johnson P.E.,  01/01/98]

Machine Vibration Triggers Hour Meter  -  This 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. Machine vibrations are detected by an off-the-shelf piezo . . . [Circuit by David A. Johnson P.E.,  01/01/98]

Measure Button Battery Cell's Internal Resistance   -  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 . . . [Hobby Circuit designed by David A. Johnson P.E.,  07/11/06]

Micro Power Over-Temperature Alarm  -  The circuit is powered for years by a single 3 volt lithium battery. It sounds an alarm when the temperature exceeds a certain point. With some minor changes the circuit could also be configured for an under temperature (freeze) alarm. The circuit uses a . . . [Circuit by David Johnson P.E.,  06/04/00]

Miniature Pushbutton On/Off Switch  -  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 . . . [Hobby Circuit designed by David Johnson P.E.,  10/16/05]

Modulated Light Monitor #1  -  This battery powered circuit will work from DC to 500KHz.  It is designed to connect to an oscilloscope to monitor the light level changes and modulated signals . . . [Circuit by David A. Johnson P.E.,  07/09/06]

Modulated 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. . . . [Hobby Circuit designed by David A. Johnson P.E.,  06/14/00]

Momentary 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. . . . [Hobby Circuit designed by Dave Johnson P.E.,  01/18/02]

Monitor Clear Fluids  -  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.,  05/01/12]

Monitor Water Seepage  -  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 . . . [Hobby Circuit designed by Dave Johnson P.E.,  01/16/11]

Monitor Water Seepage  -  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 . . . [Hobby Circuit designed by David A. Johnson P.E.,  01/16/11]

Monitors AC Current  -  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 Dave Johnson P.E.,  07/10/06]

Monitors Pump Motor  -  This circuit solves this problem in a novel way.  It clips on the outside of the pump motor’s power cable and provides a transistor switch closure whenever the circuit detects current flowing to the motor.  The circuit draws so little current that a small lithium cell will power the circuit for years . . . [Circuit Solution by David Johnson P.E.,  11/18/04]

Motion Alarm Using piezo Device  -  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]

Nine-Volt 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 Dave Johnson P.E.,  07/09/06]

Nine-Volt 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]

NMH/NiCd 6v 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 . . . [Hobby Circuit designed by David Johnson P.E.,  12/12/04]

NPN/PNP Transistor Tester  -  I designed this circuit about 30 years ago and still use it from time to time. It is powered from a 3v battery. It makes a tone when the transistor tests good. It can be used to determine device type (NPN or PNP) and the base, collector or emitter transist . . . [Hobby Circuit designed by Dave Johnson P.E.,  06/28/00]