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 A. Johnson P.E.,  01/18/02]

Car Ignition 12v Switch  -  A Discover Circuits visitor needed a way to redirect +12v DC in his car to some auxiliary loads whenever the car’s engine was running.  Many cars have cigarette lighter outlet jacks, which are either live all the time or become live when the car’s engine is running . . . [Hobby Circuit designed by David A. Johnson P.E.,  02/26/12]

Charge Coupled MOSFET Relay #2  -  This circuit acts as an AC/DC relay with a rating up to 50 volts peak and up to 10 amps of current.  The differential oscillator supplies voltage to the gates of the two FETs with good isolation will drawing only 1.5ma of current . . . [Circuit by Dave Johnson P.E.,  10/16/05]

Circuit Protects from Battery Polarity Reversal  -  This simple circuit can protect a sensitive electronic circuit from an accidental connection of a battery with a reversed polarity.  The N-channel FET connects the electronic device to the battery only when the polarity is correct.  The circuit shown was . . . [Circuit by Dave Johnson P.E.,  03/09/02]

Classic Linear 5v Supply Using 6.3VAC Transformer  -  There are times when you wish to control a pulse width modulation (PWM) signal with a DC voltage, ranging from 0v to 5v. This voltage is often created by a computer interface.  The circuit below performs this voltage to PWM conversion. PWM signals are often . . . [Hobby Circuit designed by David Johnson P.E.,  10/31/10]

Closet Light with Automatic 3-Minute Timeout  -  The circuit below is powered by three 1.5v alkaline AA cells.  With a finger tap to the pushbutton trigger switch, a cluster of 6 wide angle white LEDs is turned on. The lights remain on for about 3 minutes, then will turn off.  The circuit’s standby current is only a few microamps.  A set of fresh batteries should last at least 200 light applications.  The circuit uses a Schmitt trigger inverter and two transistors.  When the pushbutton switch S1 is pressed, the 10uF capacitor C1 is discharged . . . [Circuit by David Johnson P.E.,  11/14/10]

Computer Controlled PWM Circuit  -  There are times when you wish to control a pulse width modulation (PWM) signal with a DC voltage, ranging from 0v to 5v. This voltage is often created by a computer interface.  The circuit below performs this voltage to PWM conversion. PWM signals are often . . . [Circuit by David A. Johnson P.E.,  10/31/10]

Controlled Up/Down Ramping Light  -  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 . . . [Hobby Circuit designed by David A. Johnson P.E.,  03/22/09]

Dark Activated 12v Power Supply Switch  -  Using an inexpensive phototransistor, this circuit switches 12v to 24v power to a load, whenever the sun goes down . . . [Hobby Circuit designed by Dave Johnson P.E.,  07/09/06]

Day/Night Switch-24vac Powered  -  Many low voltage outdoor lighting systems use 24vac to power the devices.  The low voltage is much safer to use outside than standard 120vac or 240vac power systems.  The circuit below is designed to turn off power to outdoor lights during daylight and turn them back on at night . . . [Circuit by Dave Johnson P.E.,  12/26/10]

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.,  07/15/06]

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 A. 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 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 David A. Johnson P.E.,  08/21/05]

Door Knob Alarm  -  Many companies offer simple alarm devices for personal use in bedrooms or hotel rooms. A metal chain attached to a box holding the electronics is placed around the inside doorknob of a wood door. Anyone grabbing the knob from the outside is detected . . . [Circuit by Dave Johnson P.E.,  05/29/00]

Door Knock Beeper  -  In some apartment buildings and homes, not equipped with a door bell, it may be hard to hear someone knocking on the front door.  The circuit below provides a means to activate a loud beeping sound inside, whenever someone knocks on the door . . . [Circuit by David A. Johnson P.E.,  06/14/09]

Drive a LED with an Intensity Ramping Mode  -  The circuit below was designed to drive a LED with an intensity ramping mode.  Two 555 timers generate two different triangle waveforms.  The upper device generates a 10KHz signal while the lower unit produces a 1Hz signal.  The two signals are fed to a . . . [Hobby Circuit designed by David Johnson P.E.,  12/21/07]

Exciter Circuit-12v Touch Switch  -  This circuit generates a 20KHz pseudo sine wave signal that can power approximately 50 remote touch activated switch circuits.  This hobby circuit can support a cable length of approximately 2500 feet.  A typical remote switch circuit is also shown as . . . [Hobby Circuit designed by David Johnson P.E.,  02/10/02]

Finger Tap Piezo Relay  -  This circuit uses a flat piezo wafer, glued inside a plastic box, as a finger tap sensor.  With each tap of a finger to the box holding the wafer, the circuit turns on and off AC or DC power to an external device.  The circuit is powered by a 9v 9v battery . . . [Hobby Circuit designed by Dave Johnson P.E.,  12/07/10]

Finger Touch Activated Switch  -  It does not get any easier if you want a solid state switch that is activated by the touch of a finger. Two small metal pins route voltage through the finger skin to a MOSFET switch. The circuit is great for situations where a membrane type mechanical switch . . . [Circuit by Dave Johnson P.E.,  06/03/00]

Five-Watt Fluorescent Lamp Intensity Modulator  -  The circuit was designed to experiment with using small fluorescent lamps as a broad pattern source of modulated light. The circuit hits the small lamp with narrow 1us pulses at a rate of 10KHz. Each pulse launches about 10 watts of visible light. The lamp . . . [Circuit by David A. Johnson P.E.,  06/27/00]

Flashing LED Power Indicator  -  This circuit flashes a power indicator LED to keep the average current low . . . [Circuit by David Johnson P.E.,  06/04/00]

Fluorescent Lamp Intensity Modulator  -  This hobby circuit was designed to experiment with using small fluorescent lamps as a broad pattern source of modulated light. This hobby circuit hits the small lamp with narrow 1us pulses at a rate of 10KHz. Each pulse launches about 10 watts of visible light. The lamp starting method is a bit crude but the circuit does work. . . . [Hobby Circuit designed by David Johnson P.E.,  06/27/00]

Fluorescent Lamp Night Light Needs Only 1 Watt  -  If you are tired of replacing small night light lamps, try this circuit. The line powered circuit uses a long life cold cathode fluorescent lamp. A simple charge pump technique runs the lamp without any transformer . . . [Hobby Circuit designed by David A. Johnson P.E.,  03/26/99]

High Power Touch Switch Exciter Circuit  -  If you have hundreds of touch switches that need an excitation signal, then this circuit is what you need.  Its 20KHz 20v peak to peak output signal can supply up to 3 watts of touch switch excitation power . . . [Circuit by David A. Johnson P.E.,  02/11/02]

HO Model Train Lighthouse Flasher  -  HO train sets often have authentic looking scale model homes and buildings.  The hobby circuit below can be mounted inside a model lighthouse.  The electronic circuit drives a single LED lamp in such a way that it produces light which simulates the rotation . . . [Hobby Circuit designed by David Johnson P.E.,  07/11/06]