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 . . . [Hobby Circuit designed by Dave Johnson P.E.,  03/09/02]

Pushbutton Activates Service Request Beeper  -  A single press of a pushbutton switch turns on a beeper for one second but can’t be activated again for 60 seconds. . . . [Circuit by David Johnson P.E.,  09/25/05]

Pushbutton 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 . . . [Circuit by David Johnson P.E.,  03/22/09]

Pushbutton Oneshot & Latch  -  This circuit uses a single IC to convert a noisy pushbutton switch signal into a clean pulse or a sustained push on-push off signal. It can operate from 3v to 18v . . . [Circuit by David A. Johnson P.E.,  03/26/02]

Pushbutton On-Off Latch Circuit  -  Often you want to turn on and off power to a device with a single pushbutton switch.  The circuit below performs this logic function with just a few parts and can operate from DC supplies between +3v to about 15v.  It uses two inverters within a 74C14 or a CD4069 hex inverter package.  Each press of the button toggles between the two on/off logic states. . . . [Hobby Circuit designed by Dave Johnson P.E.,  01/09/10]

Pushbutton Switch works through Glass  -  This simple technique allows a mechanical pushbutton switch assembly to activate a reed type relay through a pane of thick glass.  Pushing the button moves a magnet near the reed relay, closing the relay contacts . . . [Circuit by Dave Johnson P.E.,  02/10/02]

Ramping Lamp Driver  -  This circuit drives any 12v lamp, incandescent or LED type, in a ramping fashion.  Each press of the button toggles the lamp from a fully off mode to a fully on mode.  But, instead of an instantaneous lamp intensity change, the power is slowly ramped up a . . . [Hobby Circuit designed by Dave Johnson P.E.,  12/21/07]

Ramping LED Driver  -  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 . . . [Circuit by Dave Johnson P.E.,  12/21/07]

Reduced Power Sequential LED Flasher  -  The 74HC4017 is a neat decade counter which has 10 decoded outputs.  When supplied with a low frequency clock and with a LED attached to each of the outputs, an interesting ten step sequential flasher effect can be produced.  The LEDs can be configured in . . . [Hobby Circuit designed by David A. Johnson P.E.,  11/30/08]

Reflected Infrared Light Switch  -  Infrared light reflected off a finger is used to activate this switch circuit. Drawing only 30uA from a 3v supply, this circuit will detect a human finger with a range of about 1 inch.  The sensor uses an inexpensive infrared LED and a matching photo diode . . . [Hobby Circuit designed by David Johnson P.E.,  07/09/06]

Replace a Pushbutton Switch with this Infrared Safety Switch  -  Using a slotted optical sensor, this circuit can be used as a replacement for a large mushroom pushbutton safety switch . . . [Hobby Circuit designed by David Johnson P.E.,  07/11/06]

RF 2.45Ghz Signal Detector  -  This passive RF indicator is made from a few simple parts.  A 100uA moving coil meter is used to display the relative intensity of the RF signal.  This circuit can be used with some cell phones and many cordless telephones . . . [Hobby Circuit designed by David A. Johnson P.E.,  07/09/06]

Safety Light  -  My 1200 square foot metal shop only has one small entrance door and no skylights.  If the lights are turn off while inside, there is not enough light to safely find the door to exit the building.  On a couple occasions, while I was quietly working at my test bench, my wife would walk into the shop, grab a garden tool and rush back out, turning off the lights and closing the door without thinking . . . [Hobby Circuit designed by David A. Johnson P.E.,  11/29/04]

Service Request Beeper  -  A single press of a pushbutton switch turns on a beeper for one second but can’t be activated again for 60 seconds. . . . [Hobby Circuit designed by Dave Johnson P.E.,  09/25/05]

Shadow Activated Motion Detector  -  The hobby circuit below works quite well in typical indoor room lighting.  I would not recommend its use in direct sunlight.  Two small PIN photodiodes positioned about one inch apart form a shadow detector.  With no shadow cast on the devices, both device . . . [Hobby Circuit designed by Dave Johnson P.E.,  08/21/05]

Shop Safety Light  -  My 1200 square foot metal shop only has one small entrance door and no skylights.  If the lights are turn off while inside, there is not enough light to safely find the door to exit the building.  On a couple occasions, while I was quietly working at my test bench, my wife would walk into the shop, grab a garden tool and rush back out, turning off the lights and closing the door without thinking . . . [Hobby Circuit designed by David Johnson P.E.,  11/29/04]

Simple Circuit for White LED Night Light  -  This simple circuit is designed to plug into a standard AC electrical outlet.  It uses four super bright white light emitting diodes (LED) in conjunction with a capacitor coupled full wave rectifier circuit . . . [Circuit by David A. Johnson P.E.,  07/27/03]

Single Output 120VAC 200 Watt Touch Switch  -  This simple circuit uses a single IC to form a nice touch switch circuit.  A single transistor forms the remote active switch sensor.  Multiple switches can be wired in parallel.  The switch circuit can be located about 500 feet from the control circuit. . . . [Hobby Circuit designed by David Johnson P.E.,  02/07/10]

Sink to Source Universal DC Converter  -  As illustrated below, many control systems use an open collector transistor to turn something on.Perhaps it is a LED light, a relay or a small motor.However there are othersystems which require a common ground connection instead of a common V+ supply connection . . . [Circuit by Dave Johnson P.E.,  03/27/11]

Small Pushbutton Controls 10 amps AC  -  The circuit below uses a pair of long lasting lithium coil cell batteries to power a latching relay circuit.  With each activation of the small pushbutton, the relay is either latched closed or unlatched open.  Since coin cells can’t provide the necessary peak current, a 500uF capacitor is used to store the needed energy.  The circuit standby current is kept very low by using a stack of five 100uF ceramic capacitors instead of the usual electrolytic cap . . . [Circuit by David Johnson P.E.,  01/20/12]

Solid State 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 . . . [Hobby Circuit designed by David Johnson P.E.,  06/03/00]

Sound Triggered Night Light  -  A while back, I received a schematic from a guy by the name of Karl.  He described a situation he and his wife found themselves in aboard a cruise ship in the Caribbean.  They were staying in a room toward the inside of the ship, which did not have any port holes, windows or a balcony . . . [Circuit by Dave Johnson P.E.,  08/04/13]

Switch activated by Piezo Vibration  -  An inexpensive piezo wafer is used to detect vibration and when the vibration is sufficient a switch is activated . . . [Hobby Circuit designed by David Johnson P.E.,  07/09/06]

Switch detects Audio Signal  -  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 Johnson P.E.,  07/09/06]

Switches AC or DC power using a Latching Relay  -  The circuit below switches AC or DC power to a load using an ultra low power 3v latching relay with two coils.  The relay can handle about 10 amps of current, up to 250vac.  Each time the small pushbutton is pressed, the circuit either latches the relay . . . [Hobby Circuit designed by David Johnson P.E.,  12/24/09]