Three-Minute Closet Light  -  I have a coat closet by my front door.  The thing is very dark at night and really needs some kind of illumination inside.  I could hire an electrician to install a light in the closet ceiling and then wire it to a toggle switch outside the door but that would cost me quite a bit of money . . . [Circuit by David A. Johnson P.E.,  12/25/11]

Through Glass Push Button Switch  -  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 David A. Johnson P.E.,  11/07/10]

Through Glass Pushbutton Switch  -  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. . . . [Hobby Circuit designed by Dave Johnson P.E.,  02/10/02]

Touch Activated Switch  -  This circuit provides about one watt of non-isolated DC power for an automotive type 12v LED array lamp in addition to a standard incandescent table lamp. . . . [Circuit by Dave Johnson P.E.,  01/18/02]

Touch Alarm for Door Knob  -  I designed this hobby circuit many years ago and it seems be a very popular construction project.  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 place . . . [Hobby Circuit designed by Dave Johnson P.E.,  05/29/00]

Touch Switch 240VAC  -  This is another capacitance based touch on/touch off switch, which can control power to a 200 watt 240vac load . . . [Hobby Circuit designed by Dave Johnson P.E.,  02/12/02]

Touch Switch controls a 120 VAC load  -  This is a capacitance based touch on/touch off switch, which can control power to a 200 watt 120vac load . . . [Hobby Circuit designed by Dave Johnson P.E.,  07/11/06]

Touch Switch Exciter  -  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 . . . [Hobby Circuit designed by David A. Johnson P.E.,  02/11/02]

Transistor-Switch detects 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 A. Johnson P.E.,  07/09/06]

Universal DC Sink to Source 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 other  systems which require a common ground connection instead of a common V+ supply . . . [Hobby Circuit designed by David Johnson P.E.,  03/27/11]

Up/Down Ramping Light Pushbutton Control  -  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 A. Johnson P.E.,  03/22/09]

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