This simple circuit from the May 1996 Think Tank column of Popular Electronics activates a relay when it senses a composite video signal. This allows you to use the tuner built into your VCR to turn on and off older TVs that are not equipped with a remote. It can also be used to activate surround sound equipment, turn off the room lights, turn on video game consoles, etc. For such a simple circuit, it is very versatile.
|R1, R2||2||10K 1/4 W Resistor|
|R3||1||1K 1/4 W Resistor|
|R4||1||33K 1/4 W Resistor|
|C1||1||1uF Electrolytic Capacitor|
|Q1, Q2, Q3||3||2N2222 NPN Transistor||2N3904 NPN Transistor|
|D1, D2, D3||4||1N4148 Diode|
|MISC||1||Case, wire, board|
1. Since you may be using this circuit to switch mains voltage, it should be enclosed in a case.
2. The circuit will also work with most line level audio, although you may have to adjust the value of R1.
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