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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 | |
| K1 | 1 | 9V Relay | |
| J1 | 1 | RCA Jack | |
| 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.