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.
| Part | Total Qty. | Description | Substitutions | | 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 | |
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- Since you may be using this circuit to switch mains voltage, it should be enclosed in a case.
- The circuit will also work with most line level audio, although you may have to adjust the value of R1.
