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
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.