VIDEO ACTIVATED RELAY

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.

Parts
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

Notes
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.