How many times have you wished that there was a simple way to turn on that sub-woofer or some other piece of audio equipment, simply by sending it a signal? This ability is fairly common in commercial subs and some other gear, but there seems to be a complete absence of circuits on the net, and they seem completely unavailable as an add-on device.
The circuit presented here will operate with a signal of 10mV, which will be adequate for all but the quietest listening. Even then, 10mV represents a typical power of about 12mW into a typical amp and 8 Ohm speaker. It is possible to make it more sensitive - I tested it to 1mV, but at this level even tiny amounts of mains hum or other noise will trigger the circuit.
http://sound.westhost.com/project38.htm
