|This circuit can be broken up into four patrs:
The pre-amplifier increases the input signal to a level that can be processed. The gain is set to about 100 so any signal of 10mV or above will do. If using an electret mic then use a 2.2K resistor from the supply to the input to drive it.
The demodulator rectifies any signal and when it reaches above about 0.6volts the transistor turns on and triggers the monostable.
The monostable provides a timed output to the transistor driving the relay. The contacts of this connect to the load.
While the monostable is active, the signal is fed back to another transistor which pulls the abse of the switch transistor to 0v, thus preventing any further re-triggering of the monostable until the timing period has ended.
This circuit has been tested on Crocodile Clips software - and on breadboard.
Download circuit simulation
Phil Townshend 2001.