This relay triggered by voice or by another sound was found in an Elektor from the 90's. The circuit uses a 12 V sensitive relay and has no sensitivity control.
The circuit in the figure trips with a negative transition of the input signal, its output...
The basic frequency of this circuit is determined by the capacitor of 470 nF and the adjustment of the...
The load current can be adjusted up to about 1 A. The transistor can be TIP32 or TIP42. The...