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.
This circuit was found in the Spanish magazine No longer in existence, Mundo Electrónico, in a...
This circuit that has a response from 10 Hz to 700 kHz was found in Philips documentation from 1968. Dual...
This simple sweep generator circuit has been found in older documentation, but can still be easily...