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 allows you to read temperatures with a common digital voltmeter. We found it in the...
This circuit for the 1.5 to 15 V scaled ranges was found in a 1982 documentation, but uses current...
The integrated circuit of this 1976 configuration is the 7400 and the type crystal in televisions of the...