This circuit uses two relays that switch when the voltage exceeds the value given by zener D2. The circuit is from a Radio Electronics magazine from April 1975.
This circuit was obtained from an American 80's documentation. The balanced mixer can be shown in...
This circuit was found in a 90s Electronic Engineering. The quadruple operational amplifier can be...
I found this circuit in an English publication from 1989. This configuration can be mounted with...