This circuit consists of a stereo audio compressor. We found it in Radio Electronics magazine from October 1968. The circuit uses an opto-coupler made with a neon lamp and LDR.
This circuit can be connected to the output of alarms or sirens with the 555, CMOS or TTL,...
Found in an American leaflet from the 90s, this circuit uses an AM radio out of station to monitor...
Found in the Encyclopedia of Electronic Circuits Volume 5 of 1995 this circuit has the gain...