Found in an 80's Elektor, this circuit can use any equivalent optical coupler. The buzzer is of the Sonalert type with included oscillator.
This circuit is from a July 1970 Radio Electronics magazine. The sensors are metal probes. The 18 V...
This commercial medium and short-wave radio was manufactured in Japan in the 60s. The circuit has...
General Electric suggests this circuit in an 80's documentation. The circuit makes use of PUTs...