This circuit was found in a 80’s American documentation. The adjustment trimpot is from 47k to 100k. The diodes are general-purpose types use and the supply voltage depends on the relay.
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...