This circuit is from a July 1970 Radio Electronics magazine. The sensors are metal probes. The 18 V power source must be isolated from the mains.
I found this circuit on a December 1976 Radio Elettronica. The Italian magazine no longer exists, but...
This amplified circuit was found in a magazine for radio amateurs 73 of the 90s. The circuit uses an...
This sound relay, Vox or acoustic switch has a bistable action triggering a relay when the...