This simple sensitive circuit was suggested in a July 1992 Radio Electronics issue. The relay is a double-contact changeover relay. Sensitivity adjustment is made at R2.
Although indicated in the original publication of the 1980s as a capacitance switch, the circuit...
This amplifier step with a transistor has an input impedance of 100k to 10 M. The transistor can be any...
This circuit was obtained from an English magazine from 1977. The magazine no longer exists, but...