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.
Found in 1965 General Electric technical documentation, this half-wave circuit controls shunt type...
We found this circuit in a 1991 publication. What it does is to amplify a signal with unit gain...
This circuit can generate signals from 80 to 800 Hz with an amplitude of 0.3 V. We find it in a...