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.
In the figure we have the basic configuration of an operational amplifier for audio signals. The...
This General Electric circuit produces pulses of light only when light stops shining on the...
Found in an Electronic Design of 1964 this circuit has a control for the frequency and another for the...