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 show a timer, this simple one with only one of the four gates of integrated...
This circuit is a sensor for photodiodes that can be used in optical couplers, encoders and other...
This crystal tester was found on a 90s Ham Radio. Transistors are commonly used. The circuit is for...