This simple circuit has good gain and can be implemented with any general-purpose transistor. The circuit is from Electronics Now, June 1997. The coil determines the frequency range.
This circuit is from a March 1978 Radio Electronics magazine article that showed how to build it....
This circuit was obtained from an article in Radio Electronics magazine in October 1966. It...
In the figure we have a bistable circuit that activates LEDs from touch rings and turns off data...