This common circuit using operational amplifiers was found in the 1993 issue of Radio Electronics Special Projects. The circuit is sandwiched between the pickup and the amplifier input.
The purpose of this circuit is to turn off a lamp after a few seconds or minutes depending on the...
The circuit we present is that of a good power ultrasound generator, as it uses a transistorized output...
Using the well-known 2N2646 that was popular at the time, this circuit was found in Radio...