Found in a 1964 Electronic Design this circuit compares two signals by providing the inverse of the algebraic sum of their frequencies. The circuit can be made with modern equivalent transistors.
This circuit shows how to control an astable configuration using the 555 IC. The control signal is...
This continuity tester for low resistances was found in an English magazine from 1978. The circuit...
This 1995 circuit uses an audio amplifier that is no longer common today. However, other...