This third-order Butterworth filter was found in American documentation from the 1980s. Formulas for component calculations are attached to the diagram.
This receiver for an audio link by modulated light was found in a 90s Elektor. Transistors are common...
Using a tunnel diode, this circuit was found in a 1966 American publication. The circuit is for...
This interesting configuration of optical triggering using an oscillator was found in a 90s...