This simple circuit is intended to test the zener test. RV1 can be calibrated in terms of voltage. The maximum voltage of the tested zener is that used in the circuit supply.
We found this circuit in English documentation for PCs from 1996. The ZN448 is a very uncommon...
This photo-relay uses a Darlington stage to increase sensivity. Any photo-transistor can be used....
A new version of the previous circuit triggered by the negative transition of the input signal is...