Found in a 90s Elektor, this circuit makes use of general-purpose transistors, except T4 that can be replaced by an equivalent.
This low drift or offset circuit is from an 80's documentation. The configuration can be made with...
This circuit has a logarithmic response as a function of the light falling on the sensor. The...
Found in 80's Electronic Engineering, this circuit can tune the frequency to be rejected. The power...