This circuit was found in Ferranti's 1967 documentation. The circuit drives a 40 V relay, but it can be adapted to operate with sensitive lower voltage relays.
This circuit was found in a Popular Electronics from the 90's. The transistors are general-purpose types...
This circuit was found in an English magazine from August 1982, but it can be easily assembled with...
You can measure capacitances across a large range of values using the AC Power line frequency as...