This proximity relay circuit was found in the July 1990 Radio Electronics magazine. The supply voltage should be according to the relay used.
This circuit with unijunction transistors was found in a documentation from the 80's. The trigger...
In this circuit of an old American documentation, the input impedance is the same as the output...
In the figure we have the way to obtain a voltage of 18 V from a 6.3 V transformer with central...