With this circuit it is possible to vary the resistance by adjusting the mark/space reaction of the 555. The circuit was found in the November 1989 Radio Electronics magazine.
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...