I like mechanical systems. Even when it’s not the most efficient solution, a machine with moving parts has flair that electronics often lack.
My Hand-Crank Geiger Counter is not a truly historical design, but it does use an anachronistic “saw switch” as a mechanical oscillator. While I couldn’t remove all modern semiconductors from the circuit, I kept them to a minimum and used no integrated circuits.
How the Electronics Work
The circuit applies 18V (from two 9V batteries wired in series) to the primary windings of the step-up transformer through the saw switch.
The on-off cycling of power to the transformer creates inductive high voltage in the secondary winding, which passes through diode D1 and charges capacitor C1. The Zener diodes D2, D3, D4, and D5 regulate the voltage to 450V for the GM tube.
When the capacitor is charged, we can siphon off 150V from the junction of D3 and D4 to light a neon bulb. The neon bulb indicates when the capacitor is fully charged and optimum power is going to the GM tube, at which point we can stop rotating the saw switch.