This circuit was found in the April 1971 Radio Electronics magazine. It uses common transistors and can be adapted to use an LED or audible indicator instead of a light bulb.
This circuit was obtained from an American documentation from the 1980s. The T1 triggering...
Found in a 1974 Ferranti manual, this circuit uses diodes to limit the voltage on the transistors. The...
This circuit is from an English publication of 1991. With it you can use a common multimeter on...