This two-transistor circuit was found in the September 1971 issue of Radio Electronics magazine. The sensor is an NTC and sensitivity adjustment is via R1.
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...