Found in an Electronic Design and 1970s, this circuit uses transistors of the time. The transistors can be replaced with equivalents and the relay is 12 V depending on the power supply.
This circuit is from an October 1963 Radio Electroncs. It uses a single 2N18383 transistor and a...
Found in a 1968 Ferranti documentation, this circuit can be implemented with modern equivalent...
This light meter can be mounted with equivalent integrated circuit. The power is made by a single...