This power supply protection circuit using a relay was found in a 1967 documentation. The components are from the time, but current equivalents can be used depending on the power source current.
This amplifier was found in a Japanese publication in 1972. Transistors are uncommon today, and...
The current of this source is 200 mA at the most, and the output depends on the zener that may...
Found in Mallory documentation from 1968 this circuit uses an integrated circuit from that time. The...