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.
We find the original circuit in a technical publication of 1978. Changing the components, such as...
Found in Popular Electronics from the 90s, this circuit can generate signals from 1 to 10 MHz...
Found in ancient American documentation this circuit uses semiconductors from the thyristor family such as...