Using a 555, this configuration appears elsewhere on this site. This is from American documentation from the 90s. The indicator can be an LED or some device controlled by a relay.
Suggested by an old RCA manual, this circuit makes use of the period components. Data for the...
This amplifier is from a 1990s Linear Technology component manual. The circuit requires...
This circuit is from a 1967 General Electric SCR manual. The capacitors are polyester and the...