This circuit was found in the April 1971 Radio Electronics magazine. It uses common transistors and can be adapted to use an LED or audible indicator instead of a light bulb.
In this circuit, from a 1977 publication, we have the traditional version of a filter that rejects 1...
This PA or Public Address amplifier with a few watts of power was found in the March 1986 Radio...
The sensor of this circuit is an NTC and the indication is made on a common analog indicator, and...