This sound meter or sonometer was found in a Popular Electronics from the 80's. The circuit can be easily implemented today, and the microphone is made of electret or ceramic.
This circuit was taken from a Popular Electronics from the 1990s. The circuit triggers a relay that...
The transformer inf this circuit is a 6 or 9 V unit and diodes 1N4002. The filter capacitor is 2...
This circuit causes a relay to oscillate generating a signal that turns into high voltage in a...