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 small AM transmitter was found in American documentation from the 1980s. Transistor support...
We found this circuit in a 90's Popular Electronics. The circuit produces a loud sound if S2 is...
This circuit is suggested by National Semiconductor and is based on an LM1578-A integrated circuit...