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 reader with TTL logic compatible output uses an LM139 comparator. The circuit was found in...
This circuit can be used with common transistors, from general purpose to medium power. The...
Although it is a common configuration today, these circuits were taken from an old publication. The...