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.
Common operational amplifiers like the TL series, CA3140 and others can be used in this circuit...
This high-gain amplifier was found in the March 1994 issue of Electronics Now magazine. The...
We found this timer for times of a few minutes in a 1974 Ferranti transistor manual. The relay is 37...