This circuit allows the use of an electret microphone with a 1.5 V source. We found it in the Encyclopedia of Electronic Circuits volume 5 from 1995.
This circuit for photography uses was found in the Electronics Experimenter Handbook Special...
It is like to the previous circuit with a lock that keeps the LED on at the end of the timer. The...
This circuit, with two operationals, was obtained from an old American publication. Operational...