Any small sound amplifier from 200mW to 1W can be used in this simple circuit. Or circuit of two 80s based on a trigger like 555.
This circuit was found in a 90s EDN. Its purpose is to increase the efficiency of operational...
This LED indicator circuit uses an LM339 comparator. The circuit was found in the January 1995...
This circuit was found in an 80's General Electric documentation. The circuit uses uncommon...