This wired intercom circuit was found in an English publication from the 80's. The integrated circuit can be replaced by an equivalent, just like the transistor which is a general-purpose type.
This circuit generates a siren sound. It was found in the April 1969 Radio Electronics magazine. The...
This circuit converts pulse frequency to current, which is indicated on an analog instrument. The...
This circuit was obtained in a 1991 Electronics Handbook. The publication no longer exists, but...