We found this circuit in an English publication from the 60's. The transistors are of germanium of the time and the headphones must be high impedance magnetic.
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...