This circuit was found I a 1965 Radio Schack documentation. The transistors are from that time, admitting equivalents. The transformer has 500 ohm primary and secondary depending on the speaker.
This contact or reed switch actuation was found in a 1964 Electronic Design. The circuit can be...
This is the previous version, but with a simplification given by the use of the 7404 and 7408...
In the figure we present a circuit of a microphone signal conditioner using the SSM2167 integrated...