Found in the May 1971 Radio Electronics magazine, this is the circuit for a commercial high-impedance instrument. The circuit uses 4 transistors and is powered by 2 9 V batteries.
This two-door doorbell was found in a 1990s US publication. The circuit is still current as...
Suggested by Intersil in documentation from the 90's, this circuit is valid for modern operating...
We found this circuit in an old RF documentation. The circuit must employ RF transistors and the...