This circuit is intended to detect when current flows through the opening of a car door. The circuit fires the horn for 90 seconds. The FET can be the BF245 and the NPN transistors can be the BD135.
This circuit from the 70's used germanium transistors and operated in the range of medium and...
Found in a 1965 Electronic Design, this circuit produces a signal with the waveform shown in the...
Unlike the previous detector circuit, this includes the inverter not requiring a high voltage...