This automotive grade circuit was found in a 1991 Electronics Hobbyist. The transistors are general purpose types. P1 sets the monitoring voltage.
This sensitive two-transistor circuit was found in the June 1965 Radio Electronics magazine. The...
This circuit, obtained in an old publication, causes a second telephone bell to be played in a...
Found in a1969’s neon lamp documentation, this circuit uses a high voltage transistor. The output...