This alarm circuit triggers when S1 opens. The circuit from a January 1975 Radio Electronics magazine can be used with interrupt sensors connected in series.
This small transistor amplifier was found in an ancient American documentation. Transistors can be replaced...
One touch of the X1 sensor and the capacitor C1 charges, causing the power transistor to drive by...
This circuit was found in the 1972 RCA Power Transistor Databook. Transistors can be replaced by...