Alarm circuit with central station and sensor contacts - has oscillator output coupled to voltage comparators detecting both break and short circuit conditions

The alarm circuit uses a number of sensors coupled to a central controller. It ensures that the alarm is activated if either any sensor is triggered normally of if sabotage is attempted resulting is a circuit break. An oscillator (O1, is coupled to the primary winding of a 1:1 transformer (T1) over an RC (R1, C1) circuit. The alarm contacts (R1, R2...kn) are coupled in parallel with a load resistor (R2). the signal from a pick-up point (P) is rectified (D1) and connected to two comparators (K1, K2) with high and low reference voltages (REF1, REF2). The high voltage corresponds to a circuit break condition and the low voltage to a short circuit condition.