When the sensor connected to points A and B is activated, removing the ground from the base of Q1 and thereby bringing this component to saturation, the monostable formed by CI-1 is triggered. This monostable controls the trip time of relay K1 through Q2. The time obtained with a resistor of 100 k ohms and a capacitor of 470 uF is of the order of 1 minute, but using a resistor of 1 M ohms, it arrives at 10 minutes. The resistor R4 together with capacitor C2 connected to pin 4 of the 555 guarantees automatic reset of the circuit, preventing it from being triggered when the power is turned on. The switch that turns the circuit on and off can be installed in a hidden location to activate and deactivate the alarm. The sensors can be of the type reed-switch, micro-switches or other.
