Fig. 1 shows the circuit of the smart loop-type burglar alarm. This circuit is built around quad op-amp LM324 and a few discrete components. Two op-amps of the LM324 are configured as voltage comparators.
Resistors R and R1 divide the 12V DC power supply equally. The resulting 6V is applied to the inverting input of the first op-amp (IC1A) and the non-inverting input of the second op-amp (IC1B). The non-inverting input of the first op-amp (IC1A) is kept below 6V and the inverting input of the second op-amp (IC1B) is kept above 6V. As a result, in normal condition, the outputs of both the op-amps remain low.
When the thief shorts the loop outside the area to be protected, the output of op-amp IC1A goes high, transistor T1 conducts and relay RL1 energises to sound the alarm.
Also, when the thief breaks the loop, the output of op-amp IC1B goes high, transistor T2 conducts and relay RL1 energises to sound the alarm. Thus the alarm sounds when the loop is shorted or cut.
Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. The relay should be 12V, 2C/O type having contacts of suitable current rating. Use a regulated 12V DC power adaptor or a 12V battery to power the circuit. Before connecting the circuit to the power supply, check the assembled PCB for proper soldering of the components. Connect the first C/O contacts of the relay to the 230V AC bell (for alarm) and the second C/O contacts to reset switch S1. Mount switch S1 on the front panel of the cabinet. Use a thin wire for loop connection. Don’t forget to connect sensing resistor R in series and place it in the area to be protected (refer Fig. 2).