Disconnection anti-theft protection circuit

The characteristic of this disconnection anti-theft protection circuit is that as long as the loop is damaged, the alarm circuit will immediately sound an alarm. Simply connecting the broken part of the loop with a wire or simply cutting the loop will not help at all. This is because a detection resistor is connected in series in the loop.

Figure 1 is the schematic diagram of the loop anti-theft protection circuit. It is composed of a four-op-amp chip LM324 (IC1) and a small number of discrete components. IC1 only uses two of the op-amps IC1A and IC1B to form a voltage comparator.

Resistors R and R1 divide the 12V power supply to 6V, which is then sent to the inverting pin ② of op amp IC1A and the non-inverting pin ⑤ of op amp IC1B. The voltage of the non-inverting pin ③ of IC1A is usually lower than 6V, while the voltage of the inverting pin ⑥ of IC1B is usually higher than 6V. As a result, under normal conditions, the outputs of both op amps are low potential.

When the thief shorts the loop outside the protected area, the output of op amp IC1A goes high, transistor T1 turns on, relay RL1 closes, and the circuit sounds an alarm. Similarly, if the thief disconnects the loop, the output of op amp IC1B goes high, transistor T1 turns on, and the relay closes to sound an alarm.

RL1 uses a 12V, double-contact relay, and the contacts should have a suitable current capacity. The circuit can be powered by a 12V battery or a 12V regulated AC adapter. Before connecting the circuit to the power supply, carefully check whether the soldered printed circuit board is correct and firm. Connect the first contact of the relay to the 220V alarm bell, and the second contact to the reset switch S1 on the panel of the circuit box. Use a thin metal wire to complete the loop connection. When connecting, don't forget to connect the loop in series with the detection resistor R, and place R in the protection zone, as shown in Figure 2.