Economical and practical loop anti-theft protection circuit

The characteristic of this loop 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 disconnecting the loop will not help at all, because a detection resistor is connected in series in the loop.

The following figure 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 ICIA and ICIB 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 ICIA and the non-inverting pin (5) of op amp ICIB.

The voltage of the in-phase pin (3) of ICIA is usually lower than 6V, while the voltage of the inverting pin (6) of ICIB is usually higher than 6V. As a result, under normal conditions, the outputs of both op amps are low potential.

When the thief short-circuits the loop outside the protected area, the output of the op amp ICIA becomes high, the transistor T1 turns on, the relay RL1 closes and the circuit sounds an alarm. Similarly, if the thief disconnects the loop, the output of the op amp ICIB becomes high, the transistor T2 turns on, and the relay closes and sounds an alarm.

RL1 uses a 12V, double-contact relay, and the contacts should have appropriate 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. The second contact is connected to the reset switch slo 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 above.