A novel and practical power line anti-theft and cutting alarm circuit

In mountainous areas and rural areas, due to the long transmission lines, crossing mountains and valleys, and wading through rivers, low-voltage power lines are often cut by lawless elements. In addition, due to the bad environment and climatic conditions (such as strong winds, freezing, etc.), transmission line phase failure and power outage accidents may also occur. The alarm circuit introduced in this article can sound an alarm when the power line is broken or stolen. It has two options: on-site sound and light alarm and wireless alarm, and can also perform two alarms at the same time. The circuit is simple and practical, and it uses ordinary components and is easy to make.

The circuit principle is shown in Figure 1. R1~R3 ​​and optocoupler IC1~IC3 (4N23) are used for isolated sampling. The light resistance of photoresistor RG should be less than 100Ω and the dark resistance should be greater than 10MΩ. Its function is that the resistance is very small during the day when it is illuminated, and the alarm circuit does not work. At night, the resistance is large and an alarm can be issued. If it is necessary to be in an alert state during the day and at night, RG can be removed. IC4 (NE555) is connected in an astable oscillator mode. Adjusting RP can obtain an alarm frequency of 750~1500Hz (estimated by f=1.443/(RP+2R6)・C1). S1 is a switch for selecting on-site alarm or wireless remote alarm. BL is an 8Ω/0.5W speaker, and LED uses a red flashing light-emitting diode. VT2 and others form an FM carrier oscillation transmission circuit. The parallel resonant frequency of L1 and C5 is the transmission frequency. The alarm signal is received by an FM radio 1km away.

When the transmission lines A, B, C, and D are normal, the optocouplers Ic1 to Ic3 are all turned on. Their output series resistance is less than 100Ω, and the equivalent resistance in parallel with the RG light-receiving resistance is less than 50Ω, so VT1 is cut off. The voltage at the strong reset terminal of Ic4 ④ is less than 1.3V, so IC4 and subsequent circuits do not work.

When any line of the power transmission line is cut off at night, one of Ic1~Ic3 is cut off, and its output series resistance is infinite. Because the resistance of RG is greater than 10MΩ at night, VT1 is turned on, and the high level triggers the ④ foot of Ic4 to start oscillation, the LED produces a strobe, and the BL sounds an alarm. Or after modulating VT2, the radio wave is sent to the distant FM radio for reception.