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Circuit may protect property and life

03 Jul 2015  | Vladimir Oleynik

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The circuit in figure 1 sets off an alarm whenever the perimeter-protection line—a thin copper wire of appropriate length—is breached. You can get the thin wire from a used relay coil. Place the wire around a tent, a camping area, or another area. The wire is thin and invisible in the dark, especially when you place it in grass as high as 0.5m. Because the wire's resistance is small relative to the 200-kΩ resistor, the two form a voltage divider that keeps the MOS transistor off. When a human or an animal intruder breaks the protection wire, a transistor switches on and a buzzer sounds. Because the blinking LED controls the transistor, the buzzer sounds at the LED's switching frequency. The protection wire's resistance can be as great as 33 kΩ.

Figure 1: When the perimeter wire breaks, an LED lights and an alarm sounds.

The circuit operates at a battery voltage as low as 4.8V. In standby mode, it consumes less than 50µA. When the buzzer sounds, the circuit consumes about 30 mA using a 9.6V battery; this value decreases while the battery drains. The circuit uses a 9V-dc Varta Longlife Extra alkaline battery and operates in standby mode 24 hours a day, with a test alarm switching on for 10 seconds daily. Under those conditions, the daily battery voltage drain is 0.02V, so you can estimate battery life at about eight months while the battery voltage declines from 9.6V when the battery is new to 4.8V when it is exhausted. If there were no alarm switching, the battery would operate for several years.

Figure 2: Adding a P-channel MOSFET cuts current consumption.

The circuit in figure 2 reduces current consumption to less than 0.5µA and lets you use longer protection wire, thus enlarging the protected area. The protection wire can have resistance as great as 3.3 MΩ, and the circuit still works. Thus, you can calculate a possible perimeter-protection-wire length according to the appropriate wire diameter and copper resistance (reference 1). Both circuits can stay in standby mode for years.

"Reference & Information, AWG Cable Description," American Wire Gauge.

About the author
Vladimir Oleynik is from Moscow, Russia.

This article is a Design Idea selected for re-publication by the editors. It was first published on May 13, 2010 in

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