The circuit in Figure 1 sounds an alarm whenever the perimeter-protection line-a thin copper wire of appropriate length-breaks. 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 resistancernis 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.
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