A tripwire alarm is a light line across a route that releases a noise maker when something pulls it, and it guards only the ground it crosses: three routes 3 m wide on a ring of 150 m radius are 9 m of 942 m, under 1% of the ring. Use monofilament or thread set so it gives way rather than holds, test the sound from where you sleep and expect false alarms from wind, animals and weather.
Part of Early Warning Systems
A tripwire alarm is a thin line stretched across a path, anchored at both ends and connected to a release that sounds a noise maker when the line is pulled or broken.
A Wire Watches A Route, Not A Ring
Thin line across a trail is the simplest mechanical alarm, and the parent article calls it the most reliable. The advice there is monofilament fishing line, nearly invisible and weather-resistant, set 15-25 cm above the ground, low enough to catch a leg and high enough not to be triggered all night by small animals, with extra lines at 40 cm and 80 cm and crossed lines in woods, attached to a noise maker such as cans, bells or a board that tips.
It guards only where it is strung. A person who walks round the end of the line, or steps over it with a lifted foot in daylight, passes. The place to put wires is where the ground forces people through a narrow gap: the lanes of Channeling Approach Routes, gaps in hedges and the approach to the gate.
What You Need
- Line: monofilament, thread or thin cord. Monofilament may stretch when wet and slacken, so check after rain.
- Anchors: stakes, posts or tree trunks, firm enough that wind does not move them.
- A release that gives way: a peg, matchstick or clothes-peg set so the line pulls it out of its notch.
- A noise maker that you can hear from the sleeping place. See Noise Makers.
- A map of every line, held by the household.
Method: Build The Release First, Then The Line
Step 1: Build the noise maker and the release on a bench. The line pulls a small peg out of a notch; the peg was holding up a weighted can lid or a hanging bell clapper; when the peg comes out, the weight drops a short distance onto a metal lid or flat stone. Test it with a hand pull.
Step 2: Set the release force light. A light pull should work it. The line must come free of the stake or break before it can arrest a leg.
Step 3: String the line at the heights given in the parent article across the route, with the noise maker anchored to one post.
Step 4: Tune out the false alarms. Wind and small animals trigger lines. Raise or tighten it, or add a shield, until the noise is rare.
Step 5: Walk it in the dark yourself. Anything that makes you stumble or reach for a hand-hold will also catch a child.
Worked Example: What One Wire Covers
A ring of 150 m radius has circumference 2 × π × 150 m = 942 m. Suppose three routes lead through it, each 3 m wide at the gap where you put a wire. The wires cover 3 × 3 m = 9 m, and 9 ÷ 942 = 0.0096, which is 0.96%, under 1% of the ring. The rest of the ring is guarded by the barrier, the animals and the lookouts. Wires are useful because routes are narrow, not because they cover the circle.
Failure Modes
- Stepped over or swept aside. A person carrying a light can see a taut line, and one with a stick can sweep ahead. Lines work best on the unprepared and on animals, and they are not the only layer.
- Weather. Wet monofilament sags, frost stiffens it, dew and leaf litter hold it down and growth lifts it.
- Animal triggers. Deer, dogs and stock cross at heights above the lowest line; keep a log of each trigger by time and date to separate animal patterns from the exceptions.
- Missed alarm. If the release does not reset, the next intruder passes. Reset after every trigger.
Common Mistakes
- Tripwires at a path the household uses. A line left across a route someone walks at night is a fall waiting.
- A single wire. Use crossed lines or several at different heights, so a step over one meets another.
- No log. Without a record of triggers you cannot tell the signal from the noise.
Safety
The parent article's flare and cap rigs are not recommended
The parent article suggests a tripwire pulling a flare ignition cord, or springing a mouse trap with a cap on it. I have not written those up: a flare lit by a trigger burns whoever is next to it and can start a grass fire, and a snapping mouse trap injures a hand that finds it. A tripwire should produce only a noise. Do not string wire at neck height, across a path used by riders, or where children play, and never leave a line at ankle height across a household route at night.
What’s Next
- Learn next: Noise Makers — what the line should set off
- Combine with: Track Traps — tells you what crossed, even when the wire missed it