Trip Wire Alarm

[Hoipoloi]

I made a pair of these for use when there may be someone skulking around at night while I'm fast asleep in my tent (I don't sleep with my cycle and I have concerns for it's well being).

I could probably have adapted something from a 'pull-pin' personal alarm, or maybe one of those fishing alarms that tell you when your line is taught. Problem is they all work from button cell batteries and I have a strict AA cell only policy for my torches (Eneloop cells and an AA charger running of an Anker 14w solar panel).

So I decided to put these simple alarms together powered by a single AA battery (the other alarm is identical other than a piezo sounder is used instead of a buzzer).

Basically just a series circuit with two switches, one of which is the trip switch. The buzzer (in white) is bottom right and the trip switch is center right. All else should be self-explanatory. The trip switch consists of an ordinary 1/4" audio chassis socket that incorporates contacts that make (close) when a plug is withdrawn. For the 'plug' I used a piece of plastic tubing which is pulled out of the socket when the trip wire is tugged.


To set up one end of the trip wire (10 metres of black 1mm waxed cotton thread) is attached to a tree/log whatever, then routed to where the alarm will be stationed thus determining the monitored area. Any slack in the trip wire is pulled through the hole in the plastic plug, wound around it a few times and then passed through the slit in the end of the plug. The alarm is then anchored to the ground using a small tent peg. Once this is set up the rocker switch is turned on. This rocker switch was added so that the whole thing could be deployed in silence and finally armed when everything is to your liking.

A small twig is used to lift the 'wire' to the desired height.



This unit is not waterproof as it is intended to be located under a tarp or basha, basically next to my ears as the sound is NOT car alarm loud, more like travel alarm clock loud.

[Quercus-robur]

Thanks for the info. I would love to try building one of these. Afraid I'm a bit of a noob when it comes to circuits. What sort of alarm did you buy? Did all the components come with it or did you take parts from something else? How did you put it all together?

Cheers

Qr

[preparedsurrey]

Simple and effective!

I don't know how alarming you want it to be but i have seen 'Alarm mines' used effectively for this - ie stopping people nicking your gear while you sleep, they consist of a spring loaded pin that is propelled into a shotgun blank when the trip wire is pulled. Totally legal and theres no problem buying blank cartridges, our local gun shop has them.
They not only wake you up but scare the living daylights out of whoever triggered it, just don't forget they are there!

[Hoipoloi]

Thanks for the info. I would love to try building one of these. Afraid I'm a bit of a noob when it comes to circuits. What sort of alarm did you buy? Did all the components come with it or did you take parts from something else? How did you put it all together?

Cheers

Qr

The circuit is the easiest to understand. It's a 'series' circuit which means each component is connected to the next one after the other and ultimately returns back to the first component. Effectively a 'loop'.

A battery and a bulb connected as in the diagram below is a series circuit.



The trip-wire alarm follows the same principle but changing the bulb for a sounder, and the addition of what is basically two switches (or breaks if you like) anywhere in the circuit. This means that both switches have to be closed (or ON) for the circuit to work.

The diagram below shows how the trip-wire alarm circuit is constructed. It is laid out to resemble the earlier photo above.



The only thing to be careful of is that whichever sounder is used will likely be polarity conscious, i.e. the positive side of the sounder will need to be connected to the positive side of the battery (if the sounder has a red wire then this goes towards + on the battery.

Components:
The case is what is called a 'project box', available in an assortment of sizes from outlets such as Maplin. In fact all the components can be bought from them.

Battery Holder: I used a single AA battery but using 2xAA batteries is OK (allow for this when choosing the size of box).

Switch: Any small switch will do. A rocker switch is best as it won't protrude so much when carried.

Sounder: This can be a buzzer or piezo. A 3 volt version of each should work OK for a single or two AA batteries.

Socket: A 1/4" mono audio socket. The larger type of headphone socket normally found on home stereos. These sockets have contacts that close (connect) when the headphone plug is removed. This is what we want to happen when the trip-wire plug is pulled from the socket. The mono version has four terminals of which only two are used (normally opposite each other. The stereo version of the socket has six terminals and one of these is acceptable as, again, we only use two of the terminals.

Of course you could use a 3.5mm headphone socket but soldering becomes a little more difficult and finding a suitable 'peg' to act as the plug would also be difficult. The plug (which our trip-wire is attached to) needs to be non-metallic other wise the alarm will sound while the peg is still inserted. Plastic or hardwood are both suitable.

I originally bought two of everything for this project but while cutting out the aperture for the sounder I buggered it so I replaced it with a piezo sounder. This means my two alarms make different sounds. I make the excuse that with two alarms set up I can immediately tell which side of me the intrusion has occurred by the sound emitted.

I hope this explains things clearly, if not just ask. I sometimes assume people understand me which is wrong I know.

[Wonderer]

Great info thanks. Only down side it may attract more zombies lol.

[lightningxl]

Sweet idea - thanks for sharing..