Mechanical Intruder Alarms.

Expedient intruder man traps are as old as man himself and many were reinvented during the Vietnam war. The warning device in this case being a soldier caught in a trap, screaming in pain until they died.
Mechanical Intruder alarms and trap guns preceded more recent types by several hundred years. The earliest practical examples used wheel-lock mechanisms, followed by flintlocks, but it was not until the introduction of the percussion cap around 1837, that such alarms and traps became reliable and cheap. At first glance mechanical alarms may seem archaic in the electronic age but they do have advantages, in that they do not use batteries, cannot be destroyed by an EMP, can work in the rain and not least of all … ‘Remain available’ .  
Perhaps I ought to expand on my ‘remain available’ remark.  Basically … anything that goes bang in the military is covered by Ammunition Regulations, which means live ammunition, empty cases and blank cartridges are strictly controlled and anyone coming in contact with them has to make a formal declaration at the end of any training period … “I have no live rounds, or empty cases in my possession !”. So when designing any kind of mechanical alarm for the military, one has to take this into consideration.
Another consideration is that any device used as an intruder alarm could also be used as the trigger mechanism for a bomb and this also must be taken into consideration. The generally adopted solution is to simply “meet the threat” and that means keeping counter-measures simple  and not using hi-tech devices until you have to, since one does not want to unnecessarily educate the enemy and escalate a security situation.  An example of this happened in Africa, where at one stage I found myself disarming terrorist devices, because the person who’s duty it was had no idea about IED design. At one stage I asked him what type of anti-handling devices he was finding and his reply was “What’s an anti handling device !”.  So I made him up a simple one, basically a clothes peg ‘switch’ in a Tupperware box, lift the lid and the switch operated. About two weeks later this idea turned up in a real bomb !.  He was running an explosives course for the local police and wanted to impress them with his rather limited experience … showed them how to make the clothes peg anti handling switch …  In effect educating the enemy.  To counter this the powers that be try to suppress any ‘interesting’ ideas and classify them at first sight.
The end result of the above is that in the past when troops have to go into action, they are often ill prepared, particularly in defensive perimeter intruder detection devices. This usually means that the serviceman in the field has to resort to  making his own, using empty beer cans with stones in them, tied up to string ‘trip wires’. Yes, one man RADAR intruder sets exist,  as do trip flares etc, but the only device that is effective in the field is the one the man on the spot has !.  Anything that exists, but is ‘classified’ and un issued may just as well not exist !.  At one stage a little plastic box containing a trip wire switch, an LED and a battery was so secret it could not be issued until a ‘real’ war started !.
That was why an RAF officer approached me and asked me if there was a simple replacement for the empty beer can intruder detector, when multi-million pound aircraft were deployed in the field, with never enough personnel to properly cover the tactical defensive perimeter.

Practical Mechanical intruder alarms.

With any alarm system one has to decide whether it should be ‘active’ or ‘passive’.  Active devices are normally referred to as ‘alarm guns’. Many used a slack ‘trip wire’ that when touched caused a simple gun to rotate and fire a shot along the ‘trip wire’.  Many of these were use to ‘deter’ poachers in the 1800’s. They were called ‘active’ devices because they dealt with the intruder problem, in real time and did not require additional human intervention, except for the occasional disposal of bodies. Passive alarms simply warn the human element that an intrusion has taken place and that an action is required.
If we now leap forward to present times, the world is full of electronic alarms triggered off by birds, the wind, the sun , a passing truck etc. and about 100% of these alarms are completely ignored by passers by and the police . We have simply become immune to electronic alarms and they probably frighten thieves as little as they do us.  So the question is what would an intruder respond to ?. In the civilian case an explosion that sounds like a firearm being discharged or a bomb going off, might produce a response because it is still relatively unusual (at this time). Since such a sound may elicit a police response the intruder might decide to make a quick exit, before armed police turned up.
The military situation is a little different because the opposition is known to be armed and if I was the one crawling along the ground in the dark, with no idea exactly where the enemy were and something like a rifle shot happens near me … I am going to respond with return fire … thus further adding to the alarm situation, when everyone else joins the fire fight !.  So although a percussion mechanical alarm is ‘passive’ it can cause the intruder to react in a ‘noisy’ fashion.
If we consider aircraft deployed in the field, it would be prudent to consider a 360 degree perimeter. This perimeter is then divided by the number of personnel one can have on any one stag and that gives us the length of the perimeter arc that each ‘guard’ has to cover. The next question is how many intruder devices does each individual guard have to ‘habitually carry’ in order to cover that length of the perimeter ?. Remember that anything un-issued does not count because it does not ‘exist’ and second, because it does not ‘exist’ … no-one has been trained to use it !. The point I am trying to make here is that every deployed serviceman should ‘habitually’ carry the required number of intruder detectors, to support any arc of the perimeter that they may be called upon to defend. It therefore follows the devices must be small and light … and free from ammunition regulations if they are to be  ‘habitually carried’ during their service.

Ammunition Regulations.

Maybe a good starting point is to question what items are actually covered by the existing ammunition regulations ?.  Let us specifically consider blank ammunition. I can walk into any civilian gun shop and purchase (without license) blank cartridges in any calibre  up to 12 bore and that includes  0.22”, 0.25”, 8.0mm, 9.0mm and 12 bore blanks.  Yet, as a serviceman in possession, I would be guilty of an offence that does not apply to civilians.
In response to the RAF request I did produce about a dozen  proto-type devices, most of which fired a 0.22 long rifle blank (available everywhere). Since I also have a wicked sense of humour, I also produced one that fired a kiddies toy percussion cap and that was the one they accepted !.

The one on the right was my final version that I liked  (0.22). The one on the left (and below) was the one accepted because  it used kiddies plastic caps  …  that circumvented ammunition regulations !.

As a matter of interest, I use 6 lb nylon fishing line for trip wires as they are just about invisible in use and take up very little space in a pocket. They are also much more difficult to detect with ‘feeler’ wires.  A ‘feeler wire’ is typically made of a length of 1.0mm piano wire with a finger ring formed in one end. The idea is that you place your index finger in the ring and the wire rests on the second finger which senses anything touching the feeler wire.   Normal military trip wires are designed to bring down a running man, so they hardly fall into the ‘sensitive’   category … and give a nice sensual  ‘ping’  when touched with the feeler wire.  One time when a trip wire can be best seen is just after dawn,  because dew precipitates on metal trip wires, but to a much lesser extent on plastics such as nylon.