simple loop alarm for exhibits

[Howard LewisParticipant @howardlewis46836]

If someone cuts the wire, the circuit is broken and the alarm sounds

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Howard Lewis on 01/10/2023 18:17:16:

If someone cuts the wire, the circuit is broken and the alarm sounds

.

… that one has me bewildered

If there is no current flowing in the first place, what is it detecting when the circuit gets broken.

Sorry Howard, I just don’t understand … can you explain, please ?

MichaelG.

[bernard towersParticipant @bernardtowers37738]

Pull the pin is ok but what if the wire is cut?

[Michael GilliganParticipant @michaelgilligan61133]

Posted by bernard towers on 01/10/2023 19:08:04:

Pull the pin is ok but what if the wire is cut?

.

Nothing much … That’s why I referred to it as a quick&dirty option and mentioned “chain/wire/string/ whatever”

MichaelG.

[John Doe 2Participant @johndoe2]

Obviously, any device using electrical or electronic detection, must send current through the wire, otherwise it cannot detect if the wire is cut – (unless the "wire" is an optical fibre).

With electronic detection, the current only need be a few milliamps – the amount required to hold a gate or a transistor on/off.

With an electrical system, the current will need to be more to hold a mechanical relay on/off, but still only milliamps.

[Michael GilliganParticipant @michaelgilligan61133]

That much I understand, John … but Howard seemed quite explicit:

” The alarm wire does not actually carry nany current ”

MichaelG.

[duncan webster 1Participant @duncanwebster1]

I think the alarm relies on a plug holding a N/C switch open, pull out the plug, the switch closes and the alarm sounds. The wire could be string. Not how I'd do it

[MacolmParticipant @macolm]

It seems to me that a simple wire loop alarm is none too secure these days. It would be all too easy to bypass a section, for example using a length of wire with pins attached at each end, with these simply pushed though the insulation of the loop so as to contact the wire inside. Then cut the loop wire without triggering the alarm.

A possibility to improve security would be to include a resistor at each item to be protected. Thus a length of wire, each with the resistor in the middle, for each item, would be looped through it to conceal the resistors. All would then be connected in series as before.

Now an electronic circuit would be required with a comparator against a narrow window of total resistance, so that shorting out any resistor would trigger the alarm. It would be simple to have a potentiometer that could set the window thresholds to cope with the actual number of items being protected. This addition circuit would only require one integrated circuit with ancillary components, as well as whatever is already needed for the alarm function.

[duncan webster 1Participant @duncanwebster1]

But that would take a fair bit of time, so unless the stewards area sleep the simple method is a lot better than nothing, provided it reacts to a cut wire.

[MacolmParticipant @macolm]

With a pre assembled single length of wire with resistors incorporated it would indeed be tedious. With a better detection system though, I envisaged that short lengths with central resistors, and plugs and sockets, would be procured. Each item owner would fit their wire, then simply a case of plugging together. Easy, also to remove one item, and set up the protection of the remaining items.

[duncan webster 1Participant @duncanwebster1]

I meant it would take the thief some time. Having resistors in line is potentially better as you say, but a lot of plugs /sockets if each model on display had it's own

[Michael GilliganParticipant @michaelgilligan61133]

[John Doe 2Participant @johndoe2]

I am curious, (and have obviously led a sheltered life !), What sort of a display would need this sort of protection, and are the displays often left unattended by the owner/stall holder? (toilet breaks, I suppose).

What sort of things do people steal? do they really steal things like tiny model spades etc ?

Having an alarm implies that the stall owner will not be standing next to the display, so how will they catch a thief even when the thief sets off the alarm – surely by the time the owner has returned to the stand, the thief will have mingled into the crowd?

What a sad World.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by John Doe 2 on 02/10/2023 15:52:28:

What a sad World.

.

Indeed it is

[Robert Atkinson 2Participant @robertatkinson2]

A loop alarm is a deterrent at all times and an aid when stand is attended. You can't watch everything all the time.

Electrically a loop pases current all the time and alarms if the current stops. Simplest implementation is a a relay. Loop is in series with the coil and power source. Buzzer / sounder is in series with nomally closed contact. Disadvantage of this is the relay coil uses a fair bit of current and is powered all the time so limits life on batteries.
At least one professional system I've seen used 3.5mm "headphone" jack plugs and in-line sockets with screened cable. The center conductor is the loop and the screen is connected to 0V. This can use cheap off the shelf audio extension leads. A low current (limited by a resistor) passes through the center conductor and keeps a two transistor circuit driving the sounder off. If the circuit is broken current stops and alarm sounds. The use of screened cable makes it very hard to bypass a section by pins or clips. If you push a pin through the cable it will short the center conductor to 0V diverting the current and causing the alarm to sound.
If you want I can do a sketch….

Robert.

[duncan webster 1Participant @duncanwebster1]

The problem is club stands where there are lots of valuable models. Quite a few reported instances of things like Stuart Turner Models disappearing, easily slipped into a bag whilst the steward is chatting to an interested onlooker, and then practically untraceable, sold on ebay a few months later where they will fetch Several hundreds of pounds. Perhaps we should engrave a postcode onto our models if we want to display them, but having a frame number on your pushbike doesn't stop it getting pinched. Mine was lifted out of a back garden when #2 son couldn't be bothered locking it up.

And yes, the low life will steal anything

Edited By duncan webster on 02/10/2023 17:55:07

[Jon LawesParticipant @jonlawes51698]

I always stamp my postcode into my boilers as part of the boiler number.

I better not move house!

[bernard towersParticipant @bernardtowers37738]

Sorry J D but I don’t know where you’ve been hiding but this sort of pilfering has been going on for years hence the fact that most mME clubs who exhibit at exhibitions have their own loop alarms as even the most eagle eyed stewards cannot watch everything all the time. Pretty sure I have seen a club stand with cctv at one of the big shows. I had a friend exhibiting homemade aero engines at forncett and these were wired with Bowden cable but odd cranks rods pistons and rings laid on the stand, these are the sort of things that go.

[John Doe 2Participant @johndoe2]

Strewth. Why are people such sh*ts these days? There is organised pilfering too, where people just go behind the tills at shops and take fags and booze – threatening staff if they try to prevent them.

Stop the World – I want to get off !

It would seem difficult to run a wire through every little component. I guess that exhibitors will have to build perspex boxes over their displays, and serve customers like they used to in shops; from display cabinets only open to the inside of the stand.

[Howard LewisParticipant @howardlewis46836]

Fortunately, our club has only experienced one instance of theft.

My wife fund raises for a charity and has had lots of stuff stolen, that has to be really low!

Eventually, you get a sense of who to watch, groups of youngsters, in particular.

The worrying thing is that sometimes workshop tooling, as well as models, might be stolen, and then abandoned because the thief has no idea what it is, or is for. But the owner is still deprived of the item and the material, time and effort put into making it.

As Stewards, you have to keeps your wits and eyes about you. As you say. be aware of distractions!

Howard

[Michael GilliganParticipant @michaelgilligan61133]

Howard

As you are back on this thread … I would be most grateful if you could answer my earlier question

01/10/2023 18:34:11

… it’s really niggling me … have I missed a very clever trick ?

MichaelG.

[Howard LewisParticipant @howardlewis46836]

Michael,

I don't reaslly understand details of the alarm, (The club bought it), so tend be a user rather than knowing much about it.

If the circuit is broken for any reason, the alarm sounds is about the extent of my knowledge, so assumed that it relied on a switched 3.5mm jack socket.

Howard

[MacolmParticipant @macolm]

It could be a voltage reference or a logic pattern feeding the wire loop, and a check that the received voltage or signal is the same. This could all operate at minimal current both in the wire and in the circuitry, so allowing a long battery life.

[Simon Williams 3Participant @simonwilliams3]

I'm a bit late to the party, but FWIW I'd imagine something like this:

The loop of wire carries a small current of about 2 mA until it is broken, whereupon the RON sounds. At the very least it advertises to the stewards they they need to look over their stall carefully. You can't sit there all day long concentrating on everything, you'd go barmy. You do need to test it periodically say twice a day as it's not failsafe.

If anyone wants the bits to make something like this PM me, I've got only one RON so first come first served. Alternatively if it helps I can point to suitable components on ebay if that's OK with the mod's.

Note there are two basic types of piezo squawkers, one makes a continuous noise, the edgier the better, the other needs an oscillating signal to drive it as it is intended as a low cost speaker. Only the first type is compatible with this simple cct.

I've seen something along these lines with the loop made of sections of coax TV connectors. That way you can have as many sections as you wish. The lengths of cable sold for ear phone extensions with 3.5 mm jack connectors would also suffice. You need something that shouts to the scum bags that there ain't no way you're going to =get away with anything while not becoming the focus of the exhibit.

[duncan webster 1Participant @duncanwebster1]

That's pretty much what mine is, but I think I remember using a thyristor instead of a transistor so once triggered it keeps on blaring away until you disconnect the battery. having a little local difficulty retrieving it from the ex club member who has it at his house, will send in the heavy mob soon!

I put a few banana plug/sockets in the loop to keep lengths manageable, you need a surprisingly long bit to go through a lot of models

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