simple loop alarm for exhibits

very gracious of you.

MichaelG.

Bernard, Something to be aware of with this type of security device is that if you have more than one junction in the loop wire its possible for remove an item from the loop without there being a warning if you have switched it off to remove an item for e.g. demonstration. The answer of course is reconnect asap whilst the demo takes place.

This is probably unlikely for your situation but I know of such an event happening at a show on a relatively large stand where for convenience they had several junctions in the said loop. We used to use these but stopped after this happening to the other vendor.

Forewarned is forearmed !

John

Our club alarm is acontinuous wire loop, but if the loop is disconnected from the battery box, (connector removed or wire cut ) the alarm sounds.

So it sounds like it might be a switched socket, probably 3.5 mm, so that when the plug is inserted, the connection between the battery and the alarm is broken One side of the battery is connected permanently to the alarm, and the other side is fed through the jack plug to the loop where the far end s are joined.. The alarm wire does not actually carry nany current, but if it is broken in any way, the alarm is activated

HTH

Howard

so if someone cuts the wire you won't know?

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.

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.

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.

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

” The alarm wire does not actually carry nany current ”

MichaelG.

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

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.

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.

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.

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

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.

.

Indeed it is

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.

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