Alarm reed sensors: NO or NC?

What does a test lead with a pair of crocs across the wires to the reed switch, minus reed switch, tell you , when activating the system and opening/clossing croc contact? Usual failure of reeds is permanently magnetised and so closed contacts regardless of any magnet. Degausing coil anyone?

The old old alarms monitored for an open circuit. Back when people ran tape along plate glass windows, for example, a rock through the window would likely break the tape and set off the alarm. So I suspect your reed relay is normally open, i.e. open circuited absent a magnet to hold it closed.

But in normal operation, the reed relay would be adjacent a magnet, which would hold it closed. Thus "normally" the NO relay would be closed.

Is this what actually happens when the relay is next to the magnet -- the circuit opens up? Then there would be a double loop, one where open circuits are checked and another looking for shorts.

Right.

After 15 years the spring to open it may be deformed such that it no longer works.

You must have other sensors -- windows, back door etc. -- see how they function. Do continuity checks (with the door reed relay back in place) on whatever wires come into the alarm module -- see if some are "normally" shorted while others are open. Short (through a resistor) the open circuits to see if that triggers an alarm.

Thanks.

After another hour or so of experimenting I think I've finally got a grip. The difficulty was that there are 4 wires emerging from the reed, all identical. I reckon the arrangement *should* be like this

Reed, closed if magnet is nearby (i.e. door closed), opens on intrusion (door opens). #1------------0----->0------| Active loop | #2--------------------------|

#3--------------------------| Anti-tamper loop | #4--------------------------|

But the old (unreliable) reed was behaving in contradictory fashion when I removed it and tested it. No magnet nearby, it was closed; Magnet nearby, it was open. I remain baffled by that. I can't see how it ever worked (which it did 90% of the time.) Your suggestion about it possibly becoming permanently magnetised might explain it though. Could it actually have therefore 'reversed' its type?

Anyway, I replaced it with one of my several spare NO reeds (i.e. open with no magnet). And, after some trial and error it is now working OK.

This would also explain why none of the sites or catalogs state the type. It's assumed that everyone knows these are *always* NC, and meant to be used in a NC mode.

BTW, I'd have hoped the design of alarm sensor reeds would make them immune from magnetisation!

Thanks. Saw this just after posting my follow-up, replying to N_Cook.

As you see, in that I was speculating whether, if the reed's magnetic polarity had become reversed, that would solve the puzzle. But thinking about it for a few minutes that still doesn't make sense! It would explain my bench test after removal. But it shouldn't have worked in the alarm circuit, and it did. The exception was that it sometimes got 'stuck'. So that on re-entry it didn't trigger the delay warning as it should, until I gave it a gentle tap.

BTW, it's only about 2 or 3 years old - I've been replacing these regularly!

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Typically on a loop circuit we use a normally closed reed contact. Admittedly the terminology is downright confusing but very simply put the reed is open when not in close proximity to the magnet. Therefore the alarm control panel is looking for the loop to open, (door open), and then an alarm will be generated. Most panels can also be configured to have a zone be NO or normally open. So what that means is when the magnet is in close proximity to the reed the circuit is open. Removing the magnet from the reed will then provide a short on the loop. A "closed loop" by definition is a supervised loop. What I don't like about open circuit detection is that if someone were to cut the wires one at a time to the loop, the panel would never "see" it. Open loop circuits are generally used for 24 hour temperature monitoring or holdup (panic circuits). Lenny

'Normally open' means the contacts are open with no power to the relay, = and vice versa.

Many times, relays will have both NO and NC contacts. If the cost & = dimensions are OK for your application, that gives a second option.

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Some reed relays are magnetically biased, and over time become demagnetized.

vice versa.

dimensions are OK for your application, that gives a second option.

He's talking about raw reed switches, not relays.

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Also. over time, if the contact is kept closed by a biasing magnet, the two halves of the reed switch will stick together and require some mechanical vibration to unstick them when the magnetic bias is removed.. Or, if the contacts break current repeatedly, the mating surfaces can become pitted and stick together when closed, and fail to release when the magnetic bias is removed. We used reed relays in several versions of the first generation of Bell Labs-designed electronic switching systems, and sticking switches were a continuing source of concern.

Thanks all. But see also my follow-ups to N_Cook and Spamtrap1888.

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The other thing to keep in mind with magnetic reed switches commonly used on doors and windows is fusing of the contacts due to a close lightning hit. Lenny

That was premature ;-(

It is all working OK - except for the crucial feature I took for granted but didn't test, the re-entry delay. Came home today, opened front door, and instead of a 30 second warning while I disarm it with my code, the siren sounded immediately.

I think I must have damaged that section of circuitry while soldering in the reed. I did check that the passive voltages were zero, but that's the only cause I can think of.

It's a very old model, an Autona 2000 Series. Tomorrow I'll attempt contacting Autona Ltd, Princess Risborough, but research this evening indicates they're no longer trading.

-------------------- Any UK recommendations for a simple, wired (not wireless) unit to replace it please?

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Stop right there. That is a managed sensor. Replace in kind only. = Look up make and model and then find an "acceptable" substitutes list if you cannot find an exact replacement.

Will now read the rest of the thread.

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Inaccurate. See NFPA 72.

relay, and vice versa.

dimensions are OK for your application, that gives a second option.

The form C configuration has been available in magnetic reed switches for many decades. It is not common.

?-)

In every alarm panel I have installed, the contact being closed is "secure" and open is "alarm". That way, if/when someone/something cuts the wire, it's an alarm.

Further, there is a series resistor of say 4.7K ohm. So of someone shorts the loop say while in the store during the day; you will get an fault at close up.