"N_Cook" wrote:

Terry Pinnell wrote in message
.. .
I've just removed the alarm reed relay sensor from the frame of my front
door because it's become unreliable, often 'sticking' in its passive state
instead of triggering the alarm. But I now face some ambiguity about
ordering a replacement, because none of those I've found, like these


http://cpc.farnell.com/elmdene/4bp/4...c-cont/dp/SR02
970

actually specify whether the contacts are NO or NC. And the phrase itself
appears to mean different things to different people!

The sensor I removed is what I call NC, because (with no magnets in sight)
my continuity tester gives me a signal on the two active wires. But of
course when the sensor is in position and the door or window holding the
magnet is closed, so that the magnet is close to the reed, then the state
is reversed, so the circuit is open. If an intrusion occurs, the circuit
is closed again, triggering the alarm.

When I asked the tech support guy at CPC he described the above Elmdene
sensors as NC, which was what I wanted to hear. But on further checking it
turned out that by 'normally' he meant the state when the alarm is set,
awaiting possible intrusion. So, in MY terms, these are NO!

And the guy at Elmdene confirmed this, adding that the 'normal' state
required for an alarm sensor is that it OPENS the circuit when intrusion
occurs. (Makes sense, because cutting the wires would then trigger it.)

So the only conclusion I can reach is that my 15 year old alarm is
different in that respect. Or that I installed it wrongly. Or that there
is some setting on its circuit board for switching between the two
'trigger states'.

Any insight or advice would be appreciated please.

--
Terry, East Grinstead, UK


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?


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!


--
Terry, East Grinstead, UK