On May 22, 2:19*am, Terry Pinnell wrote:
"Michael A. Terrell" wrote:











Terry Pinnell wrote:

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...-plastic-cont/....

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.

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

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

--
Terry, East Grinstead, UK

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