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...ont/dp/SR02970

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

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?

On May 21, 7:37*am, 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 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.



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.

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.


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.)

Right.


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.


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.

"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

spamtrap1888 wrote:

On May 21, 7:37*am, 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 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.



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.

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.


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.)

Right.


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.


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. 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!

--
Terry, East Grinstead, UK

On May 21, 3:42*pm, Terry Pinnell wrote:
spamtrap1888 wrote:
On May 21, 7:37 am, 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 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.

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.

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.

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.)

Right.

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.

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. 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!

--
Terry, East Grinstead, UK

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.


"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...ont/dp/SR02970

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

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...ont/dp/SR02970

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.


--
You can't have a sense of humor, if you have no sense.

Guv Bob wrote:

'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.


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


--
You can't have a sense of humor, if you have no sense.

On May 21, 5:29*pm, klem kedidelhopper
wrote:
On May 21, 3:42*pm, Terry Pinnell wrote:





spamtrap1888 wrote:
On May 21, 7:37 am, 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 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.

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.

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.

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.)

Right.

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.

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. 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!

--
Terry, East Grinstead, UK

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- Hide quoted text -

- Show quoted text -

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.

"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...ont/dp/SR02970

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

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

Terry Pinnell wrote:

"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.

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?

--
Terry, East Grinstead, UK

On Mon, 21 May 2012 15:37:41 +0100, 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...ont/dp/SR02970

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

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.

?-)

On Mon, 21 May 2012 15:29:24 -0700 (PDT), klem kedidelhopper
wrote:


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!

--
Terry, East Grinstead, UK

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.

Inaccurate. See NFPA 72.

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

On Mon, 21 May 2012 20:58:49 -0400, "Michael A. Terrell"
wrote:


Guv Bob wrote:

'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.


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

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.


--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433