Terry wrote:
Can anyone point me towards a UK source (phone or online) for an
inexpensive cylindrical, enclosed reed switch, N/C, for use in a
burglar alarm circuit please.

William Sommerwerck wrote:
Aren't there any electronic-parts distributors in the UK that consumers can
purchase from?


Plenty, but he's looking for the wrong thing.

There is no such thing as a NC reed switch. The induced magnetism in the
reeds *always* attracts them together when you bring the magnet up, and
never repels them so all two terminal bare reed switches are NO.
It is possible to put an additional non-magnetic reed in the capsule, so
that one of the magnetic reeds is in contact with it until an external
magnetic field is applied, but as the reeds have to be brought out
through the pinch of the glass capsule to support them, this provides a
SPDT switch, not a NC one although it can be used as such.

If he googles 'SPDT reed switch' (without the quotes), both Rapid and
Maplin stock them. but for alarm use, I'm fairly sure he wants a
complete reed sensor and magnet pair for a NC loop, NOT a bare reed capsule.

If he's looking for a switch/magnet pair he can mount in the edge of a
door and its frame in drilled holes, googling 'recessed reed switch'
will find plenty, all NC in the presence of their magnet (i.e. with the
door closed). Again Maplin have some.

If he wants a true NC action, he's either going to have to stick a bare
SPDT reed in a cylindrical housing e.g a plastic tube from a biro with a
dab of hot glue or mess around with a biasing magnet to cancel out the
field of the actuating magnet as it approaches. Enclosed SPDT reed
switches, although nearly as rare as hens teeth, are available but
apparently not in a cylindrical recessed format.

--
Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED)
ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk
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