Can anyone point me towards a phone or online supplier for an inexpensive, cylindrical, enclosed reed switch, Normally Closed, for use as door sensors in a burglar alarm circuit please.
I'm still googling but so far without success and want to place an order for delivery Monday. Thought I'd found one but it's a Normally Open type. (BTW, I was surprised how many don't specify that crucial detail.)
Not being funny but do you really want a NC switch ? That would infer, when the door is closed, the switch would be open.
Normally, the switch would be closed when the door is closed. i.e. a NO switch. Sorry if that's incorrect.
In message , Terry Pinnell writes
When you say Nc do you mean when the door is closed? This would be typical as it then goes open when the door is opened.
Most are No until the magnet is placed near them.
Maybe?
Normally closed (i.e. closed when the door is open) would be more likely. Maplin lists alarm reed switches here:
In message , Frazer Jolly Goodfellow writes
Eh? To quote Maplin:-
"The reed makes contact when subjected to a magnetic field."
i.e. when the door is closed and the reed next to the magnet.
So that you can have a number of them wired in series and any one opening will be detected.
Two options. Replace the reed switch in a normal one with what you want - changeover types are available. Or since most alarm cables have 12 volts available for powering sensors, use a transistor to swap the action.
Eh? indeed. Invert everything I said. Also means that snipping the wire will trip the alarm.
i dont know much about it but i think
Thanks all. I was indeed wrong about the type, of course. In mitigation, there's some ambiguity IMO over the meaning of 'normally open'. I'd forgotten that the magnet was (normally!) keeping it closed! I was regarding 'normality' as the state when my doors are closed ;-)
I did realise my mistake soon after posting and duly ordered a few from Rapid Electronics
Seems you've found the solution. However I've used these NC devices in the past and they consist of a standard reed switch with an attached 'slave' magnet which holds the contacts closed. The actuating magnet has to be correctly positioned to cancel the field of the slave when the two come together.
As might be expected, not straightforward to set up.
GrahamC
If someone really needed a magnetic switch which worked the other way around, there are changeover reed switches available, so you have a NO and NC contact. (They aren't used in burglar alarms though.)
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