"Bitzer" wrote in message
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Existential Angst wrote:
"MLD" wrote in message
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"Bitzer" wrote in message
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I'm replacing a broken surface mount magnetic switch mounted on my
garage door that controls a signal light in the house that comes on
when the door is open.

Thus, I need a switch that is OPEN (meaning no current flows) when the
magnet is near the switch and CLOSED (meaning current flows) when the
magnet is removed from the switch.

So do I need a normally open switch or a normally closed one? Different
manufacturers/vendors seem to use the terms differently. Is it "normal"
when the magnet is near the switch-- or away from it?

Forget current flows or doesn't flow--that is confusing. Instead, do
you have to close the switch in order to turn on the light? If so, then
you need a normally open switch. Conversely, if the switch is open when
the light is on, then you need a normally closed switch. From what you
describe, I would think that you need a normally open switch---light off
when door is closed.
MLD

Actually, not correct, either in analogy or in answer.

The answer is normally closed -- as others have said, the state with no
outside force acting on it.
Same thing with relays -- IF the relay is "single throw".
If the relay is "double throw", then again, NO or NC doesn't apply, as
both states can be wired in as normal.

A toggle light switch also has two "normal" states, even tho it single
throw -- because the "outside force" is, well, you, so there is no
natural "return position"..
Ditto with any rotary-type multi-position switch, for fan speeds, etc.:
every position is normal.

To the OP, curious as to where you would get this kind of switch -- oem,
from the garage door mfr??
How much $$?? Wouldn't they be able to just supply the right part, or
are you kluging your own solution?

Remarkably, Stormin brought up a good point semantically: "closed" in
switching has the opposite meaning to a valve, ito flow. Go figger --
both the semantic snafu, and that Stormin actually grokked it.

I bought the switch for about two bucks 15-20 years ago at Radio Shack--
along with the wire, transformer and LED indicator light i needed for my
custome designed system. A contractor bashed the existing switch so I've
been to a few Radio Shack stores around town but the clerks didn't know
wht I was talking about. I showed the broken one to one clerk who thought
it was a doorbell. RS seems to mainly sell cell phones and electronic toys
these days-- no more electronic components.

I see lots of them on line and on Ebay for around $10 or so-- but it's
really unclear if they will keep the circuit open when the magnet is near
or away. I guess I'll just buy one of each kind-- an NO and an NC--
and toss the one I don't need.

If you find one that says SPDT or DPDT, then it can be used as either NC or
NO.
Often, the screw terminals or lugs themselves are labeled NO and NC, but not
the switch as a whole.
--
EA