On Thu, 29 Apr 2010 11:56:39 -0400, Bitzer wrote:

Existential Angst wrote:
"MLD" wrote in message
...
"Bitzer" wrote in message
...
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.
Go to the Borg or any other reasonably well stocked shop and get a
normally closed magnetic alarm switch. Install it so the magnet (on
the door) is in proximity to the switch with the door closed, and the
light will come on when the magnet leaves the switch.