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?
Wiki didn't have the answer to this, so I provided it.
In switch language, open means to disconnect, or break the
circuit. Some people call this "turned off". Closed, is to
connect, or make the circuit. Some people call this turned
In the case of a normally open switch, the switch is open
(off) unless something takes action on the switch. Pushes a
button for example, or holds a magnet near the switch, or
other active force. A normally closed switch is closed (on)
unless some force causes the switch to be opened.
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.
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
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.
re: "Radio Shack seems to mainly sell cell phones and electronic toys
these days-- no more electronic components."
Well, since you brought it up...
I was working at an event this weekend and somebody broke the 1/4 mono
jack for one of the loudspeakers.
I went over to the nearest Radio Shack to get a new jack. Since the
young lady was nice enough to approach me and ask if she could help, I
started to explain to her what I needed - quickly noticing the glaze
that was coming over her eyes.
Her response was "You'll have to speak to one of those guys. I know a
lot about cell phones, but not much about that kind of stuff."
So what's the deal? Does Radio Shack *expect* that everyone that walks
into the store wants a cell phone? If not, why would someone who knows
nothing about their other products approach me?
On Thu, 29 Apr 2010 13:49:12 -0700 (PDT), DerbyDad03
Radio Shack should expect everyone to buy a cellphone.
They left me with a bad taste in my mouth when I bought my first
compute from them.
I used to buy everything at Radio Shack. I can't remember the last
time I have been in one of their stores. Several years.
I know your situation was that you needed something that day. I let
Newegg bring the stuff to my door.
I went to a Shack store in a small town outside of Birmingham the
other day to get a toggle switch to repair a neon sign for a store
belonging to a customer of mine. The Shack (new name) has nowhere
near the number of electronic parts they used to carry but I can
usually find what I need because the morons there have no clue
what parts they carry. I worked for their repair division more than
30 years ago, I wound up with all kinds of discrete components.
re: "I let Newegg bring the stuff to my door."
As you surmised, I needed the jack rather quickly.
"Normally" (thread-based pun intended) I go to the one remaining
*real* electronics parts store in my town where if they don't have it,
they can get it the next day - without trying to sell me a cell phone.
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.
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