RBM wrote:
"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?

As an electrician, I've been through this numerous times over the years. It
clearly depend on who you are talking to, and what trade they're in. IMO,
the "normal" position is the state the switch is in when nothing affects it.
Alarm people have the opposite take on it.

I agree.

In all the following the contacts are closed with the magnet next to the
reed switch.

From an alarm perspective *NC* ("normal" is when the door is closed):

http://en.wikipedia.org/wiki/Burglar..._control_panel
"Most switching devices are N.C. (normally closed) circuits, so when the
device is not in an alarm condition, the circuit is closed."

http://www.alibaba.com/product-gs/21...ch_5C_31A.html
"Output signal: normally closed (switch contact is closed when the door
closed)"



From a component manufacturer *NO*

C&K: "ELECTRICAL CIRCUIT: SPST NO (Contact Form A). Reed switch
opens when magnet is removed from proximity. Contacts are
held closed when magnet is within actuation range."


To know what the switch does you need to have a description like all of
the above.

My solution is to always buy
devices that come in the "form C" which is both open and closed circuits,
this way you can't go wrong

For the magnetic switches I looked at from alarm companies, none said NO
or NC. A few said NC loop. They were probably all "NC" from an alarm
perspective. Except a few were Form C.

--
bud--