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Old 03-02-2011, 12:20 PM   #1
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Magnet reed switch turns on lights under sink?


Does anyone make a magnetic reed switch which will close when its magnet on a cabinet door moves away from the switch?

We have many white LED Christmas light strings which would illuminate under-counter cabinet interiors.
One switch would be on each cabinet door.
The 50-light string draws 0.04 Amps, 4.8 Watts.

Thank you.

Robert

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Old 03-02-2011, 12:31 PM   #2
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Magnet reed switch turns on lights under sink?


you can do a simple Google search and come up with a lot.

http://www.google.com/search?hl=en&s...&aqi=&aql=&oq=

or Digi-key http://search.digikey.com/scripts/Dk...ch+n/c&x=0&y=0

a commonly used switch that would work would be a door or window switch used for alarm systems. Just make sure you get it in a N/C (normally closed) configuration and of course, be sure it is rated for the current and voltage present.

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Old 03-02-2011, 12:44 PM   #3
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Magnet reed switch turns on lights under sink?


Thanks.
According to their terminology, now realize it must be N/O - meaning the contacts are open when magnet is near.
The contacts close when magnet is away, which is opposite of what most window andoor sensors do.
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Old 03-02-2011, 01:05 PM   #4
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Magnet reed switch turns on lights under sink?


no. you want a N/C contact. That means it is in the closed position when relaxed or not being controlled by any outside force.

the magnet being near will hold the contacts open and opening the door (moving the magnet away from the switch) will release the switch to a closed position.

and yes, that is inverse to MANY alarm contacts since many alarm system uses N/O contacts that are actually closed when in the "set" or normally operating condition. They do this so the circuit can be monitored and if the wire to the switch is cut, it will trigger an alarm. The system is designed to utilize that closed condition to "see" the window as being closed. Removing the magnet that holds the contact closed will trip the alarm by opening the circuit.

but there are alarm contacts that operate as you need as well. You just have to be sure of sourcing the proper switch. In fact, some switches have contacts that are both N/C and N/O on the same switch with a common terminal used for both sets of contacts.


the only exception I have seen to this type of designation is the contacts used in a sprinkler alarm system. They label their contacts for the condition experienced in normal operating position which is inverse of the true listing type of the contact.

Last edited by nap; 03-02-2011 at 01:11 PM.
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Old 03-02-2011, 01:19 PM   #5
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Magnet reed switch turns on lights under sink?


"Reed Switch: Normally Closed (when magnet is within actuation range)"

Is this a misprint?
Thanks.
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Old 03-02-2011, 01:51 PM   #6
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Magnet reed switch turns on lights under sink?


where did you get that?

here is an excerpt from Wikipedia on reed switches:

Quote:
The reed switch is an electrical switch operated by an applied magnetic field. It was invented at Bell Telephone Laboratories in 1936 by W. B. Ellwood. It consists of a pair of contacts on ferrous metal reeds in a hermetically sealed glass envelope. The contacts may be normally open, closing when a magnetic field is present, or normally closed and opening when a magnetic field is applied. The switch may be actuated by a coil, making a reed relay,[1] or by bringing a magnet near to the switch. Once the magnet is pulled away from the switch, the reed switch will go back to its original position.
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Old 03-02-2011, 01:55 PM   #7
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Magnet reed switch turns on lights under sink?


From the specs. for one the reed switches advertised which you kindly linked.
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Old 03-02-2011, 02:26 PM   #8
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Magnet reed switch turns on lights under sink?


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Originally Posted by Robert Gift View Post
From the specs. for one the reed switches advertised which you kindly linked.
I went to a few of the sites as well. I can't account for their labeling. I saw it on a couple different sites. The terms are well defined in the industry though. A N/C contact is a contact that has continuity through the contacts when there are no forces acting upon the switch. Obviously a N/O contact would have no continuity through the contacts when the switch is not acted upon by any outside force.

placing a magnet near a reed switch would be an outside force acting upon the switch.

one of the listings I saw made the effort of stating N/O (when magnet is in near proximity)


the point of listing a switch as N/O and N/C is so you do not have to define or explain the statement.
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Old 03-02-2011, 03:16 PM   #9
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Magnet reed switch turns on lights under sink?


Yes.
Some consider the switch with it's "partner" magnet paired with it as the normal state.

Also, doors and windows are normallly closed so the magnet would be next to the reed switch.

Wish I could install a cheap light switch with a magnetipped pawl attached to the door.
(Magnet glued to the switch toggle and to the end of springy door stop.)
Opening the door moves the pawl away, which pulls the switch toggle ON as the magnet separates and pulls away.
Closing it the magnet reattaches and pushes the toggle OFF.

Last edited by Robert Gift; 03-02-2011 at 03:24 PM.
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Old 03-02-2011, 03:36 PM   #10
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Magnet reed switch turns on lights under sink?


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Yes.
Some consider the switch with it's "partner" magnet paired with it as the normal state.

Also, doors and windows are normallly closed so the magnet would be next to the reed switch.

Wish I could install a cheap light switch with a magnetipped pawl attached to the door.
(Magnet glued to the switch toggle and to the end of springy door stop.)
Opening the door moves the pawl away, which pulls the switch toggle ON as the magnet separates and pulls away.
Closing it the magnet reattaches and pushes the toggle OFF.
Some are simply wrong when defining a N/O contact as an N/C (or vice versa) simply because in that particular use, that is the typical contact position. What happens when you do not use that switch in the manner they are using it in?

I;m at a loss as to why the switches as I suggested would not be acceptable. There are myriad variations and voltage and current ratings available. Finding something that would fit your need shouldn't be difficult. Not real sure why you are wanting to re-engineer something that has already been done for you.
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Old 03-02-2011, 03:52 PM   #11
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Magnet reed switch turns on lights under sink?


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...Not real sure why you are wanting to re-engineer something that has already been done for you.
Light switches would handle many times the amperage (for incandescent rope lights) and be cheap and meet electrical code if enclosed in boxes with covers and armoured cable.

But I like the simpleasy and compact magnet switches far better.
I'd install a proper electrical outlet in each cabinet. Then plug my home-madevice, using lamp cord, into the outlet. Each light string has 3-amp fuse rated indoor and outdoor.
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Old 03-03-2011, 06:02 PM   #12
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Magnet reed switch turns on lights under sink?


i think you should go low voltage control circuitry and use a relay
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Old 03-03-2011, 07:25 PM   #13
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Magnet reed switch turns on lights under sink?


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Originally Posted by Saturday Cowboy View Post
i think you should go low voltage control circuitry and use a relay
You are correct but I wanted to do it as cheaply as possible, using an LED sting in each cabinet.
Maybe I could wire allights in parallel at 3.8 volts.

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Last edited by Robert Gift; 03-03-2011 at 08:31 PM.
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