Confused about reed switches

Discussion in 'The Projects Forum' started by AndyD273, Sep 1, 2011.

  1. AndyD273

    Thread Starter New Member

    Sep 1, 2011
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    I'm working on a garage door indicator, and I've tried push switches, but due to the door not being super dependable when it opens, I smashed the first one.

    I think a reed switch might work better, since it probably won't be effected by cold (Michigan winters) and doesn't have to touch the door, so if the door is a little jerky or whatever then it'll still work. plus depending on how strong the magnet is, it'll indicate that the door is open even if it's only a few inches open.

    I'm hoping not to spend very much on them.
    I found this one on another post, but I'm honestly not sure if it'll handle it.
    I'm still a noob with voltages.
    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=HE563C

    Also, I'm not sure what Must Operate: 20 ~ 25AT means.

    Lastly, I'm confused about NO/NC (why I chose the 3 reed). It seems like some companies reverse them, so I'm never sure if NO means that it's normally open with the magnet is next to it, or when the magnet has been removed.

    Thanks.
     
  2. wayneh

    Expert

    Sep 9, 2010
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    What are you trying to accomplish? Is this a repair to an existing system or do you want to add a functionality that was never there?

    I'd look for home-security type switches designed for exactly this type of thing. They are held closed by the opposing magnet, but I believe it would be classified NO, since that would be its state when the magnet is not present.
     
  3. strantor

    AAC Fanatic!

    Oct 3, 2010
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    Sensitivity (of reed switches) is measured in units of Ampere-turns, corresponding to the current in a coil multiplied by the number of turns. Typical pull-in sensitivities for commercial devices are in the 10 to 60 AT range. The lower the AT, the more sensitive the reed switch. Also, smaller reed switches, which have smaller parts, are more sensitive to magnetic fields, so the smaller the reed switch's glass envelope is, the more sensitive it is

    NO means normally open when there is no magnet acting on it.

    That reed switch is rated for 175VDC 5W. Are you switching DC or AC?
     
  4. gerty

    AAC Fanatic!

    Aug 30, 2007
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    Security type reed switches can be had in both NO and NC configuration. Some even offer a switch with both, the switch is condidered NO or NC with the magnet near it. you don't tell us what type of indicator you plan on using.
    Most reed switches are only rated for less than 1 amp and usually at 24v or less. If all you want to do is turn on an led, they're perfect.
     
  5. strantor

    AAC Fanatic!

    Oct 3, 2010
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    I disagree. Switches are always specified NO or NC when no force is acting on them. For example, if you were to wire a limit switch on the NO contacts, but in this switch's normal state it is held in the actuated position, there is a seperate drawing symbol to represent "Normally open, held closed".

    from wikipedia:

     
  6. someonesdad

    Senior Member

    Jul 7, 2009
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    You can also try a microswitch with a roller. Cheap and dependable.
     
  7. gerty

    AAC Fanatic!

    Aug 30, 2007
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  8. strantor

    AAC Fanatic!

    Oct 3, 2010
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  9. AndyD273

    Thread Starter New Member

    Sep 1, 2011
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    Currently, it's running off of a 9V battery with a resister in the circuit, and has two LEDs as the indicator, one inside the house, and one in the garage, so that you can tell that it's working correctly while you are still out there. That way you'll know if the battery goes dead.

    Not much voltage. I don't remember the specs on the resister, but I think the voltage that gets to the LEDs is only 4 or 5 volts, so as to not blow them up.

    You can see why the NO/NC thing has me a little confused, so I'm hoping that this three reed switch will work, so I can test it one way, then the other and make sure I'm doing it right.
     
  10. AndyD273

    Thread Starter New Member

    Sep 1, 2011
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    Thats an idea, but I can see the joints in the door causing problems, plus the fact that to the switch the door would look like a bunch of peaks and valleys, peaks for the joints, valleys for the flat parts. Thats were my home made switch has problems, the light comes on at every joint, then goes back off again when it hits a flat. I've tried adjusting the tension, but the wire arm just bends to much after a while, and needs constant readjusting.
     
    Last edited: Sep 9, 2011
  11. AndyD273

    Thread Starter New Member

    Sep 1, 2011
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    DC, from a 9V battery, with a resister to take it down to 4 or 5 volts to light two LEDs.

    So 20-25 AT means that the magnet wouldn't have to be to close to the switch, which is good, and the voltage I'm using is way below the switches rating, so as long as you can't have to little current, then this switch should be fine, and is pretty cheap...
     
  12. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    In an Alarm closed loop system, NO switches are used with magnets in place, doors closed, so that if a line is cut an alarm will be activated. One disadvantage is there is always monitiring current flowing. For monitoring a single door an open circuit system might be used with NC switch, held open by a magnet on door. So your form C reed , SP-DT, should work fine.
     
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