How to Identify Reed Switch

Discussion in 'The Projects Forum' started by Cumulonimbus, Aug 22, 2013.

  1. Cumulonimbus

    Thread Starter New Member

    Aug 22, 2013
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    I am doing an electronic restoration project and have run into a snag. I have a glass reed switch and need to find two more that are identical. The problem is there are no markings as to what type of reed switch it is. I have bought three from an electronics store that had the same size glass tube. None of them worked. After doing some research it appears reed switches have different sensitivities. So my question is how do I measure the sensitivity of the one I have or how can I identify it?

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  2. eman12

    Active Member

    Oct 26, 2007
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    I just use a magnet to do so.
    But I use a solution if the sensitivity is not proper to me. The trick is to use 2 magnets, one closer to the reed switch and the other a bit further!
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    What do you mean 'none of them worked' All the ones I have used with slightly varying sensitivity will work, just that the magnet proximity varies.
    Guessing the size of the one you have they generally trip on average around 5mm.
    And that is using a small magnet that is not is considered not extremely strong.
    Also the switch should operate regardless of where the magnet is on the circumference, it has to be in the centre of the switch, however.
    Max.
     
  4. Cumulonimbus

    Thread Starter New Member

    Aug 22, 2013
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    Here's the issue, I cannot change the magnet proximity or change the magnet itself by the design of the unit. The object of the electronic restoration is to keep it original appearing. The glass tube is 0.75" long by 0.1875" wide. I tried three different types of switches and none worked. I don't think the magnet was strong enough. If I borrow an original reed switch out of another identical unit, it works great.
     
  5. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Reed switches can be normally open or normally closed.
    You can verify the operation (out of the circuit) with an ohm meter or continuity tester.
     
  6. Cumulonimbus

    Thread Starter New Member

    Aug 22, 2013
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    It is a normally open reed switch.
     
  7. MaxHeadRoom

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  8. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    read the datasheets of the ones you bought.. look at the pull in excitation rating (AT) and get one thats lower. Its just a guessing game because you don't have the equipment to test for what you need. At least now you know what won't work :)
    Just make sure all the other specs still fit the bill.
     
  9. Metalmann

    Active Member

    Dec 8, 2012
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  10. gerty

    AAC Fanatic!

    Aug 30, 2007
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    Have you rotated the switch? The sensitivity will change as the relationship between the contacts and the magnet changes. Just roll the switch on its axis, with a meter connected. You should see the distance required to make the switch change as you roll it.
     
  11. MaxHeadRoom

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    I have not found that on the ones I use so far?
    Max.
     
  12. tubeguy

    Well-Known Member

    Nov 3, 2012
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    I have to say I have seen that on some reed switches too ... where rotation matters. Maybe something to do with the magnet ??? Honestly .. not sure...
     
  13. Metalmann

    Active Member

    Dec 8, 2012
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    I can see where the angles of the contacts, would be changed, by rotation; in relation to the magnetic forces.....

    a lot of variables.....say that the reeds were lined up (or not), with the greatest magnetic forces?
     
  14. MaxHeadRoom

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    I would think it would be part of the spec sheet data if it were a concern?
    Max.
     
  15. gerty

    AAC Fanatic!

    Aug 30, 2007
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    I've not seen a data sheet on a reed switch. When I was in the alarm business, the manufacturer would rate the switch by the gap to the magnet, among other things.
    Sometimes we had a sloppy fitting door or latch and, after tightening it all up as much as we could, had to compensate by metering several different switches out of the same box. We later found the the way the switch was orientated in the housing was the difference.
    One type that was called 'wide gap' was considerably larger than a standard one, as was the magnet.
     
  16. MaxHeadRoom

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    Jul 18, 2013
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    I have found there is a difference with the Hamlin type reed switch/magnet combo and they are made with a mating package.

    But with the glass reed switches of the type in the OP, I have done tests of my own as I rebuild rotary position sensors and the ones I obtain from DigiKey do not show any difference in switching with the switch rotated in various angles. Also the data sheets do not mention anything on this, only the PI (AT) value.
    Max.
     
  17. Cumulonimbus

    Thread Starter New Member

    Aug 22, 2013
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    Rotating the switch doesn't change anything. What surprises me is these switches have no markings on them.
     
  18. MaxHeadRoom

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    None of the ones I have ever purchased had either!
    Max.
     
  19. MaxHeadRoom

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    Incidentally at .1875" dia, that is quite a large by current common use, at that dia it probably requires a fairly strong magnetic field to operate them?
    Max.
     
  20. Cumulonimbus

    Thread Starter New Member

    Aug 22, 2013
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    The magnet isn't very big. Probably the size of a dime but thicker. The original reeds are from the 1970's. Maybe they were that large back then.
     
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