Assistance please.. Steps on how to change magnetic switch from NO to NC

Discussion in 'General Electronics Chat' started by Crea, Dec 11, 2017.

  1. Crea

    Thread Starter New Member

    Dec 11, 2017
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    Good day!

    I am asking for assistance regarding my problem. I have bought an NO magnetic switch and I am wondering how I can change it to NC. Simple reason is that the NC magnetic switches are more expensive.

    Any form of aid is appreciated and the picture of the magnetic switch that I have bought is included if it can help.

    Thank you!
     
  2. tranzz4md

    Member

    Apr 10, 2015
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    The sketch is your desired switching characteristic?
     
  3. MrAl

    AAC Fanatic!

    Jun 17, 2014
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    Hi,

    Simple answer is to bias it with a small magnet.

    The small magnet is enough to close the switch, and oriented in such a way so that when the large magnet used to operate the switch comes close, it cancels the field of the small magnet and thus the switch opens.
    It may take a little experimentation to get the small magnet oriented right and in the right position relative to the switch and larger magnet.

    If you do not want to do that then buy another switch that is NC, or else use a transistor inverter if you can. A transistor wired as common emitter creates an inverter (DC only however). The transistor circuit will require some quiescent power though so it has to be designed for low power if running from a battery.
     
    Last edited: Dec 11, 2017
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  4. tranzz4md

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  5. tranzz4md

    Member

    Apr 10, 2015
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    Oh, this all comes in less than 10 minutes, in my chair, Googling on my smartphone, while watching TV. Look around. There's lots of stuff out there.
     
  6. MrAl

    AAC Fanatic!

    Jun 17, 2014
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    Hi,

    Yes it's amazing and only 10 minutes before yours post #3 came in that said basically the same thing :)
     
  7. Crea

    Thread Starter New Member

    Dec 11, 2017
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    Thank you both for the quick replies.

    Sorry for the confusion. The characteristic that I actually want is the opposite from the sketch that I included.

    May I know what transistor to use and a simple diagram on where to connect it?

    Thanks.
     
  8. MrAl

    AAC Fanatic!

    Jun 17, 2014
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    Hi,

    Just connect maybe a 100k resistor from +Vcc to the base of an NPN transistor. Connect emitter to ground. Connect collector to load. Connect NO reed switch from base to ground.
    The transistor is on normally, but when the magnet comes close and the switch turns on, the switch shunts the current from the 100k to ground and that turns the transistor off, so the load at the collector turns off too.
     
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  9. Crea

    Thread Starter New Member

    Dec 11, 2017
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    Thank you for the suggestions MrAI.

    I tried the magnet and it worked like a charm. I'll also try this idea of yours as well. Thank you once again!
     
  10. MrAl

    AAC Fanatic!

    Jun 17, 2014
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    Hi again,

    You're welcome and good luck with it.
     
  11. DNA Robotics

    Member

    Jun 13, 2014
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  12. killivolt

    Distinguished Member

    Jan 10, 2010
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    You could go with a Transistor and a Hall Effect sensing switch or Momentary contact Hall Effect, either way $1.25 and a PNP general purpose Transistor and resistor.

    kv
     
  13. MrAl

    AAC Fanatic!

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    Hi,

    Sounds interesting, i just have to ask how much quiescent current will the hall device draw?
     
  14. killivolt

    Distinguished Member

    Jan 10, 2010
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    I'm no engineer nor do I claim to be, I've just been playing around with these little guys in another circuit so when I seen he wanted something activated by a Magnet I jumped in, he would need someone to guide him maybe you could look into them to see how he could do this electronically.


    kv

    Edit: Mouser
    785-SS466A-L

    785-SS466A
     
  15. MrAl

    AAC Fanatic!

    Jun 17, 2014
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    Hi,

    Oh no problem i just thought you might know offhand. I didnt mean to put you on the spot or anything, just thought you might know from experience.

    I see what you are talking about now, and i think that is a very good idea if the battery can take it or it is run off the mains.

    For some applications we need very low quiescent current, like 100ua, so 5ma would be too high because it would run little batteries down too fast. For bigger batteries though or for mains driven apps it should be ok though.

    To compare, using a reed switch and bias magnet requires zero quiescent current. Using a transistor with reed switch uses 4.5/100000 amps or 45ua so you see how low that is and this allows for very long battery life even with small AAA batteries.

    My refrigerator monitor uses batteries and they last for 2 years because of the low current draw. For AA batteries we have roughly 2000mAhr capacity so at 1ma they last 83 days, but at 100ua they last for 830 days which is over two years. So you can see how much difference this makes. At 5ma they would last about two weeks. On the other hand, if it was driven from the mains then 5ma would not matter very much. In that case a hall device and maybe one transistor would be ideal i think.

    Note the Honeywell version could use as much as 7.5ma supply current.

    I found a low current version though:
    DRV5032

    with very low current around 2ua or less depending on the version. This of course does not include the load itself or a load resistor.
     
    Last edited: Dec 13, 2017
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  16. killivolt

    Distinguished Member

    Jan 10, 2010
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    I had to google "Quiescent" at rest meaning quiet while off. Now your explanation makes much more sense to me. Thank you for the explanation, I think many could benefit from this I know I sure do. Doesn't mean I could possibly do anything with it but, ask more questions I'm sure. :p

    Thanks again,
    kv
     
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