Magnetic field sensor

Discussion in 'The Projects Forum' started by Willy77, Dec 21, 2014.

  1. Willy77

    Thread Starter New Member

    Aug 14, 2013
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    Does anyone know how to build a circuit that would sense just the magnetic field of a relay by proximity to see at a glance if it is working or not?
    To sense any applied linear DC voltage. Not an oscilating one.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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  3. Willy77

    Thread Starter New Member

    Aug 14, 2013
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  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Automotive relays are usually sealed, so you would need to 'uncase' them.
    Reed switch:
    upload_2014-12-21_14-52-50.jpeg

    Max.
     
  5. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Use an hall effect sensor.
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    True.
    There is the Honeywell SS400 series that should work?
    Max.
     
  7. Willy77

    Thread Starter New Member

    Aug 14, 2013
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    Thanks Max.
    It would be nice to avoid to uncase them and instead having a device (sensor) that would tell me with an led if the relay is active or not by just putting the sensor near close by the relay.
    An instrument just like those neat pen like AC line voltage sensors only this one senses a DC magnetic field of a relay. Avoiding off course having to connect your voltage probes in hard to get places.
    I tried some experimenting but could only sense DC oscilating voltage magnetic fields.
     
  8. Willy77

    Thread Starter New Member

    Aug 14, 2013
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    :)Ok thank you guys. I will try an hall effect sensor like Dodgydave says. Thanks a lot.
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    You never mentioned where you were, but Digikey have the Honeywell, you need to watch the part number as there are unipolar, bipolar and latch.
    Max.
     
  10. Willy77

    Thread Starter New Member

    Aug 14, 2013
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    I'm from Santa Cruz Bolivia.
    Not sure if I will order the parts just yet or which I would need. I have a Digikey parts catalog and will look up into it. But first I will investigate for a circuit that would suit my needs.
    Anyway I will apreciate any info. Never had an order from Digikey yet, nor I know if Digikey deliver policies reach these places.
    Thanks Max.
     
  11. tom_s

    Member

    Jun 27, 2014
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    check to see if one of your phone apps can do it.

    on android, there is smart tools (multifunction application)
     
    GopherT likes this.
  12. ebeowulf17

    Active Member

    Aug 12, 2014
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    If you just want the simplest on/off LED indicator, a hall effect switch or even a reed switch would make a simpler circuit than a linear sensor like the 490 series.

    Having said that, the switches have pretty wide tolerances on their trip points and that circuit wouldn't be adjustable.

    If it were for me, I'd use the 490 series sensor with a 10k pot providing a reference voltage for a comparator so that you can adjust the trip point to suit your needs. You might also need an extra resistor or two to add some hysteresis. I recently made almost exactly this circuit with the Microchip MCP6542-E/SN with great results.
     
  13. ebeowulf17

    Active Member

    Aug 12, 2014
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    Here's a version of my circuit with the unnecessary bits trimmed out. This one was for external 5VDC power supply, so I'm not entirely sure what it would take to adapt it to battery power.

    [EDIT:] U1 isn't properly called out in the drawing, but it's a Honeywell SS495A.

    Relay-Field-Sensor.png
     
  14. Willy77

    Thread Starter New Member

    Aug 14, 2013
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    Thanks Tom. There are some cool apps ( and installed some) but no luck for what I want.
     
  15. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Do you have a compass app? That will detect nearby magnetic fields.
     
  16. Willy77

    Thread Starter New Member

    Aug 14, 2013
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    Great! Thanks ebeowulf17 you put me in a good position to start experimenting. I will check this out and comment later on. Thanks again!
     
  17. Willy77

    Thread Starter New Member

    Aug 14, 2013
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    My phone does'nt take this app, but I do have a compass, I will try with it. Good point. Thanks Alec t.
     
  18. ebeowulf17

    Active Member

    Aug 12, 2014
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    Max brought up a good point that I forgot to touch on:
    If you use my circuit with the linear, proportional SS495A, the sensor will only do a good job detecting your magnetic field in one orientation, with the face of the sensor perpendicular to the field lines, either face up or face down depending on whether it reads positive or negative gauss.

    If you went with the right type of hall switch (omnipolar if I remember correctly,) you'd be able to detect the field regardless of sensor orientation, but then you can't adjust sensitivity/trip point. You'd need to have a reasonable idea of what field strength you're expecting and buy a switch with the appropriate trip point, or buy a variety of them and see which one works best through trial and error.

    So I guess the question is which level of flexibility is more important: the ability to sense a field from any orientation, or the ability to adjust your sensitivity as needed.
     
  19. Willy77

    Thread Starter New Member

    Aug 14, 2013
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    Hi eveowulf17.

    Thanks for pointing this up. I wasn't considering the gauss polarity. Actually I'm more concern about sensitivity and being able to adjust it as needed.
    This can be very handy if for example there's a situation of a few relays lined up and you're testing a switch to check wich relays are on and which are not.
    I will try to get a variety and experiment through trial and error as you said.
    Thanks again.
     
  20. ebeowulf17

    Active Member

    Aug 12, 2014
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    Didn't have any dc coils to test until just a few days ago, but here's some real world data that may be useful for comparison purposes. I tested the field strength on both ends of a 24V, 8W solenoid, using a Honeywell SS495A and an Arduino.

    On the more open, exposed end, I found:
    500+ Gauss touching the end
    100 Gauss @ 1/4"
    40 Gauss @ 1/2"

    On the business end, which was pulling a small plate tight against itself, I measured:
    103 Gauss touching the plate
    54 Gauss @ 1/4"
    28 Gauss @ 1/2"

    Depending on what size relays you're testing, their coils may be weaker or stronger than this.
     
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