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Home made hall effect pot

Discussion in 'Electronic and hardware generally' started by h106frp, Aug 7, 2015.

  1. h106frp

    h106frp New Member

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    Recycle old/damaged/unwanted 10 turn pots (these are spectrol ones) as endless hall effect devices. I am using 2 of these in my rig for feedback and they perform really well. The mechanism is endless and gives 0-5 volts for the first 180 degrees then 5-0 volts on the next 180 degrees from a 5 volt supply so they are a 180 degree potentiometer and you can choose the polarity sense output. No gear cogging issues, true analogue output, no wear and difficult to damage

    Device is Allegro a1301, the 2 neodymium magnets attached to the pot inner stem were very cheap off the web. Debris in the background is the original resistive wire track and pick up slider that have been removed.
    [​IMG]
    The Hall device is placed in the pot base and the 2 flat bar magnets are glued to the original potentiometer inner body and aligned to pass a North-South field through the device (wide faces are poles). The yellow plastic inner stem only needed gentle scraping down with a craft knife to ensure clearance between the magnets and outer body.As the field rotates on the pot stem which fits over the hall device body the voltage output changes. I feed the voltage to an offset and gain op amp circuit to give any feedback voltage profile i desire, currently -10 to +10 volts for my servo drivers.

    Reassembled Hall effect device under test;
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  2. RacingMat

    RacingMat Well-Known Member Gold Contributor

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    Interesting!
    Could you tell us what is your amplification board schematic?
  3. h106frp

    h106frp New Member

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    I 'borrowed' this idea from the web for offset and gain (span).

    [​IMG]
    text from original article gives a better explanation of operation;
    Quote..

    Since your full scale output from the DAC is 5V, and you want a full scale output from the opamp of 20V (-10 to +10) then you need to have a gain of 4 in the opamp. That gives you 0-20V. So you want (DAC x Gain) - Offset.

    In a non-inverting op-amp, the + input has a gain set by the (feedback resistor/input resistor)+1. However, the negative input has a gain of simply the (feedback resistor/input resistor). So a non-inverting input with a gain of 4 will have a a gain of 3 from the negative input. You can use the negative input to offset the fullscale 20V to -10 to +10. So you want 10/3 = 3.33 volts at the negative input.

    At the point I marked 3.333V, for a standard non-inverting op-amp configuration, that point would be GND. But you want to add the negative input's gain to the output too, so you need 3.333V there.

    Unquote...


    Any 'low offset' or precision type dual op amp should work better than the original LM324, i used OP200 because i have some to hand. OA2 establishes a low impedance reference voltage for OA1 so you can use an adjustable multi turn trimmer for R3/R4 divider to make it adjustable without upsetting the gain structure and OA1 sets the span. Works well and has been very stable with the Hall devices as the input.
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    Last edited: Aug 7, 2015
  4. tombo

    tombo Active Member

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    Hello,
    very nicely done. I did somesthing simmilar in the past but never got a perfect output, because i can't find a good housing to rotate the Magnets.
    Your solution looks very good. Thanks for sharing this Information over here. :thumbs
    Maybe I try it again. Have some unused 10turn pots, and also old magnets and chip :)
  5. h106frp

    h106frp New Member

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    Thanks. I did see a few different versions of this idea around before i tried mine but they were either bulky, difficult to manufacture or apply to the rig. Recycling the potentiometer body solved all these issues and keeps everything aligned nicely in a familiar package.

    I finished mine with a piece of 3 track strip-board (vero) soldered to the device legs and a pin header for easy electrical connection, it also serves to fix the Hall device tightly in place.

    One of the nice advantages of the Hall device is low-noise as its very difficult to stop 'pot crackle' when you have DC voltages present on normal potentiometer tracks, especially cheap carbon track types.
  6. Bord-Ing.

    Bord-Ing. New Member

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    Nice idea! I built a quick setup to test with a popular SS39ET and an Arduino Uno
    pic01.jpg

    and got almost the same result as you.
    0° -> 180° => 1 - 4V
    180° -> 360° => 4 - 1 V

    The output wave form is sinusoidal:
    pic02.jpg

    This could be used to compensate for the nonlinear movement of crank gears like wiper motors.

    pic01.jpg
  7. Jerry Atrick

    Jerry Atrick this is my sim Gold Contributor

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    Hi h106frp, add yourself to the list of Genius, well done!:thumbs

    Jerry.
  8. Gadget999

    Gadget999 Active Member

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    what happens if you exceed 180 degrees - will the swap from 0-5v to 5-0v confuse the program ?
  9. Bord-Ing.

    Bord-Ing. New Member

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    Yes, would be the same problem as an inverted connected pot. BUT, after another 180° the direction swaps again, and all should go fine :)


    ... hopefully :rolleyes:
  10. OZHEAT

    OZHEAT Member

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  11. Bord-Ing.

    Bord-Ing. New Member

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    I would rather use one with a digital interface, preferable ISP. Why using a 12 bit device with analog output, when the Arduino is only capable of sampling 10 bits, maybe 9.5 bits in fast mode?
  12. Michael898

    Michael898 To build or not to build.

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    I really do not understand!
    In my understanding we could use the as5600 as a regular analog potentiometer. 0 volt - 5 volt ???
    Correct me please if i am wrong!!!
    I was looking to replace my pots with these. (AliExpress €16,- for 5 pieces with the magnets)
  13. Bord-Ing.

    Bord-Ing. New Member

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    Of course you can do this :) You can even program an angle range to use with the 0-5 V output (see this video). In your case you would replace the mechanical part of the pot with the contactless generation of the output voltage.



    But imho there are to much A/D- and D/A-conversions in this sensor chain:

    Magnetic field -> hall sensors [AS5600] -> A/D [AS5600] -> scaling [AS5600] -> D/A [AS5600] -> analog voltage (cable noise!!!) -> A/D [Arduino/SMC3]


    I would use it like this with an ASxxxx with digital interface, avoiding the analog voltage signal transfer:

    Magnetic field -> hall sensors [ASxxxx] -> A/D [ASxxxx] -> scaling? [ASxxxx] -> SPI (digital interface) -> Arduino SMC3.x
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  14. Michael898

    Michael898 To build or not to build.

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    Okee, that’s informative. Thanks alot. I didnt think that way.
    I Will have a try, because they are dirty Cheap.
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  15. MarcoMade

    MarcoMade Active Member

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    you can print these in 3D and even modify them as you like to adapt them to your needs
    https://www.thingiverse.com/thing:2862716
    Marco