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Hall Effect Sensors

Discussion in 'Electronic Basics' started by Michael, Apr 8, 2007.

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  1. Michael

    Michael Guest

    Hi,

    I'd like to interface Hall Effect sensor to a uC and have picked out the
    A1301EUA-T (datasheet:
    http://www.allegromicro.com/en/Products/Part_Numbers/1301/1301.pdf ) with no
    particular reason - just seems easy to get hold off.

    Now based on the graph at the bottom left on page 5, am I right in thinking:

    Output from HES fed through a non-inverting amp. with gain of 250 would
    produce a voltage of 625mV at 1G when the supply voltage is 5.0V? It just
    seems a bit too easy (I'm guessing changes in temperature etc are going to
    have a significant effect but right now just trying to get hold of the
    basics.)

    If I told you the application was going to be a motor tachometer - is it
    'better' to feed the (amplified) voltage through a comparator and so convert
    the analog signal into digital signal (when above a theshold go high, below
    or at theshold go low type setup) or feed it into a ADC pin and work out the
    digital signal using software? I'm worried that if I tied that to a ADC pin
    then it could go over the pin's max voltage (5V) so would I need something
    along the lines of a zener diode to dump any 'extra' voltage over say 4.5V
    to be on the safe side?

    I've only just started learning electronics so don't know if the circuit's
    above are 'suitable' and so would appreciate any help. ( It's not a homework
    assignment :) )

    Apologies if that's a few too many questions.

    Thanks in advance,

    Michael
     
  2. Jamie

    Jamie Guest

    I wouldn't worry to much about going over the voltage of the ADC pin how
    every, I would be concerned about the response time of the ADC when
    using it as a tech (HI speed input)..
    you would be better to use a simple Op-amp with hysteresis, or I
    think the input's of the PIC already have some hysteresis in them, so
    you might be just fine.
    I would look into assigning an input to one of the counters in the
    micro. (High Speed COunter).
     
  3. Chris

    Chris Guest

    Hi, Michael. In fact, it's not so simple, and unfortunately, I think
    you've taken a wrong turn here.

    The nominal output of the sensor you're looking at is around 1/2 Vcc
    (around 2.5V for a 5V supply). Now apart from any drift caused by
    variations in supply voltage, you'll have to apply the sensor output
    voltage to an op amp to subtract out the 1/2Vcc.

    The data sheet says this quiescent output voltage is stable over time
    and temp, but considering you're talking about such a small sensing
    voltage to begin with, you are heading for problems here.

    The good news is that there are hall effect sensors with digital
    outputs that react to the presence of a magnet quite nicely, and will
    have fast enough reaction times for a tachometer. You can forget the
    analog stuff, hang a pullup resistor at the output of the sensor, and
    just count the digital pulses. This is how it's done.

    If you'd like further information or need help in sensor selection,
    please post back with more info.

    Cheers
    Chris
     
  4. Michael

    Michael Guest

    Hi,

    That sounds perfect - any keywords I should be using?

    Michael
     
  5. Chris

    Chris Guest

    Hi, Michael. Since you've found a UK distributor for Allegro, I'd
    suggest just going to the Allegro Unipolar Hall-Effect Digital
    Switches page, and taking your pick. As I said, just have a magnet go
    past the IC, use a 1K pullup, and just read the digital output with
    your uC. If it's a very fast application and you don't have a
    hardware counter on board, you might want to use a flip-flop to make a
    divide-by-2 counter, then you can just see every transistion as 1
    revolution.

    http://www.allegromicro.com/en/Products/Categories/Sensors/unipolar.asp

    You could do worse than choose one of the A1101-1104 and A1106 family:

    http://www.allegromicro.com/en/Products/Part_Numbers/1101/1101.pdf

    For all you want to know about digital unipolar hall effect sensors,
    look at the appnote listed on p. 8, the Hall Effect IC Applications
    Guide:

    http://www.allegromicro.com/en/Products/Design/an/an27701.pdf

    Cheers
    Chris
     
  6. jasen

    jasen Guest

    If it's to be a tachometer on a gasoline engine the best way is a 1M resistor
    from the switched terminal on the low voltage side of the spark coil.
    If you want to use a hall sensor it may be easier to use one with a
    digital output eg: take the phase sensor from an old computer fan.
    It's better than too few.

    Bye.
    Jasen
     
  7. Michael

    Michael Guest

    Thanks Chris, that's perfect.

    Is there an IC with a single flip-flop on or are they abit like socks?

    Michael
     
  8. Chris

    Chris Guest

    Hi, Michael. For a bit more cost, you can get a bipolar hall effect
    sensor, that will turn on in the presence of the south end of a
    magnet, and remain on until in the presence of a north. If you can
    arrange two magnets of opposite polarity at 0 degrees and 180 degrees,
    you'll have the whole solution.

    http://www.allegromicro.com/en/Products/Categories/Sensors/bipolar.asp

    Cheers
    Chris
     
  9. Michael

    Michael Guest

    Hi,

    Yeah I noticed those in the app notes I think they called them latching hall
    effect switches.

    I take it these also need a pullup resistor?

    Now I guess I have to choose one....

    Cheers,

    Michael
     
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