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UGN3503 Hall effect sensor -> mosfet -> parallel port

Discussion in 'Electronic Basics' started by Emil Johnsen, Nov 16, 2003.

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  1. Emil Johnsen

    Emil Johnsen Guest

    I'm using a UGN3503 Hall effect sensor to detect the presence of a magnet.
    The signal should be half the supply voltage when no magnetic field is
    present. Actually I get ~2.35V with a 5.1V supply. With the magnet I'm using
    the output rises to 3.2-3.5V when the magnet is close to the sensor.

    When the signal is high I want to draw one of the input pins of the paralle
    port to gnd.

    I eventually got this to work using a mosfet with a Gate-Threshold voltage
    of 3V (BUZ21). Obviously using a (somewhat costly and bulky) 20amp mosfet
    when the load will be at most a few mA is not a very elegant solution. Also
    I migth have been lucky that this mosfet was close to the typical
    Gate-Threshold voltage rather than the min (2.1V) or the max (4V).

    Are there any small (preferably TO-92 or SMT) mosfets with a Gate-Threshold
    voltage of 3V with thighter tolerances? If min and max were withing 2.5-3.2V
    I could be sure it would work. I have looked at a lot of datasheets but I
    have not found anything yet.

    Maybe I'm geting needlessly attached to mosfets.. Is there a better way of
    doing this?
     
  2. There is a component specifically to do a 1 bit analog to digital
    conversion (which is what you are doing. It is called a comparator.
    It has two high impedance inputs, labeled + and -. When the + input
    is more positive than the - input, the output goes to a logic high.
    Otherwise it is a logic low.

    All you have to do is provide a reference voltage (from a voltage
    divider, for example) to compare your signal to.

    They come in 5 pin surface mount packages and draw very little
    current. An opamp is similar, except that the output has been slowed
    down enough that it can be tied back to the - input, and produce a
    voltage equal to that applied to the + input, without oscillating. If
    you are not worried about the microsecond or so this slow down delays
    the output, you could use an opamp for your job, also.

    Here is one example of a comparator:
    http://cache.national.com/ds/LM/LMC7211.pdf
     
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