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Detecting a magnet passing through a coil using a darlington pair?

Discussion in 'Electronic Basics' started by Kieran Simkin, Aug 4, 2006.

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  1. Hi Guys,
    I'm trying to build a simple project to get some pratice in analog
    electronics. The intention is to make an electronic cat flap that only
    allows entry to a cat wearing a magnetic collar (yes, you've seen them in
    pet shops).
    I a 403Ohm coil around the entrance to the cat flap, passing a magnet
    through this coil generates approximately 1-20mV and varies the resitance of
    the coil by about 20Ohms in either direction. I need to turn this signal
    into a digital on or off, with on indicating that a magnet is moving through
    the coil. I'm not concerned about directionality, and the more sensitivity I
    can get the better. Neither does the signal need to remain on for very long,
    as I have a latching circuit in place that will work with a very short
    signal from the detector.

    I attempted to built an A.C. coupled darlington pair as described on the
    following page (following the resistor and capacitor values exactly):
    However, this did not work as the output remained on in every instance.
    Measuring the coil and the 22nF capacitor in series gave me a resistance of
    infinity - I don't know why because I'm not quite sure of the function of
    the capacitor in this circuit.
    I tried removing the coil+capacitor pair and replaced it with a normal
    potential divide with the coil on one side and a fixed resistor and a preset
    on the other side of the divide. Even with the resistance on either side of
    the divide matched, I'm still unable to get any negative output from the

    Can somebody point me in the direction of a circuit that should work for my
    project? I'm not sure whether I should be using a darlington pair or an
    op-amp, or whether I need to A.C. couple the coil to the collector in my
    first transistor, if so, I'm unsure of the reason for A.C. coupling and I'm
    also unsure of why it does not work for me.

    I've tried doing a search of the internet but cannot find anything more than
    a basic description of a darlington pair and how to wire one to an LDR,
    presumably a coil is significantly different to an LDR?

    Any advice you guys could give me would be greatly appreciated.

    Best regards,

    ~Kieran Simkin
    Digital Crocus
  2. Guest

    The collector value is wrong try 10k instead.

    The capacitor blocks the dc current, the text tells you that, so it
    will allways read open circuit.
    You will upset the dc conditions.
    You will only get a very small voltage so it will need a lot of
    amplification. The cicuit gives you an ac singnal which you will need
    to process in some way before you can operate a latch. Overall it's a
    scheme that is unlikely to work very well if at all.
    indeed it is.

    Considerably more research effort, read some books.
  3. ...Ad!

    ...Ad! Guest

    Hi simkin,

    Did u try magnetic switches? these tiny things could work for you...
    just put a few as reqd under the catflap, or somewhere it can catch the
    cat's collar!

  4. Mike

    Mike Guest

    It sounds like maybe the coil voltage would be changing fairly slowly,
    so You may need to increase the value of the 22n cap to something in
    the range of 1uf-10uf. Also try values in the range of 10k-33k instead
    of the 100k from the collector of the 1st transistor. The transistors
    are not biased very well so the actual resistor values needed will
    vary from those shown. If you are consistantly getting 10mv-20mv from
    the coil then it might work. If you don't consistently get enough
    voltage you add some turns to your coil, but too many turns and you'll
    have trouble with false triggering from stray magnetic fields mainly
    from AC power wires.. Keep in mind that the 47k from the collector of
    the 2nd transistor is a fairly high value so you won't be able to put
    much of a load on it. When you get it working, try lowering the 47k to
    a 4.7k or 10k. FYI, those 2 transistors are not configured as a
    darlington pair. It is a 2 stage direct coupled common emitter
    amplifier with the input AC coupled. I'm not sure what you mean by
    "negative output". The output should sit at approximately 9v and go
    toward 0 then back to 9v as the magnet passes through the coil.
    Hopefully this is what your latching circuit requires to operate. The
    input capacitor is required so that the low resistance of the coil
    does not short the emitter-base junction of the 1st transistor which
    would keep it from operating. One other thing, The resistance of the
    coil does not change as the magnet passes through it. The coil just
    generates a voltage as the magnet moves through it.

    "The scientist is possessed by the sense of universal
    causation...His religious feeling takes the form of
    rapturous amazement at the harmony of natural law,
    which reveals the intelligence of such superiority
    that, compared with it, systematic thinking and acting
    of human beings is an utterly insignificant reflection."
    Albert Einstein (theoretical physicist)
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