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Linear movement detector using magnetic, wire coil and transistors

  1. Jul 28, 2012 #1

    I have cool idea for a device and one hardest part of it is to detect the movement of the object. I think I can use a wire coil, and a small magnet attached to the moving object. When the object passes the wire coil (up or down), there will be small voltage inside the wire coil. It will be positive or negative voltage depending on the direction of the movement. When no movement, no voltage generated. This is my understanding so not sure it is correct or not :D

    Then, the wire coil is connected to a circuit board with few transistors on that, if positive voltage (object move up), led 1 turn on, if negative voltage (object move down), led 2 turn on. When no movement, no led on.

    I'm IT developer, physic and electronic are not my strong fields so I am seeking for help from you guys to confirm my knowledge and help in calculation, also the PCB design. I want to mass produce this device and may be we can be partner in future.

    Thanks a million !

    Attached Files:

  2. jcsd
  3. Jul 29, 2012 #2
    Google differential transformer.
  4. Jul 29, 2012 #3


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    When you move a magnet past a coil, you get a brief pulse while the magnet is moving, then it dies down and you get nothing.

    It would be possible to wind all those coils, take these pulses, amplify them and then make them longer using monostable chips, or use other chips to light a LED until another pulse was generated.

    However, that is a lot of hard work when there is a simpler solution.

    You can get Hall effect sensors.
    These will detect the presence of a magnet, moving or not. You can get some that switch when a certain level of magnetic field is present and you can get others that give an output that is proportional to the magnetic field.

    For example, you can get 10 for US$2.50, delivered, here:

    These are the switch type. As long as they get a sufficiently strong magnetic field of the right polarity passing through them, they switch on.
    The field from a small neodymium magnet would be plenty to switch one of these from half an inch away.
  5. Jul 30, 2012 #4
    Thanks Carl Pugh and vk6kro, some problems I found:

    1. If use wire coil (Linear variable differential transformer), it seems that the moving object must be inside the coil, if outside the coil it does not work very well or generate too low voltage. My device only works outside the coil.

    2. If use Hall Effect sensor or Reed switch, the problem is when the object stops near the sensor, it will keep sending ON value, which will kill the battery. This device uses CR2032 battery and sends its value wirelessly.

    Is there any special hall sensor than senses the movement of magnetic field ?
  6. Jul 30, 2012 #5


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    Hall effect devices have to have a supply voltage and they take a continuous current.
    In the 3144, this is given in the data sheet as being between 4.4 mA and 9 mA.

    So, if you had 20 of them, this could need about 200 mA whether the device was switched on or not.

    The output is an open collector transistor, so the extra current could be another 1 mA or so depending on the load. They can switch up to 20 mA.

    Passing magnets past the outside of a coil is going to give poor output and you will have to amplify the small pulse to get anything useful. Amplifiers use current, too, so the humble CR2032 battery really isn't going to be good enough.

    I don't think Hall effect devices come in movement sensing versions, but you can get Pyroelectric motion sensors like this:

    This is a low voltage version of the motion sensors you see everywhere on houses.
    These need about 4.5 volts minimum but only draw 50 μA on standby.
    Depending on how many you used, this could be an option.
  7. Aug 21, 2012 #6
    I, too, am looking into building a device of this nature.

    I simulated the field of the magnet that I am required to use. When the NdFeB magnet is directly over a proposed sensor, it has a B-field of about 10 mT and a density (H-field) of about 8.5 [itex]\frac{kA}{m}[/itex]. But it also has a B-field of about 3 mT and density of about 2.3 [itex]\frac{kA}{m}[/itex] when at it's furthest possible distance. Will Hall-effect be able to detect this low intensity field's changes enough to provide an accurate output of the magnet's position?

    If so, which ones would you recommend? If not, what are some other techniques for doing this measurement?
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