I have a coil with solid core, there is an small magnet that pass over it (like on the image) but not touching the coil (there is an small plastic between them, no more than 0.5mm thick). I need to detect when the magnet pass over the coil, i've investigated the hall effect devices but i'm not sure if they will work here as the magnet pass over the coil and not over the sensor.

I've connected an oscilloscope to the coil and watched while the magnet pass but i didn't recognize anything over all the noise (I connected the probe to one side of the coil and ground to the other side).

Any help or ideas will be highly appreciated.

Edit: The coil is not made from 4 turns, it has way more than that (around 50) but i was too lazy to draw 50 turns on the image. Also on the best escenario the coil should remain there, but i'm open to other devices like the reed switch (that i'm going to try soon).


  • \$\begingroup\$ This is essentially what guitar pickups do, you need a different coil geometry (wide loops, loop opening oriented towards the moving magnet) \$\endgroup\$
    – crasic
    Jul 29, 2013 at 8:12
  • \$\begingroup\$ Does the detection definitely have to happen via the coil itself ? If not, just place a reed switch to detect magnet passage. However, you need to be sure that the magnetic field generated by the coil (when energized, without magenet being present) doesn't close the reed-switch. \$\endgroup\$
    – jay
    Jul 29, 2013 at 9:36

1 Answer 1


If you are looking to see an induced voltage on the coil when the magnet moves over it you may find that you need higher speed. Induced voltage is proportional to speed and therefore if you move it slowly, your oscilloscope may not see the tiny change in induced voltage.

If your coil has only 4 turns this won't help pick-up the movement. Induced voltage is proportional to number of turns so try winding with a hundred turns to see what you get.

Alternatively, if you set up a free running oscillator that uses the coil inductance and move the magnet towards the coil, the material (not the magnetism) that forms the magnet will alter the field pattern produced by the oscillator and you may see a change of frequency.

  • \$\begingroup\$ Pedantic footnote: Induced voltage is proportional to differential of magnetic field, I suspect, not actually proportional to speed :-) \$\endgroup\$ Jul 29, 2013 at 10:40
  • \$\begingroup\$ @AnindoGhosh Hmmm, a differential magnetic field.... Strictly speaking it's a change in mag field with respect to time but I'm sure that's what you meant. The "speed" in which the magnetic field changes (also speed of magnet moving) dictates the magnitude of the induced voltage. \$\endgroup\$
    – Andy aka
    Jul 29, 2013 at 10:59
  • \$\begingroup\$ Thanks, I will try the free running oscillator as i've seen some 555-based circuits to detect this but i wasn't sure how they worked. Also, my coils has way more than 4 turns, i forgot to comment it on the question (Edited). \$\endgroup\$
    – kR105
    Jul 29, 2013 at 18:51
  • \$\begingroup\$ @kR105 I meant an oscillator that relied on L as part of its tuning rather like a BFO oscillator (just the oscillator part) as used in a metal detector. There should be plenty of circuits on the web. \$\endgroup\$
    – Andy aka
    Jul 29, 2013 at 19:09
  • \$\begingroup\$ The induced magnetic field from a current in the coil is in the middle of the windings, and inversely, the strongest current is generated by magnetic field gradients (i.e. motion of the manget) in the middle of the windings, moving the magnet on the outside of a short coil will produce only a small signal. \$\endgroup\$
    – crasic
    Jul 29, 2013 at 20:02

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