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I'm trying to design a speed trap for an airsoft BBs. My original idea was to use IR photosensor. But this solution is prone to fail if dirty.

I'm playing with the idea to use a coil on a PCB with hollow centre where a BB would fly through. Can I use change of relative permeability of coil's core to detect proximity of a plastic BB? Would it be practical?

BB's diameter is 6mm and it should fly parallel to PCB coil with hollow center.

Thank you

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  • \$\begingroup\$ How would a plastic BB cause a significant change in the permeability of the coil's air core? How can a BB fly parallel to a coil and through its "hollow center" at the same time? No, I don't think this will work. \$\endgroup\$ – Elliot Alderson Aug 15 at 18:08
  • \$\begingroup\$ I meant parallel to the hole. So the plastic would not make significant change? \$\endgroup\$ – Adrián Paníček Aug 15 at 18:15
  • \$\begingroup\$ Plastic is non ferrous and non conductive, so both mechanisms for external influence on a magnetic field are missing. \$\endgroup\$ – Aaron Aug 15 at 21:20
  • \$\begingroup\$ You could try making a small capacitor, then as the plastic is between the plates it will cause a change in the permittivity which will cause the capacitance to change. However, that all depends on the plastic having a different enough permittivity compared to free space (air). \$\endgroup\$ – Aaron Aug 15 at 21:30
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So you need to measure the speed of a fast moving object.

This requires that you measure its position and the time at that position twice and precisely. Or if you know the weight of the projectile you measure its energy and hence derive speed.

Neither of these things can be done with a plastic projectile through an inductive sensor.

Go back to the classic two position opto sensor. Alternately shoot the projectile into a block on a string and measure it's weight and height rise to get energy.

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