I want to design a paintball counter. I have a metallic or plastic tube through which a paint ball passes.

How can I detect that the ball passed? What kind of sensors do I need?

I cannot see the ball from both ends of the tube, thus I need a sort of non mettalic detector. I dont want holes, and not microphone because of the tube will shake. And no obstacles in the tube.

  • \$\begingroup\$ Can holes be drilled in the tube? What is it's diameter relative to the tennis ball? \$\endgroup\$ – Matt Young Mar 31 '14 at 15:55
  • \$\begingroup\$ No drilling. The diameter fits perfectly the tube \$\endgroup\$ – Napster Mar 31 '14 at 16:25
  • \$\begingroup\$ Would be interesting to know what you plan to achieve overall. \$\endgroup\$ – Rev1.0 Mar 31 '14 at 18:26
  • \$\begingroup\$ I am trying to design a paintball ball counter without using IR sensors nor vibration sensors \$\endgroup\$ – Napster Mar 31 '14 at 18:48

If the tube is plastic you could arrange plates of a capacitor around the periphery of the tube - under normal circumstances the dielectric of the cap is air but as the tennis ball passes, it is likely that the permittivity of the material of the ball will increase the capacitance - it might increase the capacitance by 30% if care is taken.

Use a 1 MHz oscillator with the capacitor as part of the tuning - probably an LC resonant tank type oscillator would work just fine - count the oscillation cycles over a short duration - as the ball is passing, the count should drop measurably.

  • \$\begingroup\$ Great idea. A little bit like a stud finder. What can I google to have an idea on how to build the circuit? \$\endgroup\$ – Napster Mar 31 '14 at 17:08
  • \$\begingroup\$ I mean, what diy electric circuit example is there on the internet based on what you said? \$\endgroup\$ – Napster Mar 31 '14 at 17:26
  • \$\begingroup\$ @Napster any LC oscillator would do AND come to think of it any RC oscillator would do which relies on one capacitor doing the tuning - a simple schmitt trigger relaxation oscillator. Hey, even a 555 would work I believe. \$\endgroup\$ – Andy aka Mar 31 '14 at 18:54

Some possible alternatives:

  • pressure sensor (if the tube is not too much larger);
  • some sort of mechanic switch (not good if you don't want to obstacle the ball though);
  • photo detector (may require a hole, like Matt suggested);
  • microphone (record vibrations from the passage, need to define a pattern).

To me the microphone sound the least invasive, I'd go for that one. But it takes some engineering to figure out the sound pattern.

There are more for sure, but some more specifications may narrow down the list.

  • \$\begingroup\$ I dont want holes, and not microphone because of the tube will shake. And no obstacles in the tube. I forgot to mention. \$\endgroup\$ – Napster Mar 31 '14 at 16:26
  • \$\begingroup\$ Do you have other suggestions? \$\endgroup\$ – Napster Mar 31 '14 at 16:27
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    \$\begingroup\$ @Napster Do you have any other details? ;) It's not productive to play guessing, try to figure out all the details and edit the question accordingly. \$\endgroup\$ – clabacchio Mar 31 '14 at 16:27
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    \$\begingroup\$ Use a clear (or even translucent) plastic tube and an optical sensor \$\endgroup\$ – W5VO Mar 31 '14 at 16:35
  • \$\begingroup\$ I want to design a paintball ball counter \$\endgroup\$ – Napster Mar 31 '14 at 18:49

What is the nature of the tube shaking? If it shakes because a ball passed through, there's your answer. If it is due to something else, you may still be able to distinguish a ball passing from the shaking--the frequencies should be different.

For many plastic tubes (white PVC included), you can shine a bright light through it and see a shadow as the ball passes.


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