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I'm trying to create an ammo counter for an airsoft gun. I'm going to control it with an attiny series MCU and i want to detect the projectiles exiting the barrel using a light based sensor. Now that's the question:

Which method of detection is the best for such application? A photodiode, a phototransistor or other, perhaps laser based sensors?

The projectile is 6mm in diameter and at 115 m/s after exiting the barrel which means that the sensor will have around 70μs to detect it.

DELTA12
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  • Can you say something about how it would be mechanically mounted? At the exit of the barrel? I think it is important to mention if ambient light will be reaching the sensor or if it could be in the dark, protected from ambient light. – payala Apr 10 '16 at 21:11
  • I have 3D printed a mount for a photodiode + led couple which i tried using before but the resutls weren't so great (the circuit couldn't detect such fast moving object) so i want to rebuild the circuit. It had no trouble with ambient light back then. – DELTA12 Apr 10 '16 at 21:13
  • As aleternatives, you could consider detecting the acceleration or sound caused by the bullet being fired. – Wouter van Ooijen Apr 10 '16 at 21:17
  • Yes but direct projectile detection is the most reliable way since it can as well find out whether the weapon did actually fired or just shot blank. – DELTA12 Apr 10 '16 at 21:19
  • Using a CW laser, it should be no problem to measure even a 26x faster projectile (chronographs measure rifle bullets at >2600fps). **However** attempting to "catch" the projectile amid the cloud of vapor right at the muzzle will likely prove quite problematic for you. There's a reason why nobody fires with the muzzle of their weapon 1" from the first "screen" of their chrono. Maybe it would be more fruitful to place your counter between the hopper/magazine & the chamber; catch them as they're loaded into the barrell @ lower speed & without the vapor 'noise'. – Robherc KV5ROB Apr 10 '16 at 21:21
  • Hey we're talking about an airsoft gun not an actual firearm. There's not muzzeflash explosions or vapors. The bulled is fired by compressed air nothing more. – DELTA12 Apr 10 '16 at 21:26
  • Can you post a schematic of the circuit you used last time?, maybe you already had an interesting approach, and just need some running for the speed. – payala Apr 10 '16 at 21:50
  • @DELTA12 compressed air often causes condesation of water vapors, thus a spray of water mist may be visible if you watch one on high-speed video. Either way, the loading round moves slower & is very easy to detect optically ;) – Robherc KV5ROB Apr 13 '16 at 01:14

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You can try a Photo Interrupter. Such self contained emitter / detector devices are common and can be found in a variety of configurations. This one has a 10mm opening (ball is 6mm wide) and a max low to high response of 9us + a max high to low of 15us. Assuming this represents the shortest (worst case) possible detectable obstruction of 24us. Which, if correct, is << 70us.

https://www.sparkfun.com/datasheets/Components/GP1A57HRJ00F.pdf

st2000
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