I'm trying to make airsoft chronograph/chronometer (a device that measures airsoft bullet speed). The idea is to measure time in which bullet is traveling from point A to point B, and then I can calculate bullet speed.
Bullet is shot into plastic pipe with diameter of 32mm. I've tried to put on one side IR LED and on the other side phototransistor (BPW 40), both at the start and end of the tube. Something like this:
Blue circles are phototransistors, red circles are IR LEDs and white circles are airsoft bullets.
When bullet passes through area where bullet 2 is drawn (just in between IR LED and phototransistor), everyting works perfectly. But when the bullet passes where bullets 1 and 3 are drawn, it isn't detected. That is expected, but unwanted behavior.
So my question is: how can I detect bullets regardlessly of it's position ? I've thought about putting IR LEDs and phototransistors around the pipe (not only in one place), like this:
but this solution isn't cheapest: 5 IR LEDs + 5 BPW 40 = cca $12 times 2 (because I need it on both sides of the pipe) = $24. Is there any cheaper solution ? Is BPW 40 good choice for phototransistor ? I don't know which IR LEDs I'm using (and salesman in the store also doesn't know – he told me these are generic ones for remote controllers, like TV or DVD controller).
Diameter of the pipe is 32mm and it will be long about 14cm (sensors will be 10cm apart).
Diameter of airsoft bullet is 6mm.
I'll go for my second idea. I just have one more question:
is it better to arrange IR emitters and photo transistors this way:
or this way: