Although I never get time any more to go airsofting (safe and legal team BB gun shooting in forests and fields), I thought it would be useful to have my own chrono to test and verify BB gun velocity and power. Whilst thinking about how to make one, I came across a disused solar garden light. The light was broken, but the plastic pipe used to stake it into the ground dismantled into sections, one of which fitted perfectly over the barrel and looked like a silencer.
After finding IR LED transmitters and receivers in Maplin that I could use as light gates, I found online that the Raspberry Pi could cope with the timing in nanoseconds required to measure the potential BB velocity. Next I then created a breadboard circuit example so that I could create some python code that would control the IR LEDs and return when a light gate was broken.
I had to decide a distance between the light gates that would provide accurate results. Using that distance, I then created 2 small prototype boards, one for the IR transmitters and one for the IR receivers. I then drilled appropriate holes in the plastic pipe and hot glued the boards to the outside of the pipe. Early testing showed it worked fine when simply dropping BB's vertically through the chrono. When I then moved to testing actual shots, I found that only one of the light gates triggered an event back in python. This is as far as I got on this project...so far. I think the distance between the gates is too small. If/when I get some time, I will have to create a slightly longer silencer pipe and extend the distance between the gates so that the Pi can handle the high velocity.
Project by or assisted by David Booth