In the study of ballistics, you can do very little without knowing the velocity of a projectile. Whether you need to hit a target at over a mile, check if a paintball gun is safe for opposing players, or photograph high-velocity objects, you need a way to measure that velocity. [td0g] enjoys the challenge of photographing bullets impacts, and has created an open-source ballistic chronograph to help achieve this.
[td0g]’s design makes use of two light gates spaced some distance apart, and the time that an object takes to travel between the two is measured and used to calculate velocity. Most commercial ballistic chronographs also work in this way. [td0g] created the light gates using pairs of infrared photodiodes and LEDs. When there is a sudden dip in the amount of light received by the photodiode, the Arduino control circuit knows that an object has passed between the photodiode and LEDs and triggers the timer. An LCD shield on the Arduino is used to control the software and display velocity. As you probably guessed, clock accuracy is very important for such time measurements, and [td0g] demonstrates a simple technique using a smartphone metronome app to manually calibrate the clock to acceptable accuracy for his purposes.
This is the second such chronograph that [td0g] has built, and he changed the frame to be mostly 3D printed for easier construction, and upgraded the sensor boards to custom PCBs instead of the perfboard. If you want to build your own, all the design files are up on Github, and the light gate sensors should be for sale on Tindie soon. He has successfully used the rig to measure various projectiles ranging from 100m/s (paintball) to 875 m/s (rifle bullet). With a high power rifle, the chronograph needs to be at least 2 m from the muzzle to avoid damage or false readings from the muzzle blast, which also means the careful aim is required to put the bullet through the sensing area, without killing the chronograph in the process.
Getting the shutter to trigger at just the right moment is probably the biggest challenge of high speed photography. We’ve a number of different triggers, including for water balloon photography, and a laser for droplets.