Photograph Crashing Bullets with This High-Speed Chronograph
Measuring the speed of something like a paintball gun a relatively simple concept. Pick up the object in flight with a sensor, then measure the time it takes to hit a second sensor a known distance from the first. After some simple engineering math, you have the speed. Things become more complicated though when what you’re measuring is traveling at extremely high speeds, such as the velocity of a rifle bullet in the 1000 meters per second range. Normal phototransistors used in “lower” speed applications just aren’t able to pick up these microsecond-range optical interactions.
In order to photograph rifle rounds crashing into other objects, Tyler Gerritsen took on this high-speed challenge. His chronograph uses an array of photodiodes configured so that they can pick up voltage fluctuations occurring in the few microseconds that this type of bullet generates, and arranged in such a way that a .177 caliber bullet can’t pass through the sensing window without being picked up. Other tricks were needed to make the setup work correctly, such as measuring the variation of the Arduino Uno’s clock speed used for control, and calibrating the delay between trigger and flash on his photography equipment.
If you’d like to learn more about the concepts that Gerritsen used in the build, be sure to check out his excellent write-up. Or, if you’d just like to see what happens when a bullet hits a glass full of water, this album has the oddly beautiful answer!