Planet Arduino

[Electronoobs] built a coil gun and the obvious question is: how fast is the projectile? To answer it, he built a chronograph suitable for timing a bullet. The principle is straightforward. A laser and a light sensor would mark the entry and exit of the projectile over a known distance. As it turns out, there are some issues to resolve.

For one thing, a laser is too narrow and might miss the projectile. The first attempt to rectify this used mirrors, but the loss was too great — we suspect he was using a second surface mirror. The final answer was to use an array of detectors and removed the laser’s collimation lens to cover a wider area.

That worked, so all that was left was a nice mechanical design to allow changing the height of the sensors and the distance between the sensors. After that, an Arduino can take over.

We liked the mechanical design and the way he managed pushbuttons in the 3D printed case. We couldn’t help but wonder if a first surface mirror might have worked better. We also thought it would be nice to add some sort of encoder to let the device measure the distance between sensors automatically since it is adjustable. We also thought the response time and wavelength sensitivity of light-sensitive resistors might be a bit off. It seems like a photodiode or transistor would be more accurate and have better sensitivity to the laser or even just a conventional light source. But this does seem to work.

How fast was the coil gun? Well over 100 meters per second. For a point of reference, a .22 caliber round will have a muzzle velocity of well over 300 meters per second, but, still, 120 to 130 meters per second is nothing to sneeze at.

If you need a coilgun, we always liked the looks of this one. Or, you might prefer a more futuristic look.

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.

To capture images of bullets “interacting” with various objects, photographic hacker Tyler Gerritsen created an impressive chronograph rig, able to measure the speed of a bullet launched from a rifle at 1000 meters per second. While the concept of measuring time from one sensor to another isn’t new, implementation at this speed required some interesting tricks.

To accomplish this feat, Gerritsen designed his own sensor array using photodiodes in a reverse-biased setup, and even calibrated the clock speed of the Arduino Uno for control in order to account for any variation. Finally, the time between triggering a flash and light actually appearing had to be compensated for in the code, a different value for each type of equipment.

The project write-up is a great read for anyone interested in this type of photographic or measurement technique, and the resulting photos can be seen here.