https://github.com/Hermann-SW/Raspberry ... m-exposure
Knowing the length of the bullet allowed to determine length per pixel. Distance between successive exposures of bullet can be computed from length per pixel. Finally multiplying with frequency resulted in speed of bullet. Intially I did those experiments with v1 camera and global external shutter. Later with just an HQ camera, even at daylight:
https://github.com/Hermann-SW/Raspberry ... t-inflight
A modern gun chronograph does determine bullet speed differently:
The modern chronograph consists of two sensing areas called chronograph screens, which contain optical sensors that detect the passage of the bullet. The bullet is fired so it passes through both screens, and the time it takes the bullet to travel the distance between the screens is measured electronically.
I did buy a 32$ gun chronograph, and did not notice the word "mini" -- it is really a small device, 75x43x33 mm³, with tube diameter of 12mm:
The bullet has to be shot through the tube. I can see the two screens from the side, but they are roughly 1cm inside the tube. I will try to use v1 spy camera to get detailed photos from the screens.
After turning on the gun chronometer, I tried to get a first measurement. I took a pointed bullet and just tried to throw it through the tube -- always 0 was displayed. Then I placed chronometer on floor with tube vertical. After a few tries letting fall the bullet from 50cm height, it passed both screens in tube and resulted in 3.3m/s measurement: I did not correct the weight of the bullet, used a milligram precision scale to see that pointed pellet (8mm long, left of chronometer) does weigh 620mg, so the 0.20g weight on display is wrong.
After playing with the chronograph I ended up in a different display, showing multiple measurements. That photo revealed that my previous 5 attempts resulted in measurements as well, but speeds less than 2m/s were not displayed in the other display. On the left side you can see the tube in this photo as well:
Product description states:
Measurement speed range: 1-999.9 m/s (fps range 30-999.9)
Not sure yet whether screen is a single row camera, or even a single pixel "camera". The airgun of a friend I use has shown speeds of 109m/s, far below maximal speed that can be measured.
I will try to repeat Raspberry camera PWM multiple exposure speed deterrmination just after pellet leaves the chronograph tube, in order to verify that both speeds determined match.
The device runs on 5V, later I will try to open it and get hold of the 2nd screen signal (with voltage level adjustment to Pi). I did captures with sound trigger before:
https://github.com/Hermann-SW/Raspberry ... nd-trigger
This is a 5 exposures photo of an inflight softair pistol pellet taken with sound trigger:
Plan is to use 2nd screen signal (instead of microphone) to trigger capture of single digit microsecond flash after delay so that a single exposure photo at high speeds becomes possible.