Tripwire Trigger System Functional

A project log for Edgerton, A High-Speed LED Flash

Affordable photography tool used to capture images of bullets with no apparent motion blur

tyler-gerritsenTyler Gerritsen 07/01/2019 at 13:220 Comments

Various methods of triggering are available for high-speed photography, but the triggering systems are not cheap.  In an effort to make the hobby accessible and affordable, I've added a built-in triggering system that doesn't require any major hardware.  The 'Tripwire Triggering System' works by placing a special jig on the muzzle of the rifle.  A fine wire is positioned in front of the barrel, and when the projectile breaks the wire, Edgerton will strobe after a pre-programmed delay.

Here's a photo with the triggering system set to 0 delay (triggering as soon as the tripwire is broken).  You can see the tripwire breaking just left of the projectile.

Part of the challenge was to add the functionality without affecting the intuitive user display.  The limitations of the 7-segment display really become apparent here.  The delay setting can be selected by pressing the encoder button, then turning the encoder to adjust the delay.  At delays shorter than 90 milliseconds, the display shows the delay in the format XX.XX milliseconds - although the decimal point is actually a colon.  I wish that the points on this cheap display could be controlled independently so that it could show an actual decimal point.  At 90 milliseconds and above, the display shows the delay in the format XXXX milliseconds (no decimal point).

This triggering system requires that the user knows the projectile's speed beforehand.  Without knowing the speed, the projectile's position after a given delay isn't known.  It is possible to calculate the projectile's speed by taking a photo of a free-flying projectile after a given delay and determining how far it traveled from the muzzle.

Very fine wire (such as the strands in 22-guage silicone wire) is required.  As shown below, the wire can affect the bullet's trajectory.