Chrono trigger

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Problem Statement

The Auto_Trigger works really well as a trigger for high speed photography where the aim is extremely well controlled and/or there is a lot of spall from the target. However, it was discovered that rifle bullets (CZ-5M, 7.62x39mm) vary their trajectory by several bullet diameters even as close as 5 feet from the muzzle. Certain targets also cause the bullet to fly off course. This results in numerous "misses" where the bullet misses the laser and forces the spall or target to trigger the photo. The bullet is usually long gone by then.

I want a system where the velocity of the bullet can be measured and used to 'place' the projectile at a programmable distance from the detectors. A Chronometer based trigger system, or a Chrono-trigger if you will ;)

System Overview

The system uses an EZ430RF2500 board which contains an MSP430F2274 and a CC2500 radio chip. A "shooting chrony" was donated to the proejct after its owner shot the unit. While still functional, I think he was too embarassed to bring the JB-welded monstrosity to the range in front of his peers! I pulled the sensor modules out and interfaced them with the timer capture/control peripheral of the MSP430

The MSP430 is clocked at 16Mhz and the capture/compare timer is run at 4Mhz with a distance of 241mm between the gates. The system then has a trigger distance granularity of 250µm and a max range of 16 meters at 1000 m/s (3280fps) and a granularity of 25µm and a max range of 1.4 meters at 100 m/s (328fps). Repeatability under real world conditions is still unknown.

The wireless radio is used to relay the chronometer data back to a PC or other interface and make it useful as a stand-alone chronometer. Also, good data to have so that the speeds can be correlated to actual photographs!


The photogate sensors were reverse engineered to determine the best way to interface. A falling edge is provided during a fast change in light (differentiator trigger). If the light level is too low or is held low for too long the AGC will begin to oscillate at the extreme giving false-signals. Since shooting must be performed in the dark, the sensor tunnel is flooded with two 100mA high brightness IR LEDs.



A slideshow of the build and photos taken using the Chrono-trigger are shown below.

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March 25th 2011 - Sensor schematic reverse engineered. Pretty slick design, uses a JFET for AGC and a differentiator for triggering on fast events only.

March 27th 2011 - Sensors incorporated onto a board and interfaced with MSP430. Software written and tested in chronometers mode. Trigger line appears to work but I still need to interface it with a flash unit and get some actual photos. Building a shooting enclosure with IR leds is the next step.

March 30th 2011 - Laser cut a simple tunnel shaped box 12 inches long x 6 inches square.

March 31th 2011 - Mounted the photogates and wrote more code. First working test with photo!

April 2nd 2011 - Laser cut a press fit wooden box for the circuitry and tried some impact photos. Jordan and I ran into some problems but once we got a feel for its quirks the chrono trigger seemed to work well. First good impact photo!

April 3rd 2011 - Laser etched Chrono onto the top of the box. Lookin good!

June 18th 2011 - Went to the desert and tested it with the REAL guns! Found that the "sweet spot" is smaller than I had anticipated. Need to mask out the areas that will result in a mised trigger or possibly recess the sensors.

MSP430 Source Code Can be found here(For reference only, all rights reserved)

2010 Jasper Nance KE7PHI