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This chronograph project consists of a sensor assembly and timer circuit box that can be used to measure the muzzle velocity of a projectile fired from an air cannon. 

I originally built this setup for use with a large air cannon with a 3" bore.

The sensor assembly is attached to the end of the air cannon barrel. It contains infrared sensors used to detect the passage of the projectile. A microprocessor measures the time it takes for a projectile to travel a known distance and displays it on an LCD. From this information, the velocity can be easily calculated.

Step 1: Overview of Operation

Below is a block diagram of the timer box and sensor assembly system. The sensor assembly is shown at the bottom of the diagram, with a round projectile traveling through it. The functional blocks within the grey box are located on the PCB. The switches, connectors, and LCD module are mounted on the case of the timer box.

The sensor assembly consists of a length of 3” diameter PVC pipe, with the two IR sensor pairs mounted 6 inches apart. The sensor assembly has a threaded fitting on one end, which allows it to be easily connected to another threaded fitting on the end of the barrel.

The timing circuit initially monitors the first sensor, waiting to detect that the beam has been broken by the projectile. After the first beam is broken, the processor starts an internal timer to record the time that has elapsed since the beam was broken. The software then begins monitoring the second sensor. When that beam is broken, the timer will be stopped. The processor then displays the time elapsed between the breaking of the beams on the LCD.

About This Instructable

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Bio: "But I was going to Toshi station to pick up some power converters!"
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