Shown here is the Mark II detector. It is smaller in diameter, shorter and more sensitive than the first prototype.

This detector is suitable for airgun use and clamps onto the end of the barrel. Each gate circuit has a potentiometer for sensitivity and uses a transistor to amplify the signal from the optical trigger and an SCR to trigger the digital pin(s) of the Arduino.

The length between the first and second optical gate is 12" (1 foot). This makes the FPS calculation exceedingly simple:

FPS = 1000000 / microseconds

To get meters per second we simply multiply our FPS by 0.3048.

MPS = FPS * 0.3048

I should have the source code up on Github in a few days so stay tuned!