I've always wondered exactly how fast potatoes leave the barrel of my potato gun, so I built this device to find out once and for all. At it's core is a high speed timer. It's driven by a 1 MHz quartz oscillator, which makes it accurate out to a microsecond. The timer is triggered by infrared photogates, placed at the end of the barrel. The first gate starts the timer and the second stops it again. That gives me a very accurate measure of the time it took for the potato to travel from the first photogate to the second, which allows me to calculate the velocity.
Once the timer has been triggered, it cannot be triggered again until the reset button is pressed. This prevents an oddly-shaped projectile from triggering it twice.
There are 3 different speed settings. The speed is indicated by the position of the decimal point point on the display. In the picture above, it is on the fastest speed.
The first digit is one millisecond, and the last is a microsecond. On the slowest setting, the first digit is a 10th of a second. This allows me to adjust the timer speed to account for the distance between the photogates. Regardless of the speed, the display always shows milliseconds.