In this project, I wanted to make a device to accurately measure the velocity of a projectile. The approach I took was to use two pairs of infrared LEDs and phototransistors, with each pair facing each other, spaced a fixed distance apart, along with a microcontroller (Teensy 3.2), to determine the time it takes for a projectile to travel from one of the sensor pairs to the other. Whenever a projectile travels in front of the LED and phototransistor pair, the beam through the tube holding the sensors is temporarily broken, and with a pull-up resistor, a pin on the Teensy is given a high digital signal. This happens again 4 inches from the first sensor, and the Teensy records the difference in time from one sensor to the other. From this, a fairly accurate measurement of the speed of a projectile is calculated and returned to a plugged-in computer via USB serial.
