This was my project throughout AS level back in 2009. It was a relatively simple design as chronographs go, but proved very effective and accurate when compared to commercial chronographs. The input stage was comprised of two pairs of photodiodes and infrared LEDs set 10cm apart in a tube, which were sampled by two op amps configured as comparators. These comparators were then linked to the interrupt pins on a PICAXE18A, which was used to translate the timing signals into a speed. Finally the microcontroller outputted this speed in the format for 7 segment displays to three shift registers, which drove the displays without any microcontroller input.