High resolution stopwatch for cents
  A very low-cost, easy-to-make stopwatch is presented to support various experiments in mechanics. The high-resolution stopwatch is based on two photodetectors connected directly to the microphone input of the sound card. A dedicated free open-source software has been developed and made available to download. The efficiency is demonstrated by a free fall experiment. 
Zoltan Gingl and Katalin Kopasz
physics.pop-ph physics.ed-ph physics.ins-det