Sub-$10 sound card photogate variants
  Recently, a simple and very low-cost photogate has been shown by Horton (Phys. Teach. 48, 615, December 2010) as an efficient experimentation tool in physics education. The photogate connects to the microphone input of a personal computer and a free software can be used to visualize the light interruptions caused by a moving object like a pendulum. Although the device works properly, there are further possibilities of improvement and similar alternatives also exist. The following brief review may help teachers to pick the one that best fits their needs and possibilities. 
Zoltan Gingl
physics.ed-ph physics.ins-det