i need to create a system that has two gates. when the first gate is broken it starts the timer and when the second gate is broken it stops the timer. i was thinking about using something like this
http://www.electronic-kits-and-projects.com/3130.htm
or like this
http://www.maplin.co.uk/Module.aspx?...1800&doy=15m10
or like this
http://www.maplin.co.uk/Module.aspx?...7513&doy=15m10

oh and i need it to do it through the computer.
does anyone think its possible and quite cheap?