One hobby that has maintain popularity through the years is slot car racing, and it also lends itself nicely to modification with the Arduino system. With some simple external circuitry accurate timing systems can be developed, and this was the purpose of the SlotCarDuino:
The cars are detected by a break in an infra-red beam that is vertically below the display gantry. The sketch uses the finite state machine library which doesn't require the use of hardware interrupts. Apart from its usefulness as a timing system, we're very impressed with the level of work taken to integrate the circuitry and especially in camouflaging the Arduino board:
For more details and instructions on how to build your own timing system, visit the project page.