Goal counter and speed sensor for fussball tables
Built using an Arduino Mega 2560. In each goal, there are two small lasers (red laser pointer-style), and two photodiode sensors. The sensors are powered, and have a TTL output. When a ball passes through the goal, it will hit the beam and cause the output from that sensor to change (go high). By measuring the time between the front and back triggers and knowing the distance between them (which is about 3cm), you can calculate the speed.
Photo sensors: http://dx.com/p/photosensitive-diode-light-sensor-module-140676
Lasers: http://dx.com/p/6mm-5mw-red-laser-module-3-5-4-5v-13378
Arduino pins 18-21 are used for input. These stay clear of any regular-sized shields, and have interrupts on state changes.
John Stäck john@stack.se