A cheap laser break beam sensor.
Step 1: Parts
Parts: Laser module (recycled but new ones available @ https://www.adafruit.com/products/1056)
Light sensor (I used the one that came in the iot dev kit, but the cheap ones work fine @ https://www.adafruit.com/products/161)
Intel Edison Arduino @ https://www.adafruit.com/products/2180
Step 2: Program Intel Edison
Download the Intel Arduino IDE from the link above.
Download the light sensor code
Upload the code to the edison
When installing the light sensor, it may require a shroud of some type, too much ambient light can throw off the sensor.