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Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
@Gordon
Yes - in that case you have a transmitter and a receiver, and the hand is just cutting the path of the light between the two.
A kit like this would have all the parts you'd need for something like that, plus a few!
http://www.ebay.co.uk/itm/Sensor-modules-kit-37-in-1-arduino-raspberry-pi-pic-arm-mcu-/222618981407?hash=item33d520141f:g:ZLsAAOSwilBZmaaQ