Instructables user Adum24 has devised a simple and effective laser-tripwire system based around an Arduino board. By using a laser diode and a photoresistor you can replace the physical 'trip wire' with a laser beam, and use this as a base for various security functions, counters, or anything that requires detecting motion in one direction. It is easy to assemble, fits in a small tin - and as shown in the video below it works well:
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If this sort of project intrigues you and you're new to Arduino, the first step is a solid board for your projects - our Freetronics Eleven - the Arduino-Uno compatible with low-profile USB socket, onboard prototyping space and easy to view LEDs: