"A person with that level of smarts, apparently, has enough brain power leftover in his spare time to invent tricorders, not to mention the greedlessness to share the blueprint with DIYers who want their own. Instructions are available from his Tricorder Project website. Like the Trek devices, Jansen's gadgets will measure the environment, things such as ambient temperature, humidity and magnetic fields, as well as take spatial readings for distance, location and even motion. They won't, however, identify aliens for you."
How to make your own tricorder, Smart planet
"Take one part open-source code, two parts OLED display, mix in a generous helping of curiosity and serve with a side of extra-delicious geekiness. That's your basic recipe for a real-life tricorder, as envisioned and modeled by Dr. Peter Jansen. The result is a handheld device with sensors for reading atmospheric, electromagnetic and spacial properties in the surrounding environment. Dr. Jansen's tricorder won't necessarily detect alien life-forms, but there's plenty of room to expand on his open-source design. Your mileage may vary."
"The Mark 2 tricorder, which is the more sophisticated of the two devices, runs Debian Linux on an ARM920T-based Amtel microcontroller. It is designed in a clamshell form factor, with a pair of OLED resistive touchscreen panels on the inside. Jansen's Mark 2 tricorder is powered by a lithium polymer battery (more energy efficient than the Mark 2 EMH, which is powered by Andy Dick) that fits inside the device's housing. The built-in sensors can measure temperature, humidity, atmospheric pressure, magnetic fields, color, ambient light level, GPS location, and distance to a surface."