A good introduction, but a couple simplifiers:<p>1) You can read a photocell directly from a PC without an Arduino. The PC serial port and a couple of passive components can be used as a simple analog-to-digital converter for a cost of pennies. One of many examples can be found here: <a href="http://www.cedarlakeinstruments.com/blog/archives/10" rel="nofollow">http://www.cedarlakeinstruments.com/blog/archives/10</a><p>2) Similar idea using a Beaglebone or Raspberry Pi would eliminate the Mac/PC for a more compact solution.