Thanksgiving is past and Christmas is almost upon us.  That means it's time to pull out your tangled mess of lights and decorate your room or apartment.  But this year, don't settle for the same boring chaser light setup you always use--throw some electronics know-how into the mix and make a display you can be proud of.
      Most light strings plug directly into the wall, meaning they expect to see an AC voltage.  Unfortunately, the digital circuits you'll want to control the lights with run on DC voltages.  Thankfully, a device called a triac bridges this gap for us.  It's a semiconductor device that acts as a switch for AC current that's turned on and off with a DC voltage.  The MAC15A8 is a triac you can buy for about $1.20 at the ECE stores on campus which will easily handle a string of lights.
      Another important component is the optoisolator.  Since you'll probably want to control your lights with a computer via the parallel port, you'll need to somehow isolate the computer from the light strings in case something goes wrong.  A simple component like the MOC3041 will do this job for you, and they sell for about $1.00 each from any electronics supplier.
     So what would a typical light control circuit actually look like?  See the example below.
     This circuit uses a single data bit of a PC parallel port to control a string of lights.  Please note that ACM cannot take responsibility for the use of this circuit.  Also, make sure that the side of the circuit plugged into the wall is physically separated from anything you might touch.  It may be wise to add a fuse to this circuit, though the light strings usually provide this for you.
     To turn on the lights, simply set the least significant bit of the parallel port.  Clear the bit to turn off the lights.  Or, substitute the parallel port interface with a microcontroller.  For more fun, connect multiple strings of lights to the other data bits of your parallel port by repeating this circuit.  Cut up strings of lights for more possibilities.
     Triacs can be used for other control projects as well.  Check out the Linux Coffee how-to at http://www.linuxdoc.org/HOWTO/mini/Coffee.html for a particularly good example.