Chad writes “For a while I have wanted to control things with a serial port. It was pretty easy to control a relay with a serial port. With a standard serial port you can control 2 relays. (with a parallel port you can control 8 relays, but I don’t have a parallel port on my system). A standard PC serial port has 9 pins. Pin 4 – DTR (data terminal ready) and Pin 7 – RTS (request to send) can be used to control a relay. These two ports don’t actually send data. They are used to signal the other device to tell it when to send data.” Here’s the HOW TO – Link.
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There’s a great book for this stuff, Controlling the World with your PC, by Paul Bergsman. I loved this book when I was a young tinkerer (alas I have much less time for tinkering these days, which is why I live vicariously through Makezine.) Mr. Bergsman also used to write for Circuit Cellar, I think. The book has general tips for interfacing with the parallel port, as well as designs for driving LEDs, relays, stepper motors, and lots of other stuff. Great fun!