SerialChad 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.