X10 is a popular technology that allows you to control electrical devices throughout your house with existing 110-volt wiring, eliminating the need to run new wires - a costly enterprise in some cases. Because X10 equipment simply plugs into wall outlets, it's usually simple to install and use. But it's far from simple in its design. For those who want to understand what's going on inside those little boxes, here's a quick tutorial. X10 is both a patented line of home automation products and a communications protocol for remote control of electrical devices. It is designed for communications between X10 transmitters and X10 receivers using household wiring. Although transmitters and receivers generally plug into standard receptacles, some may be hardwired into electrical boxes, screwed into lamp sockets or built into appliances. Transmitters send commands such as "turn on," "turn off" or "dim," all of which are preceded by the identification of the receiver unit to be controlled. This broadcast goes out over the electrical wiring in a building. Each receiver is set to a certain unit ID and reacts only to commands addressed to it. Receivers ignore commands not addressed to them. Each X10 system is capable of using 16 "house" codes (labeled A through P) and 16 "unit" codes (labeled 1 through 16). Since each device address is a combination of the house and unit codes - a lamp, for instance, might be addressed as J9 - there are 256 possible commands in each system. The reason for the house codes is that the commands can feed back through the utility lines to any other house served by the same transformer. There are usually half a dozen homes on any one transformer, so the inventors were looking forward to the day when many homes were outfitted with X10 devices. In use, an X10 receiver module is plugged into the wall outlet near the device you want to control. Using a screwdriver, the house and unit codes are set to a unique address. That address then is programmed into the transmitter unit. The "AC" in "110 volts AC" stands for alternating current. The voltage (and current) in the line swings from positive, through zero, to negative and back to zero 60 times per second. It thus passes through zero 120 times per second. The instant the voltage passes through zero is called the zero point. At that point, X10 devices communicate, because that's the quietest time on the AC line for the receiver. The X10 signal is in binary form - that is, it consists of "1" and "0" bits. Each "1" in a transmission is a 2-volt, 120-kHz burst superimposed on the AC line for .001 seconds, and each "0" is the absence of that burst. The next bit is sent on the following zero-crossing. A single command normally takes eleven cycles of the AC line to finish. X10 has no effect on the operation of electrical devices in the home, although appliances starting and stopping can occasionally introduce noise that affects X10 signals. The X10 market is burgeoning. More and more devices are using the technology, including those tiny cameras whose ads constantly pop up on your computer screen. And the systems can be controlled using a home computer that creates schedules for devices to be turned on and off.