Creating sensor networks is a lot of fun and tremendously useful. There are plenty of reasons to collect data from multiple nodes and bring it to a central location. There are equally great reasons to take commands from a central location and create real events in multiple physical locations remotely. The XBee radio is capable of receiving commands that set its digital I/O output pins to trigger real-world events without the use of any external microcontroller. By itself, the XBee can power an LED, sound a small buzzer, or even operate a tiny motor. Many more devices can be operated directly from the XBee with the use of a relay. Relays are really just electrically operated switches. They allow low currents to ....