The expansion board’s power comes from the three power rails of the 3pi robot base: VBAT, VBST, and VCC. VBAT is the battery voltage, which is supplied by the 3pi base’s four AAA cells. VBAT will change over time as the batteries discharge, and the nominal voltage depends on whether you are using alkaline or NiMH cells. VBST is the output of a 9.25 V boost regulator supplied by the battery voltage; this voltage remains constant even as the batteries discharge. VCC is the output of a 5 V linear voltage regulator supplied by the boosted voltage. All square pads on the expansion PCB are ground.
The mbed is powered from VBST, the Wixel is powered from VBAT, and the USB A connector gets its 5 V from VCC. The XBee receives its power from the mbed’s 3.3 V VOUT, which means . If you want to use an XBee without an mbed, you will need to provide your own 3.3 V power supply for it.
If you want to add your own sensors or custom electronics, you can access these power rails using the set of 3pi pins near the center of the board (see Section 4.i for more information).