This kit includes a round printed circuit board (PCB) with holes spaced on a 0.100" grid, one extended 2x7 male header, two extended 1x2 male headers, one 2x7 female header, two 1x2 female headers, four 7/8" nylon spacers, four 1-1/4" screws, and four nuts.
The expansion PCB matches the diameter of the 3pi chassis and mounts just above the tops of the wheels using the four included screws and spacers. Once assembled, the PCB has electrical connections to the base that allow you interface your own electronics with the 3pi robot, which is sold separately. These connections give you access to the ATmega168's free/jumpered pins, as well as to the three on-board voltages: VBAT (battery voltage), VCC (regulated 5 V), and VBST (regulated 9.25 V that is supplied to the motors). Additionally, the expansion PCB connects to the base's power button and battery charge port, allowing you to add your own power buttons and charge ports.
This expansion kit's PCB has cutouts that allow you to view the LCD below and access the power button, reset button, and ISP programming header. If you want additional I/O lines or extra prototyping space and you do not need the LCD, please consider the version of the expansion kit without cutouts, which replaces the LCD. For a more advanced expansion kit, please consider the m3pi expansion kit, which lets you turn your 3pi robot into an m3pi robot.
The expansion PCB is designed to provide plenty of prototyping space for your components. It has room for one 0.6" 40-pin DIP (dual in-line package) component, such as the ATmega32 in the picture below, or for numerous smaller DIP components. The prototyping space extends all the way to the edge of the PCB, allowing you convenient points to mount a variety of sensors such as bumper switches and range-finders. The silkscreen shows how the pads are connected; the electrical connections are on the bottom side. You can cut the copper traces on that bottom side (with a sharp knife or a small rotary tool cutoff wheel) if some of the pre-made connections interfere with your desired layout.
The two unused I/O lines on the 3pi's microcontroller are it's serial transmit and receive lines. This means that you can add a second microcontroller or microcontroller board, such as a Baby Orangutan, Basic Stamp, or Arduino Nano, to the expansion PCB. This second microcontroller would deal with all of the sensors and additional hardware on the expansion PCB and control the base via serial commands. We have released a serial slave program for the 3pi base that turns it into a serially controlled platform that can be driven at the whim of another microcontroller.