Page 397 - Hacking Roomba

378 Part III — More Complex Interfacing

There are two mystery parts inside the battery pack. The first is a little flat rectangle connected
in series with the cells. This is a positive temperature coefficient (PTC) resistor that acts as a
sort of resettable fuse. If a short happens across the battery terminals, the cells heat up as they
dump current. This heats the PTC resistor, increasing its resistance and limiting the current
output. It’s a great failsafe device. The other mystery part is the temperature sensor, usually
located in the middle of the pack. In Figure 16-15 it’s the small black protuberance in the
center.

Watch out! Even apparently discharged battery packs can contain a lot of charge and cause a
nasty shock. The Roomba battery pack is designed to prevent rapid discharge, but if you start
taking it apart you’ll bypass that feature. Be careful.

Remote Control Hacking

The remote control that comes with Roomba is a standard circuit internally, but outputs custom
infrared remote codes. You can use a learning universal remote to record the Roomba remote’s
commands and then take apart the remote and use it as another way of computer-controlling
the robot. Figure 16-16 shows what the remote looks like taken apart. Each button circuit con-
sists of two circuit traces interleaved and a rubber button containing a conductive coating on its
bottom surface. When the button is pressed, the conductive coating makes the connection
between the two interleaved traces.

FIGURE 16-16: Roomba remote taken apart