Page 63 - Hacking Roomba

44 Part I — Interfacing

And you will need these tools:

Soldering iron, stand, and solder, Jameco part numbers 170587CK, 192153CK, 141795
Hot glue gun and hot glue
Wire cutters and wire strippers

IC Hook test leads, Jameco part number 135298
Third-hand tool, Jameco part number 26690
Digital multimeter
DC power supply (wall wart) between +9V and +24V, Jameco part number 199566PS
Mini DIN 8-pin socket, Jameco part number 207722
Keyspan USA-19H or similar USB-to-serial adapter
PC (Mac OS X, Windows, Linux) capable of running Java programs
RoombaComm software package downloaded from www.roombahacking.com/
Terminal emulation program (ZTerm for Mac OS X, RealTerm for Windows, minicom
for Linux)

The Circuit

Figure 3-2 is the schematic of the entire circuit to be built. There are essentially three circuits
in that schematic: a power supply, an RS-232 transceiver, and an LED lamp. The power supply
converts the unregulated approximately +16 VDC Vpwr power line from the Roomba into the
+5 VDC needed by the RS-232 transceiver. The RS-232 transceiver converts the 0-5 VDC
signaling used by Roomba into the +/-12 VDC used in RS-232. And the LED circuit is there
to let you know that power exists (and, besides, everything needs an LED).

If Figure 3-2 looks like hieroglyphics to you, see Appendix B for how to read schematics.

Understanding Voltage Regulators

The voltage regulator circuit, shown in Figure 3-3, is the same voltage regulator circuit seen in
countless hobbyist projects. The 78L05 voltage regulator takes any voltage input between 7 and
35 VDC and converts it to 5 VDC. And it can supply up to 100 mA (0.1 Amp) of current. Its
brother, the 7805, can supply up to 1 Amp of current. Why 100 mA of current? Why are the
capacitors there? And why were those particular capacitor values chosen?