Page 356 - Hacking Roomba

Chapter 15 — RoombaCam: Adding Eyes to Roomba 337

If OpenWrt is updated from White Russian RC5, you should try the updated version. It will have
more features and work better. If it’s not out yet, but you want to get a taste for what’s in store,
seek out how to build Kamikaze from the OpenWrt wiki.

Adding a Battery Pack or Roomba Power
Externally the WRTSL54GS has a 12 VDC input using a standard barrel power connector
and a cheap 12 VDC wall wart. Those wall warts put out noisy power, so there must be some-
thing like a 7805 voltage regulator inside the SL. When you open it, you find that instead of a
simple linear voltage regulator like the 7805, there’s an MP1410ES DC-DC switching con-
verter IC. This chip takes a wide range of DC input between 5 and 20 V and outputs a clean
3.3V. This means you can forgo building an external voltage regulator and instead just build a
cable going from the battery pack to a barrel connector.
Figure 15-3 shows the simple schematic and Figure 15-4 shows the complete power adapter.
There are two things to note. First, a 9V connector is used because the 8-cell AA battery holder
used has a 9V-style snap connector on it (8 × 1.2V = 9.6V, close to 9V). Second, a power diode
is inserted in series with the positive power lead. Those 9V snap connectors are easy to plug
and unplug, but they’re also easy to temporarily mix up and apply a reverse voltage to your
gadget. By adding the $0.20 diode, you can avoid blowing up your $100 router. The diode does
end up making the output voltage about a volt lower, but the SL doesn’t seem to mind.

+
1N4004
–

9V connector Barrel connector
FIGURE 15-3: Schematic for power adapter

In Figure 15-4 it’s hard to see where the diode is installed. It’s within the heat-shrink tubing
on the red lead (the one that is above and to the right). Figure 15-5 shows the diode being sol-
dered in between the two power leads. Make sure the white bar of the diode points away from
the 9V connector when soldering.
Figure 15-6 shows the power adapter and battery pack in use. The WRTSL54GS is totally
portable and pretty easy to carry around. A portable wireless router not connected to the
Internet may not seem all that useful until you hear about mesh networking. A mesh network
is a group of routers communicating in a manner that enables them all to provide Internet con-
nectivity when only a few of them at the edge of the group have Internet access. A portable and
quickly deployed mesh network could be the critically needed infrastructure during emergen-
cies that wipe out normal cell and telephone service.