FIGURE 4. Assembled
FIGURE 5. Assembled
have on hand. You could also have
your own bases custom laser-cut by
Pololu. For the purpose of this article,
the only requirement is that the chassis
includes two small DC motors and
enough breadboard space to accommodate M-bot’s circuitry.
Use the photo in Figure 4 as a
guide in assembling the bottom base.
First, using small nuts and bolts ( 4-40
work well), attach the Double Gearbox
to the bottom base. Next, attach the
Tamiya ball caster to the (front) underside of the bottom base. Insert the bolts
in a downward direction to minimize
their protrusion on the top of the base.
If you are using a base of your own
design, you will need to add a spacer
between the base and the caster to
enable the base to sit in a level position; a suitable spacer is included in my
laser-cut set. Finally, attach the two
battery packs using Velcro® strips.
Place the batteries forward enough
so that the weight distribution keeps
M-bot’s front down.
M-bot’s breadboards are ordinary
400 point boards ( 3. 25 inches by 2.125
inches) that are available from Pololu,
as well as many other vendors. If you
look closely at Figure 5 — which is a
photo of M-bot’s top base with the
breadboard assembly attached — you
will see that the breadboard area consists of 2-1/2 breadboards. The one-half
breadboard at the bottom of Figure 5
provides some flexibility in connecting
M-bot’s circuitry to the power and
motor wires from the lower base.
To make a half-breadboard, simply
cut a breadboard right down its centerline. No embedded metal parts run
underneath the indented centerline of
a breadboard, so it’s a simple matter to
cut one in half. You can use a handsaw, hacksaw, or band saw and then
clean up the cut by sanding it smooth.
Orient the 2-1/2 breadboards in the
same direction (so that they will properly interlock when you are finished), and
remove one double power rail from
each breadboard by slicing through the
double-backed tape on the bottom
directly underneath the junction of the
main board and the double power rail.
Be sure to remove the same double
power rail (for example, the left one) on
each board. Finally, join the boards to
form one unit, as shown in Figure 5.
Attach the breadboard assembly to
the upper base by removing the backing from the tape on the bottom of the
breadboards and sticking them to the
top base as per Figure 5. Be sure not to
block the long, narrow slot that is cut in
the top base; the wires from the
bottom base will be routed through this
slot to the bottom of the breadboard
unit. The two double power rails that
you removed earlier can be attached as
shown in Figure 5, so that the power
and ground lines can be easily routed to
each of the breadboards.
Next, install the two SPST switches
in the 1/4 inch holes at the rear of the
top base. Orient the switches so that
“on” for each switch is in the forward
position. Finally, connect the two bases
using 1-1/2 inch standoffs or two inch
pieces of 6-32 threaded rod.
Now that M-bot’s chassis is fully
assembled, we are ready to begin the
chassis wiring. If you are using the type
of battery holders specified in the Parts
List (Figure 2), they each accept
two-wire assemblies that snap onto
their connectors — if not, adjust the
following to suit your parts.
First, solder the positive lead from
the six-volt battery pack to one
terminal of the switch on M-bot’s right
side. Similarly, solder the positive lead
from the nine-volt battery to one
terminal of the switch on M-bot’s left
side. Be sure to leave enough slack to
allow for changing the batteries — if
necessary, extend the leads with
stranded hookup wire.
Next, solder a six inch piece of #22
solid wire to the other terminal of each
switch and route it up through the slot
on the top base. (You may actually
want to solder two leads to each
switch at this point, in case you later
decide to add additional circuitry to M-bot in the large empty space in the rear
of the bottom base — the extra power
connections could just be taped off
and tucked somewhere convenient for
possible use later.) Insert the end of the
wire from the nine-volt battery near the
SERVO 09.2007 53