FIGURE 4. Attach the L298 module using 4-40
hardware, spacers, and standoffs. A small deck
fits on top for the mini breadboard.
Mounting the L298
The first order of business is to remove the switch
panel and wiring from the Phase 1 Beginner Bot. These
components won’t be used this time around, but don’t
discard or recycle them just yet! You’ll have the option to
reuse these components in later installments of this series.
Note: Rather than unsoldering the wires to the motors,
completely remove the motors from their gearbox housing.
Merely pull back on the retainer clip on the side of the
gearbox, and lift out the motor. Set the motor/switch
assembly aside for the moment.
Drill four holes into the Beginner Bot base for mounting
the L298 module board. Be sure to position the board on
the end opposite the skid. The reason: You want the
heaviest components over the skid to keep the robot from
tipping the other way. You’ll be placing the batteries —
which weigh more — on the skid end.
Orient the L298 module so
that the black metal heatsink
faces toward the far end of the
robot. We’ll call this the “front” of
the Beginner Bot.
Use 4-40 x 3/4” machine
screws, spacers, and threaded
standoffs to mount the board as
shown in Figure 4. The Seeed
Studio L298 module leaves plenty
of room for mounting the board,
but not all H-bridge boards are so
cooperative. If mounting space is
tight, you may need to use
smaller 2-56 size hardware which
is harder to find.
Also shown in Figure 4 is
how to mount a separate small
deck for a mini breadboard. See Figure 5 for a cutting and
drilling template for this deck. Use 1/4” plywood or
expanded PVC plastic. Don’t attach this deck quite yet,
because you need to first wire up the L298 module.
Replacing the Tamiya
The Beginner Bot uses a pair of low cost gearbox
motor kits from Tamiya. The small DC motor that’s included
in the kit is rated for operation at just three volts. As
motors go, it’s not terribly efficient. As a result, these
motors consume a lot of current, especially if they get
bogged down under lots of weight or strain.
All H-bridge circuits are rated to handle a certain
amount of current before they overheat — and either shut
off, fail, or become damaged. The L298 module can handle
up to two amps per motor. Yet, the Tamiya motors can
consume in excess of three or
four amps when powered at just
four to six volts.
Fortunately, you can get low
cost drop-in replacements for the
stock motors provided in the
Tamiya gearbox kits. For
example, the Pololu #1117
replacement motor is rated for
operation at six volts (can be
operated at up to 12 volts), and
at that voltage consumes a
maximum of only 800 mA (0.8
amps) when stalled — the motor
is still powered but cannot move.
When replacing the motor,
you’ll need to pull the small spur
FIGURE 5. The cutting and drilling
layout for the small breadboard
deck. Be sure the holes in the
deck match the position of the
holes in the L298 module board.
38 SERVO 09.2011