instructions. The board is good quality and clearly marked
which aided in assembly.
Caution: Don’t lose microscopic SMT capacitors C4 and
C5 like I did. They were loose in the bag!
Lightly clean the copper PCB pads with a Scotch Brite
pad. Solder all the components except for the motors,
battery holder, and line following LEDs and
phototransistors. Do the hard stuff first. Start on the
bottom with the SMT components: C4 and C5; a pair of
L9110 H-bridges; and an LM339 quad comparator. Refer to
instruction photos, the component list, and PCB markings.
Use an ohmmeter to verify resistances since four
color bands are used. Don’t forget to install the
unmarked half inch long jumper wire near pot R16!
Since I lost SMT caps C4 and C5 almost
immediately, I soldered 0.1 µF disk caps directly
across the motor terminals, which might be more
effective anyway (sour grapes). The forward-facing
obstacle detectors use clear LEDs (outside) and black
phototransistors (inside). Watch the polarity; match
the flats on the components to the image printed on
their PCBs. I’m not a fan of that little tiny power
switch awkwardly adjacent to a tall filter capacitor,
but what’s a “girl” to do?
The $5 line follower kits use preassembled gear
motors with spur gears, but this robot uses worm
gears to drive the wheels. Assembly is a bit tricky, so
once again refer to the photos in the instructions to
properly position those small yellow spacers.
Light taps from a small hammer will get the
shaft through the gear and wheel. Get the
wheel/gear/shaft and supports installed properly,
with the gear centered in the PCB slot. Make sure
the whole axle spins perfectly freely before installing
the motor with two screws.
One axle hole was tight, so I ran a #46 drill
through it by hand and then it was perfect. Press the
worm gear on the motor shaft just far enough to
retain it so it is centered on its mating gear.
I’m not a huge fan of worm gears, which have
lower efficiency than spur gears and can wear quickly
under load — especially when the gears are exposed
like this and easily pick up dirt and hair from the floor
(don’t add any lube here, it will just hold the dirt on).
However, I was impressed with the precision of this
mass-produced PCB mounted assembly. It’s a light
duty application and the gears should last a long
time if properly assembled and kept clean.
Once both wheels and motors are mounted,
install the front screw skid. This establishes the ride
height so you can start the somewhat tricky
installation of the hang-down line-sensing black
phototransistors and clear LEDs. The instructions say
to mount with 5 mm ground clearance, although mine
worked fine at 3 mm.
Warning: Don’t overheat these LED/PTX PCB traces
when soldering! You’ll risk breaking them off.
The location and mounting of these four parts is the
Achilles heel of this design IMO. Analog calibration depends
on all parts maintaining fixed positions, yet they are subject
to abuse in a crash. They hang out in the breeze where a
light impact with a low-lying object may bend something or
break off the PCB trace, undoing all of your careful
10 SERVO 08.2017