Experiment 32: A Little Robot Cart
276 Chapter 5
5-94 and 5-95 will help to make this clear. Initially, when you install Part F, use
only two screws, one each side, so that you can adjust its angle a little. This will
be necessary to optimize the contact of the wheels with the floor.
The side wheels and rear wheel must spin freely, but on the other hand, they
shouldn’t wobble. I simply tightened the nuts on the bolts that serve as axles
for the wheels, until there was maybe half a millimeter of clearance. I added a
drop of Loctite to stop the nuts from getting loose.
The plans don’t show precisely where to drill holes for the axle bolts, because
the location will depend on the size of your wheels. You can figure this out as
you go along. Just make sure that the side wheels aren’t mounted too low. We
don’t want them to lift the front wheel or the rear wheel off the floor. If the
side wheels are a fraction higher off the ground than the front and rear wheels,
that’s good.
If you have tile or wood floors, your cart may acquire better traction if you
wrap a thick rubber band around each disc that you use for the drive wheel
and the steering wheel.
The most important aspect of the construction is to place microswitches
where they’ll be triggered when the cart runs into something. I placed mine
at the front corners, as shown in Figures 5-96 and 5-97. And that brings me to
the electronics.Figure 5-96 Figure 5-97. Two microswitches with metal
arms are mounted on each side of the
cart, where they will sense any obstacle.The Circuit
The schematic is very, very simple, with only four principal components: two
microswitches that sense obstacles in front of the cart, one relay, and one 555
timer. You will also need a small power switch, a battery or battery pack, and
a resistor, and capacitors to go with the timer. A trimmer potentiometer will
allow you to adjust the “on” time of the 555 timer, which will determine how
long the cart takes to back up. See Figure 5-98.Figure 5-94. The assembled body of the
cart, before adding control electronics.
The wheel at the righthand end will pull the
cart from left to right. The hinged trailing
wheel will allow the cart to move in a rela-
tively straight line when it moves forward,
but will tend to turn it when it backs up.
Figure 5-95. A closeup of the hinged trailing
wheel, which rotates freely and can flip
from side to side with minimal friction.