What Next? 277Experiment 32: A Little Robot Cart0.01uF47uF100K50K10KMotor1
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4 5678Figure 5-98. This ultrasimple schematic is all the cart needs to enable it to back up when it
hits an obstacle.
The motor I chose requires 5 volts, so I had to use a voltage regulator with a
9-volt battery. If your motor uses 6 volts, you can wire four AA batteries to it
directly. If you have a 12-volt motor, you can use two 9-volt batteries in series,
supplying power through a 12-volt voltage regulator.
Assemble the components, mount them on the cart, and switch it on, and it
should move forward slowly in a more-or-less straight line. If it moves back-
ward, reverse your connection to the terminals on the motor.
When the cart bumps into something, either of the microswitches will con-
nect negative voltage to the input pin of the 555 timer. This triggers the timer,
which runs in monostable mode, generating a single pulse lasting about 5
seconds, which closes the relay, which is wired so that it reverses the voltage
to the motor.
When the voltage is reversed to a simple DC motor, it runs backward. So the
cart backs up. Because the rear wheel is mounted in a yoke that pivots, the
yoke will tend to flip one way or the other, causing the cart to describe an
arc as it moves backward. At the end of the timer cycle, the relay relaxes and
the cart starts moving forward again. In forward mode, the rear wheel just
follows along without applying any steering force, so the cart tends to follow
a straight line—until it hits another obstacle, at which point it backs up, and
tries another path.