Okay, we
have to admit this right upfront: This project is Earl’s
absolute favorite in this whole
book. Sensitive Sam is a motorized cart. You stick
black electrical tape on the floor to create a little
path or track for Sam, and Sam uses
his sensors to follow the tape around corners and in interesting
loops you devise for hours of fun. He also has a little horn you
can blow (to warn the cat that he’s
coming). What’s not to love?
In this chapter, you discover how to give Sam
the “eyes” he needs to sense where he’s going and also how to
build a radio remote control device to tell him what you want
him to do. Although you’ll find lots of little components and
connections going on here, don’t be intimidated; after you get
going, we think you’ll find it’s a pretty fun project. (Earl
did!)
The Big Picture: Project Overview
You’re probably wondering what this Sam guy
looks like and what he’s capable of. Glad you asked. Here’s the
low down on Sam, who
Has three wheels:
This design makes
the cart stable. If you use four wheels, you need to include a
suspension mechanism to ensure that
all wheels stay in contact with the floor at all times. We use
one unpowered wheel in front and
two independently powered wheels in back. This way, if the motor
for one of the wheels in back is shut off, the cart turns
in the direction of the motor that was shut off (left or
right).
Sports two eyes:
These eyes help Sam figure
out where to go. Sam’s
eyes
— phototransistors pointed
at the floor — sense infrared (IR) light
that is sent out by IR LEDs and then reflected by the
floor. By laying down a track of black electrical tape on the
floor, you create an area that reflects less of the IR light.
We set up the circuit so that when Sam’s eyes hover over
reflective floor, the motors turn.
If one eye is over the black tape or other nonreflective
surface — for example, where a bend comes in the tape
track — the motor connected to that
eye shuts off, causing the cart to turn and follow the tape.
When the eye is back over the reflective floor, the motor
turns back on, and the cart goes in a straight line
again.
Responds to a remote control:
This remote uses radio
waves to send its commands to
Sensitive Sam. (This is the same technology that a key
chain remote control device uses to open the doors on a
car.) You can set the switches on
the remote control to tell Sam to turn on/off, slow down or
speed up, or honk the horn. When you flip a switch and press
the transmit button, that effect kicks in.
Sensitive Sam is shown in Figure 13-1 in all his
glory.
Here are the types of activities you’ll do in
this project:
1. Put together the electronic circuit for the
remote control transmitter and then fit the breadboard into a
plastic box with switches.
2. Put together the electronic circuit that
decodes the radio signal and controls the movement of Sam in
response to his sensor eyes.
3. Mount the circuit onto a chassis along with
DC motors, wheels, and a few switches.
In the end, you create a cart that follows a
track all by itself and responds to your every command. It also
has a cute little horn that you can toot.