Make Electronics

Experiment 33: Moving in Steps

292 Chapter 5

the chip functions as an inverter, so that when the input goes high, the output

goes low and sinks current. This is of course just what we need for our stepper

motor that has a common positive connection.

The ULN2001A is only an amplification device, so you have to precede it with

a counter that runs from 1 to 4 and then repeats. You can stick with your 555

timers, as you’ve already assembled them, or substitute almost any CMOS oc-

tal or decade counter that sends its output pulses to a series of pins. Just use

the output from the fifth pin as the “carry” output to restart the counting se-

quence. I suggest a CMOS counter simply because it will run on 12 volts, so

you can use the same power supply that suits your stepper motors.

If you switch to CMOS counters, you will still need a pair of 555 timers send-

ing pulses to the counters. The timers will be free-running in astable mode,

and your photoresistors will control their speed. Figure 5-120 shows the

configuration.

One last item: you’ll need a 12-volt battery. You can of course put eight AA cells

together, but I think you should consider a rechargeable pack from a source

such as http://www.all-battery.com, which has a section entirely devoted to “ro-

bot batteries.”

If you put it all together, you should find that when you place your robot cart in

a very dimly lit room, it will turn toward the beam from a bright, well-focused

flashlight. To get reliable results, you may have to recess each of the photo-

resistors in little tubes, so that they receive much more light when they face

your flashlight than when they face away from it. Figure 5-121 is a 3D render-

ing of the concept.

Another idea is to rewire your cart so that it actually runs away from the light.

Can you imagine how this might be done?

Just one more thought: if you use infrared photoresistors, you can control your

cart with beams from infrared LEDs, in normal room lighting. If you and a cou-

ple of friends all have infrared transmitters, you can get your cart to run from

one of you to the next, like an obedient dog.

This takes us about as far as I’m going to go into robotics. I urge you to check

out the sites online if you want to pursue the topic further. You can also buy a

wide variety of robot kits, although of course I feel that it’s more fun to invent

or develop things for yourself.

All that’s left now is to perform one last introduction: to a device that should

make your life much easier, even though the device is much more complicated

than anything we have dealt with so far.

Photo-

Resistors

Stepper

Motors

555 Timer
Counter
Amplifier

555 Timer

Counter

Amplifier

Figure 5-120. A more efficient way to drive
the motors is to use just one timer to set
the speed of each, with a counter and
amplifier (such as a Darlington array chip)
sending the pulses down the wires. The
principle is still the same, though.

Figure 5-121. This 3D rendering shows a
possible configuration of the light-seeking
cart, with two photoresistors enclosed in
small tubes to restrict their response to
light.