You will need:
• 555 timers. Quantity: 8.
• Trimmer potentiometer, 2K linear. Quantity: 2.
• LEDs. Quantity: 4. If you get tired of using series resistors to protect LEDs in a 12-volt circuit, consider buying 12-volt LEDs such as Chicago Miniature 606-4302H1-12V, which contain their own resistors built in. However, the schematic in Figure 5-108 assumes that you will use regular 2V or 2.5V LEDs.
• Stepper motor: Unipolar, four-phase, 12-volt. Parallax 27964 or similar, consuming 100mA maximum. Quantity: 2.
• Photoresistors, ideally 500 to 3,000Ω range. Quantity: 2.
• ULN2001A or ULN2003A Darlington arrays by STMicroelectronics. Quantity: 2.
• CMOS octal or decade counter. Quantity: 2.
• Various resistors and capacitors.
Exploring Your Motor
I’ve specified a unipolar, four-phase, 12-volt motor because this is a very common type. A typical sample is shown in Figure 5-105. If you can’t easily find the one that I’ve listed, you should feel safe in buying any other that has the same generic description. “Unipolar” means that you don’t have to switch the power supply from positive to negative and back to positive again, to run the motor. Four-phase means that the pulses that run the motor must be applied in sequence to four separate wires. Because you will be running your motor directly from 555 timers, the lower its power consumption, the better.
First, though, we can apply voltage to the motor without using any other components at all. Most likely it will have five wires already attached, with the ends stripped and tinned, so that you can easily insert them into holes in a breadboard, as shown in Figure 5-106. Check the data sheet for your motor; you should find that four of the wires are used to energize the motor and turn it in steps, while the fifth is the common connection. In many cases, the common connection should be hooked to the positive side of your power supply, while you apply negative voltage to the other four wires in sequence, one step at a time.
The data sheet will tell you in what sequence to apply power to the wires. You can figure this out by trial and error if necessary. One thing to bear in mind: a stepper motor is very tolerant. As long as you apply the correct voltage to it, you can’t burn it out.
To see exactly what the motor is doing, stick a piece of duct tape to the end of the shaft. Then apply voltage to wires, one at a time, by moving your negative power connection from one to the next. You should see the shaft turning in little steps.
Inside the motor are coils and magnets, but they function differently from those in a DC motor. You can begin by imagining the configuration as being like the diagram in Figure 5-107. Each time you apply voltage to a different coil, the black quadrant of the shaft turns to face that coil. In reality, of course, the motor turns less than 90° from one coil to the next, but this simplified
model is a good way to get a rough idea of what’s happening. For a more precise explanation, see the upcoming section “Theory: Inside a stepper motor.” Bear in mind that as long as any of the wires of the motor are connected, it is constantly drawing power, even while sitting and doing nothing. Unlike a regular DC motor, a stepper motor is designed to do nothing for much of the time. When you apply voltage to a different wire, it steps to that position and then resumes doing nothing.
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