Experiment 33: Moving in Steps
290 Chapter 5
Speed Control
If you are a truly exceptionally observant, you may have noticed that I left pin 5 of
each of the timers unconnected in the schematic for driving the stepper motor in
Figure 5-108. Normally, pin 5 should be grounded through a capacitor to prevent
it from picking up stray voltages which can affect the accuracy of the chip.
I left the pins unconnected because I had a plan for them. In fact, changing the
timing of the chip is exactly what we want to do now, as a way to change the
speed of the stepper motor.
If you tie pin 5 of all four timers together, as shown in Figure 5-116, and put
a 2K trimmer potentiometer (shown in Figure 5-117) between them and the
negative side of the power supply, you’ll find that as you turn the trimmer
to reduce its resistance, the timers start to run faster. Figure 5-118 shows the
breadboard layout. Eventually, when the resistance goes below around 150
ohms, everything stops. The LEDs go dark, because you’ve reduced the volt-
age on pin 5 below the threshold level that the 555 timer finds acceptable.55510K10K8K222uF1K1K1K1K22uF22uF22uF0.01uF0.01uF0.01uF0.01uF10K10K10K10K10K555555555Stepper
Motor2KFigure 5-116. To adjust the speed of the sequence of 555 timers, their control pins (pin
5 on each timer) are linked together and attached to a trimmer potentiometer that
adjusts the resistance between the pins and the negative side of the power supply.
Figure 5-117. Close-up of a trimmer potentiometer
with pins spaced at 1/10 inch for insertion in a
breadboard or perforated board. The brass screw,
at top-left, turns a worm gear inside the unit, al-
lowing precise adjustment of internal resistance.Figure 5-118. The trimmer potentiometer has been
added to the circuit, allowing motor speed control.