BY ELZA SIMPSON
I love making unique clocks.
I think the reason is because
everybody knows what a clock
is and when they come across a
clock style they’ve never seen before their eyes light up and they
want one. The clock presented here is one of my favorites.
The face of the clock consists of two upside down
arches: one for hours and one for minutes. The hours
arch is evenly marked out between 1 and 12. Likewise, the
minutes arch is marked out between 0 and 60. Each arch
has a steel ball bearing which is allowed to roll freely on
the arch. The arches are rotated by stepper motors. As the
angle of the arch changes, the ball bearing rolls to the
lowest point, showing the current time. Of course, the
most exciting time to watch — if watching time go by can
be exciting — is when the time changes from 12: 59 to
1:00 o’clock as both arches rapidly reverse direction.
Choosing the Motors
My first choice was to use R/C servo motors. These
have been used to control R/C airplanes, cars, and boats
for years, and more recently they’ve become very popular
in robotics. I tried analog R/C servos but found there were
dead spots in some of the positions displaying the time
which caused the motor to jitter. I may not have had this
problem if I used digital servos. However, since I didn’t
know that for sure (having never used a digital servo), I
decided not to take the chance. Instead of spending $60
for two Futaba S3151 digital servos, I decided to spend
$40 for two stepper motors which I knew would work.
The $20 I saved on motors would more than pay for the
extra circuitry required to drive the stepper motors vs.
the servo motors.
The final reason I decided to use stepper motors is
that they turn quietly. I have another clock which I spent a
considerable amount of time and effort on, only to have it
sit doing nothing. The reason is because the servo motors
make too much noise when you’re trying to fall asleep.
(Music to my ears, but not so with other household
38 July 2009
Stepper Motor (Very) Basics
As the name implies, stepper motors turn in small
steps. The step angle is measured in degrees, and motors
come in a variety of step angles. For example, Jameco
sells stepper motors with step angles of 0.9, 1.8, and 7. 5
degrees. The one I used for this clock has a step angle of
1.8 degrees. This means that a single rotation of the
motor’s shaft takes 200 steps.
There are basically two categories of stepper motor:
bipolar and unipolar. A simple bipolar stepper motor has
two coils while unipolar stepper motors can have four
individual coils, two center tapped coils, or two center
tapped coils with the center taps tied together internally.
To make the motor rotate, the coils are energized in a
specific sequence. The difference in energizing the two
types of motors is that the voltage polarity on the bipolar
motor coils needs to be reversed while the polarity on
unipolar stepper motor coils does not.
Another thing to consider when choosing a stepper
motor is the holding torque. This is the amount of torque
the motor has while standing still with the motor windings
energized. The faster you energize the coils, the faster the
motor turns. The faster the motor turns, the less torque
you will have. One thing to remember when sequencing
the motor’s coils is that you must give the motor time to
turn. If you sequence the coils too fast, the motor won’t
turn at all. Generally, the rpm of stepper motors is
relatively slow. To reverse the motor’s direction, you
simply energize the coils in the reverse sequence.
Figure 1 shows the complete schematic for the clock.
U1 is an 18X PICAXE microcontroller and is the brains of
the clock. Programming the PICAXE is done through a