introduced in 1965 for use in model aircraft. In 1965,
integrated circuits had only just been introduced; the
first microprocessors were still a number of years away;
battery technology consisted of disposable carbon cells
(alkaline batteries weren’t available until the 1970s);
rechargeable Nickel-Cadmium batteries were known as
“NIFE” cells, and weren’t available to consumers in the
“AA” form factor until 1970. The technology of the
time is important to keep in mind when you are
planning on using servos, as it affects how you
interface to them.
The first misconception that many people have
about servos is that they are digital devices. They are
actually analog devices. This is confused by the name
that is applied to the servos. They are called “digital
proportional” which is really marketing speak to
make servos sound like modern devices, and not
something that came out when the Beatles were still
playing live shows.
In regard to the control signal passed to servos,
I’m sure that you’ve seen something like Figure 2
showing that the length of the pulse which is sent to
the servo is used by the servo to set its output
position. The pulse length is a continuous function; it
is not broken into discrete steps (as in many software
representations, including one of the Arduino
libraries that I use in the example programs discussed
below). This continuous output is both a blessing and
a curse.
Being an analog device, servos are not
completely accurate nor are they precise. If you have
two servos, chances are the same pulse width will
not set both of the servos to the same position, and
if you move them to new positions and then back,
you’ll find that they may not return to exactly the
same place.
This was compensated for in radio control
models with “trim pots” which minutely varied the
timing of the signal sent to the servos to ensure the
model would return to a neutral position. I’m
pointing this out because it can be an issue in
applications that require very precise and accurate
actuator movements. If that is the case, then you
should be looking at stepper motors or more modern
servos which provide much more accurate
positioning.
I’m sure that by now you’re starting to get
bored with my prattling on, so let’s change the pace
and start building some circuits and gain some empirical
knowledge regarding servos.
The first bit of wisdom that I want to impart to you is
the need for a servo connector that you can plug into a
breadboard. Servos require header pins that are at least
1/4” ( 6 mm) to 3/8” ( 9 mm) in length, and consist of three
pins that are on 0.1” ( 2. 54 mm) centers.
As I can never reliably find headers that are long
enough on both sides, I usually make my own from
soldering together the short ends of two standard printed
By Myke Predko Post comments on this section and find any associated files and/or downloads at
www.servomagazine.com/index.php/magazine/article/may2015_Predko.
Figure 2. The servo pulse stream with timings.
Figure 3. Solder two three-pin headers together to make
servo adapters.
Figure 4. A 556-based servo generation circuit.
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