degrees (usually 60°). Small servos turn at about a
quarter of a second per 60°, while larger servos
tend to be a bit slower. The faster the transit time,
the faster acting the servo will be.
You can calculate equivalent RPM by
multiplying the 60° transit time by six (to get full
360° rotation), then dividing the result into 60. For
example, if a servo motor has a 60° transit time of
0.20 seconds, that’s one revolution in 1.2 seconds
(. 2 x 6 = 1.2), or 50 RPM (60 / 1.2 = 50).
Just know there are variations on the standard
themes for all R/C servo classes. For example,
standard servos are available in more expensive
high speed and high torque versions. Servo
manufacturers list the specifications for each
model, so you can compare and make the best
choice based on your particular needs.
Besides overall size, another common trait in
R/C servos is the connector and even wire colors.
There are three primary connector types found on
R/C servos: “J” or Futaba style; “S” or Hitec/JR
style; and “A” or Airtronics style.
Servos made by the principal servo
manufacturers — Futaba, Airtronics, Hitec, and JR
— employ the connector style popularized by that
manufacturer. In addition, servos made by
competing manufacturers are usually available in a
variety of connector styles, and connector adapters
are always available. For robotics, the physical
shape of the connector is often not an issue as the
connector is used with a simple three-pin header
to make electrical contact. The pin spacing is the
standard .100”; where the connector type comes
into play is when using an R/C receiver designed
for one servo type or another.
The pinouts of the three wires used with
servos is the same, except for the “old-style”
Airtronics servos (and the occasional oddball four-wire servo). This pinout is shown in Figure 6.
With very few exceptions, R/C servo
connectors use three wires providing ground, DC
power (+V), and signal. The +V DC power pin is
almost always in the center; that way, if you
manage to plug the servo in backwards, there’s
less chance of damage caused by reverse polarity.
Most servos use color coding to indicate the
function of each connection wire, but the actual
colors used for the wires vary between servo
makers. Here’s a general idea of what you’ll find
on most servos out there:
• White, orange, or yellow: Signal.
• Red: +V (DC power).
• Black or brown: Ground.
60 SERVO 07.2014
connector used on
the vast majority
of R/C servo
connector may or
may not be
"keyed" using a
groove or notch.
Figure 7. General connection diagram for attaching a microcontroller
to a servo motor. The resistor is optional, but is included to prevent
excessive current draw by the servo.