between brands, so servo horns and accessories are not
necessarily interchangeable. Medium sized Futaba servos
use 25 splines, while Hitec servos use 24 splines. Not
compatible. There are also some minor differences between
various brands of female connectors, but nothing that will
prevent most brands from plugging into a standard three-pin male connector or PCB (printed circuit board) header.
Watch out for pre-1998 Airtronics servos, however, which
have reversed power connections and may “release the
magic smoke” if hooked up to a standard servo controller.
Electronic servo testers (Figure 4) are very handy and
cheap — some as low as $2.
Several servos can be
connected at the same time.
Most testers have three
modes, selectable by a
pushbutton switch. Mode 1
allows manual servo control:
Twist the knob and the servo
rotates accordingly. Mode 2
locks the servo in its center
position. Mode 3 oscillates
the servo back and forth.
Some simple projects might
not even require anything
beyond a servo tester!
The cheaper testers only
send 1-2 ms output pulses
which rotate most servos
less than 180 degrees. That’s
still useful for checking servo
motion on a project before a programmable controller is
A tester with a built-in display showing the pulse width
is extremely valuable. To avoid damaging the tester and
servos, always double-check polarity when connecting the
battery (also 4.5V-6V) and servos. Connect the battery first
to verify the LEDs come on properly before connecting any
servos. DIYers can build the simple servo tester shown in
Standard servos rotate their output shaft about 180
degrees, or a half rotation. Another way to say it is they
move ±90 degrees from their center position. They
are used for precise rotational positioning; perhaps
moving a robot arm or leg joint, steering a wheel,
scanning a sensor, or aiming a laser. An internal
potentiometer (rotary sensor) detects the output shaft
position and provides feedback to the internal control
circuit. This is called a “closed loop” control system.
A servo knows its position and whether it needs
to move based on the incoming control signal. It will
fight and “growl” to maintain position — even up to
the point of breaking or stripping its gears. Extra
power is consumed to move a heavy load or to hold a
fixed position when strong forces are involved.
Standard servos have internal mechanical stops at
each travel limit; often a tab on the output gear
which hits a part of the case. On some servos, the
output gear is only half a gear (a.k.a., sector) since it
only rotates 180 degrees. You’ll waste power and
possibly damage a servo if you send a control signal
which makes the servo try to rotate beyond its
mechanical stops. Every servo is slightly different, so if
70 SERVO 07.2015