The wiring of an RC servo
Those little servos are rather cheap ($ 30),
accurate (about 1°), lightweight (20 to 50 gr),
rather strong, and easy to operate.
There most certainly is a model kit retailer in
a radius of a few kilometer around your place. If
you have no function generator, you can control
it by generating pulses with a RS/232 interface,
a resistor and a zener diode. You may also use the VGA
signal of your monitor, by displaying the correct paterns
of lines on the screen.
A drawing of a standard servo:
Their only disadvantage, as quoted in this
article, is that they always move at roughly the
same speed, whatever the force they have to
give... (Their inside electronic also takes more
volume than it should.)
They usually have three wires:
At about 100 Hz, a TTL pulse between 1 and
2 millisecond is send to the servo. The length of
the pulse tells the servo at which angle it should
put its action lever. 1 ms means 0°, 2 ms means
120°. 1.5 ms means 60°, 1.2 ms means 24°, and
so on... (Servos are counter-reactive: they will
try to maintain the lever at the asked angle,
whatever the forces acting upon it.)
- A black one. This is the ground.
- A red one. This is the power source. 5 to
- A bleu one. This is the control. It's protocol
is as follows:
As a standard servo for model kits does not
have any sort of memory, it has to receive its
control pulse very often. When receiving no
pulse, it won't use it's motor any more, and just
follow the mechanical forces acting on it. Receiving
too many pulses, it will go crazy.
Definition of a TTL signal: about 0 V means
logic state 0, and about 5 V means logic state 1.
So sending pulses of 1.5 ms means that the
control wire normally has the same voltage as
the ground, but sometime, regularly, during
1.5 ms, is connected to 5 V.