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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.)

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.

Eric Brasseur  -  1994