How servo’s work

Servo’s also work with Pulse width modulation. The length of the pulse is between 1ms (fully left) and 2ms (fully right.) Of course I might be wrong about the left and right, but it boils downto this: 1ms is maximum one direction and 2ms is maximum the other direction.

You will notice two differences between the pulse width modulation of the servo and that of a speed control:

  1. For a servo the pulses don’t follow imediately after one another – they are spaced 25ms from each other.
  2. For a servo there is no such thing as a pulse width of zero length – each pulse is at least 1ms long.

The job of our speed control will be to convert the servo PWM (pulse width modulation) to a motor PWM, as well as make the motor run from the 12V or whatever battery instead of the 5V logic signal..

To summarise:

  • Get the servo signal from the receiver using a microcontroller
  • Calculate the throttle (1ms = motor off, 2ms = full throttle, 1.5ms = half throttle etc.)
  • Output the motor PWM. We’ll use a Mosfet as a switch.

An interesting Servo note:

Servo’s can also be used as a motor for robotics projects. To do that the servo has to be modified to allow it to keep on turning in any direction. If you open a servo you’ll notice a potentiometer (variable resistor) connected to the shaft. The servo uses this as an anolog feedback to know how far it has turned. To get the servo to keep on turning you have to trick it into thinking that it must always turn a little bit more to reach the desired position. Unfortunately a pot cannot turn full circle, so you have to hack it to make this work.

If you’re interested in such an operation, just Google for it – I know there’s at least one site with step-by-step instructions (and photo’s) on how to do this.

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