PWM to RC servo pulse width

Discussion in 'The Projects Forum' started by Simon0362, Sep 21, 2011.

  1. Simon0362

    Thread Starter New Member

    Sep 17, 2011
    Hi All,

    I have a pulse width problem that is bending my tiny brain at the moment - so if anyone can suggest some solutions I will be grateful...
    The requirement is to convert electronically from a nominal 2-95% PWM duty cycle pulse width into a RC servo compatible pulse width (where 2% = 1ms servo pulse and 95% = 2ms). I can set the output frequency of the PWM pulse to more or less any frequency up into the KHz whereas the RC Servo frequency is at 50Hz.

    I have built a 556 unit to manually produce the servo pulses and to change the pulse width, now I need to interface either that unit or an alternative solution with the output from the PC with it's much longer duty cycle.

    I was going down the route of PWM output converted using a F-V followed by using that to adjust the control voltage of the pulse width part of the 556, but I wondered if anyone has a more elegant solution.
    Just to make it clear, I am NOT interested in PIC or other ucontroller based solutions, this needs to be a pure hardware option.

    Thanks in advance,
  2. MrChips


    Oct 2, 2009
    Why not just output the PWM signal directly from the PC?
  3. Simon0362

    Thread Starter New Member

    Sep 17, 2011
    Hi MrChips,
    because it is part of an embedded program that I have no control over - I can instruct it to send out the speed control signal between 2% and 95% of the maximum range but it will always be in this form - and not in the form that a RC servo expects (1-2ms on a 50Hz signal).

  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    Well you ruled out the most "elegant" solution, the microcontroller.

    Your idea of PWM->V converter to control an oscillator is pretty much the way to do it.

    I would porbably use a dual comparator IC instead of a dual 555 timer as the comparator IC gives you easier control of the exact threshold setpoints where your oscillator will switch. It's a similar parts count; RC filter; 8pin dual comparator IC, and a few resistors and caps.
    Simon0362 likes this.