Help with Pwm/ driving servo with xbee

Discussion in 'The Projects Forum' started by jdevereaux, Apr 10, 2013.

  1. jdevereaux

    Thread Starter New Member

    Mar 25, 2013
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    I would like to use an xbee module to directly drive (without mcc/mcu) a common hobby servo. As I understand it the xbee pwm pin puts out a pwm signal with a period of about 64usec, which for all effective purposes is more like an analog signal (especially with a simple rc filter)... From 0-3.3v

    I'd like to be able to use that "analog" voltage to drive a typical servo that requires a 1-2ms pulse width. Without resorting to a microcontroller, is there way to use that variable voltage signal to control the rotation of the servo by getting the correct pwm signal? Extreme positional accuracy isn't really a concern, as I'm only really interested in 0, 90, and 180 degrees or there about.
     
  2. jdevereaux

    Thread Starter New Member

    Mar 25, 2013
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    I found this circuit http://www.nlvocables.com/blog/?p=368
    [​IMG]

    He mentions that the signal input voltage is between 3.2-3.5v to get the appropriate 1-2ms pulse width for the servo. Unfortunately my available signal voltage tops out at 3.3v.

    I'm not savvy enough to understand it all, can anyone suggest modifications that would bring the necessary signal voltage down to the 0-3.3v range to still achieve the correct pwm out, or alternatively boost the output of my 0-3.3v signal line to push the upper edge into the 3.5v range without sacrificing it's variable nature.
     
    Last edited: Apr 12, 2013
  3. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Might try a noninverting OP Amp adder, O-3 V gain of 1/10 so output would be +- 150mV, second input gain of 1 to adjust output to 3.35V or 1.5ms, so if I' not totally screwed up , output should swing between 3.2 & 3.5V = 1ms to 2ms.??
     
  4. jdevereaux

    Thread Starter New Member

    Mar 25, 2013
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    That sounds like a great option, especially with the availability of quad op amp packages. As you might have guessed, electronics isn't my strong suit, I appreciate the help and will probably be back with more questions.
     
  5. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    OP Amp not needed, just resistive mixer. For test used Ry = 47k, Rx = 75k, R pot = multiturn 30k. 500k pot from signal to comp. neg, pot center tap. Signal line open, adjusted R pot for 3.55V, 0 sig adj. 500k [ 294k ], to 3.2V . 3.5V sig gave 3.5V out. R values not critical, Pots interact but they do converge.
     
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