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Turning optocouplers into servo amplifiers

Posted by Chris Anderson on January 29, 2010 at 11:30pm

If you're using the UAV DevBoard with Futaba equipment or any other modern RC gear that outputs servo voltage of less than 3v, you're going to need a servo amplifier (also known as a booster or buffer) so the autopilot can reliably read your signals. You can buy one from a commercial supplier like FMA for $15, or you can use our dual optocoupler and save yourself $10 for each pair of servos you amplify. Out of the box, our optocouplers are designed just to clean up the RC signal, which is handy if you've got RF noise messing with your RC system. But if you solder two rails on the board (shown below), they become a very effective two-channel servo amplifier.

Make sure that the "input" side goes to the RC receiver and the "output" side goes to the servos or UAV DevBoard, like this (servo option shown):

When you're done, the servo amplifiers will take any input voltage and output a healthy 4.8v. Here's my scope data, from a Futaba FASST 2.4 Ghz system. Before amplification:

After amplification:

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Comments

  • Developer
    Philip May 28, 2015 at 3:27pm

    Is there an updated link to the store on this one?  this thing would be good for using some of the large 5V servos that do not work with Futaba or Pixhawk.

  • Uzair February 3, 2010 at 2:42pm
    Handy tip.
  • Greg January 31, 2010 at 4:41pm
    Great Idea.... After seeing this I realised I could use an opto coupler to convert 3.3V TTL serial to 5V TTL serial for a GPS output. This method could use 2 resistors and a 6 pin Optocouler.... I currently use 2 transistors and 4 resistors to do this. This idea could also be used for a TTL to RS232 line converter (once again 2 or 3 components ) - beats using a Max232 line converter and all the associated components.

    Thanks for the Great Idea Chris!
  • T3
    William Premerlani January 30, 2010 at 12:18pm
    Hi Chris,

    One more thing, just to be clear: The booster connects between the Rx and the UAV DevBoard.

    You do not need a booster between the UAV DevBoard and the servos, that interface is just fine. The DevBoard output voltage is nominally 5 volts.

    Best regards,
    Bill
  • T3
    William Premerlani January 30, 2010 at 12:08pm
    Hi Chris,

    Thank you very much for this posting this information. Its a great idea.

    Best regards,
    Bill
  • 3D Robotics
    Chris Anderson January 30, 2010 at 8:53am
    The older FM Futabas, such as the 6xa, output 3v, which is better than the 1.8 of the newer FASST ones. I'd still amplify for the UAV DevBoard

  • Craig Palmer January 30, 2010 at 5:58am
    how about a Futaba 6xa-fm ?
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