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Regular old 5V hobby servos can usually be safely connected directly to a microcontroller without any issues. When it comes to 6V servos (that also might be drawing sizable current), there doesn't seem to be as much information.

I've read that you can simply provide 6V to the + and - pins on the servo from a separate source and then provide the regular 5V control signal, but I doubt that this is the ideal solution.

What's the right way to control a (potentially high current-draw) 6V servo from a 5V microcontroller? Should the 6V supply just come from a voltage regulator and a couple of caps or is it important to have a more sophisticated voltage source?

Thanks!

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2 Answers 2

up vote 3 down vote accepted

So in general when you have a noisy actuator and a sensitive you run them on separate supplies and try and keep the two electrically isolated. I use a circuit like this when I try and opto-isolate a transmitter and a receiver: opto-isolator example schematic This will give you a non-inverted output of the input waveform, and will actually work with any voltage on the input and output side so long as you can still turn on the LED (i.e. the input voltage is high enough) and you are within the operating voltage of the output transistor.

Isolating the two supplies is actually good in a lot of ways. It means you don't have to worry about surges in current on the actuator supply causing issues on the microcontroller (fewer decoupling caps, etc), and it also means that if the actuator battery dies, the actuator won't try and run off the input signal from the microcontroller.

Hope that helps!

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Thanks! So I'm assuming the red box there is an optoisolator? –  Computerish Nov 15 '12 at 14:16
1  
@Computerish : Yes exactly. The one shown here was the PC457L0NIP which is a surface mount component for high-speed (i.e. data), but pretty much anything opto-isolator should meet your needs. –  Kit Scuzz Nov 15 '12 at 17:07

The servo control signal on servos does not generally draw any significant current. Also, the 6 Volt servo datasheets I have seen indicate that the control line (Signal) is 5-volt friendly.

So long as the ground of the servo main supply and the microcontroller's supply are interconnected, you can safely hook up the microcontroller's output to the servo's control input.

The servo thus takes motive power off an independent 6 volt power rail, and servos are typically not sensitive to regulation quality / ripple on that rail, so no sophisticated voltage source is needed.

The control circuit inside a servo typically isolates the Signal line, and will not use it for power, nor will it feed back any significant motor generated EMI back through the Signal line. At worst, adding a small resistance in series decoupled with a small capacitor to ground, on the line connecting the microcontroller to the servo Signal input, is sufficient.

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Thank you! Good to know that a simpler design should work. I wish I could accept both answers. –  Computerish Nov 15 '12 at 14:17
    
There will always be other questions, and other answers as well. :-) I appreciate the thought, and the up-vote, thanks! –  Anindo Ghosh Nov 15 '12 at 14:19
    
So while noise over the signal line shouldn't be significant, you must connect the ground line from both the 5V and 6V supplies together in order for the signal from the microcontroller to be detected. And at least in my experience, big actuators lead to a lot of noise on the ground plane. –  Kit Scuzz Nov 15 '12 at 17:10
    
@KitScuzz Yes, that was the first line of my answer: "So long as the ground of the servo main supply and the microcontroller's supply are interconnected". Yes, there will be ground plane noise, but on a 6 volt servo, that noise will be in the quarter-Volt range worst-case, I believe. Servo electronics almost always have excellent motor EMI filtering, else the servo's own little controller would go nuts. –  Anindo Ghosh Nov 15 '12 at 17:16
    
@AnindoGhosh : I think that largely depends on who makes your servo, and how many servos you're interfacing with your microcontroller. I have had a couple situations where the microcontroller's supply got too messed up to properly operate, but your milage may vary, just throwing out my personal experience. –  Kit Scuzz Nov 15 '12 at 17:29

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