Electronics > Electronics

$50 robot power wiring


Happy Nick:
Hi all, I am new so please be gentle :D

Ok, I'm sort of following the $50 robot tutorial but I'm using an Arduino uno board which, for now, I'm happy to have weighing down my robot. My confusion is thus: If I'm reading the tutorial correctly then it wants me to connect the negative wire from my battery pack( providing power for my servos) to the negative/ ground of my regulated 5 volts (which for me is on my Arduino). I'm fine doing this but I'm not sure why I can not have two separate systems?

Is it because the PWM signal from the micro feeds into the servo and will only do so if the grounds are common?

I hope my explanation of my confusion makes sense and that someone out there can bring me some clarity.


Negative/ground becomes a reference for the PWM signal. All signals need a reference or they don't convey information.

It seems natural to understand that on a ruler/rule that is 12 inches long if you want to cut material to a five inch length you start at the 0 end of the rule and measure to 5. The 0 end is the reference in the way that the negative end of the battery is used as a 0V/ground reference. Imagine that you want to cut this 5 inches of material and someone just hands you a broken segment of a ruler that just has a "5" on it. That marked "5" doesn't do you any good in measuring without the "0" end of the ruler in place.

So there is a single wire that has PWM on it. The signal may toggle between 0V and 5V on that wire, but it is only useful if you know what that source thinks is 0V and 5V. Voltage is potential DIFFERENCE between two wires - signal (or source voltage) and a ground reference.

Happy Nick:
Hi,  thank you for responding. I understand about potential difference ect... I did not understand why the negative/ground of the Arduino would not be good enough as a reference point for the PWM signal it outputs. By making the reference point common I can see that what ever signal voltage is output by the Arduino will also be relative to the servo.
    I'm trying to understand if there is something happening inside the servo. For instance, the square wave from the output from the Arduino should cause a similar current to flow and if so this flow of current travels down, in my case, the yellow wire gets to the servo and needs the servo ground to complete its circuit.                     
Would it work if I did not tie the grounds together but 5 volts coming out from the Arduino might be seen at the servo as something different, albeit slightly?

ARRRRRRRRRRRGGGGGGGGGHHHHHHHHHHH! I'm normally really articulated but even I'm laughing at my attempt at an explanation! I will common them up regardless but I'm the sort of person who likes to understand what is happening...


--- Quote ---Is it because the PWM signal from the micro feeds into the servo and will only do so if the grounds are common?
--- End quote ---


If you only tie the "5V" from the Arduino to the "input" of the servo, then the servo will not see any change at all. If you hooked up a multimeter, you'd see that the potential between the two grounds jumped up/down by 5V, but because the grounds are not connected, the servo PWM input doesn't see that at all.

There are differential signalling systems where something like an optocoupler is used for input, and two wires go from the sender. Those systems allow full galvanic isolation. However, those systems require more wiring, and cost more in interfacing, and are only really needed when you deal either with unknown voltages, long transmissions, or dangerous high voltage systems.

Happy Nick:
Thanks very much.  :)
Now to write the code....


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