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Basically, what are the ratings for the Ground GPIO pins? I guess nothing above 5 Volts and 1 Amp, but knowing what they are would be helpful (for a similar project I am planning to do when I have time later).

I wanted to know anyway, but the other reason is that I want to do this:


I am finishing off my Raspberry Pi powered robot, now based on an old Cybot base, which has motors, and a motor controller.

Cybot base Motor Controller

There is a detailed schematic with lots of info on the motor driver board here - here is is a modified copy (It should say left motor and right motor, but I can't be bothered to change one the 'right's): My schematic

Here is also a schematic for a Arduino board: arduino circuit

I have successfully powered ONE of these motors, not using the motor controller circuit, a TIP31C NPN transistor - which was being controlled by a GPIO on Pi through a a 1K resistor- the motor being powered by 6 Volts, and the grounds of both circuits connected.

With four GPIOs, I should be able to connect to the first 4 pins of the motor controller (which contains 1K resistors on the input anyway), and then connect the last pin of it to the Ground of the Raspberry Pi - but can I do this without putting too much on the Pi? - I guess not, but I currently have only the one (working) Pi?, and I don't really want to blow it up.

In other words, what are the ratings for the ground on the Pi? (Voltage, Current etc). Searching does not seem to find anything.

Any corrections welcome.


When I get around to testing it, I may provide the results as an answer - this could be used for reference anyway. reading


Update: 02/12/13

It is now working, though I still have a problem with a dodgy connector head. You do need to connect the grounds by the way.

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    Connecting grounds doesn't result in any current flow. It just makes sure the voltages on both boards are relative to the same ground. As far as I know it's perfectly safe, and even required for the system to work. – Gerben Nov 22 '13 at 20:27
  • Would using one Ground pin work if using more than 2 GPIO pins outputting into a circuit? (another question, sort of). I cannot find the ratings for these pins, that is what I am interested in. – Wilf Nov 22 '13 at 21:09
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    Only 1 ground connection is needed. What are you planning to do with the 1K resistors? The above circuit seems to have resistors in series with the inputs. You should be able to safely connect to the Pi GPIO pins – Milliways Nov 23 '13 at 7:25
  • I noticed that after posting my question, but could not be bothered to modify it. I shall also edit thequestion slightly. – Wilf Nov 23 '13 at 15:08
  • Ooh, problem. The Arduino Uno's (Like in the above diagram) outputs 5V, not 3V3 like the Pi. This means I will need something to interface between the motor controller and the Pi - I shall do another post on it. By the way, I am still doing this for anyone's else's future reference, and for myself when I forget bits after having built the thing =) – Wilf Nov 30 '13 at 21:32
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If you connect it as stated, then the current returning through the ground pin is only the current to control the transistors. You have at most 4 lots of 3.3V through 1K to ground, which is 13.2mA total, well within the capability of a single ground pin. The S8050 used at the inputs of the control board is a lower power transistor than the TIP31C so you may increase the resistor values a bit and still switch them on, but as the resistors are part of the board I wouldn't bother.

If the control board is sending 5V signals back to the raspberry pi, then you need a level converter, but from the circuit provided that doesn't seem to be the case. The 5V regulated power could be connected to 5V on the pi for powering the pi of the same battery if required.

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