Why do we connect Grounds when connecting (while hacking) one circuit to another?

I am learning how to Hack Wheelchair joysticks and other simple controllers..
problem is I don't understand "Why do we connect Grounds of the controller and Arduino in these cases ?"

Like here (example A):

and here (example B):
source: http://jbprojects.net/articles/programmable-rc/

in Example A the circuit is powered Via Wheelchair batteries, Example B via Arduino ..

Thanks :)

frollard4 years ago
When dealing with any CIRCUIT, as the name implies, current must always be able to flow from a source, through a load, and to ground. For example, a light bulb won't light up if you only hook a high voltage to one of the terminals, it needs a return path for the electricity to get back to the source.

When sending digital or analog voltages between circuits, the reason for having a single ground shared between the circuits is that then one circuit can input or output to the other circuit because they can sense and react to voltages in relation TO THAT GROUND. If the grounds are not tied together, then 5v which doesn't have a return path is seen as 0 volts.

In your wheelchair example A, the ground of the controller board and the arduino are tied together directly. Single wire - just so that when the other signals hit the arduino pins, the pins will actually sense the voltage.
Mikuchan (author)  frollard4 years ago
I see thank you I now fully understand "why" :D !
The ground acts as a path for the current to return to the supply, and as a zero reference voltage for any signals being passed between components. Without a ground or other reference, there is no way to tell what the level of a signal is.
Mikuchan (author)  The Skinnerz4 years ago
Thank you :)