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I'm having a hard time understanding how a circuit can have multiple power sources and have them all tied to the same ground. See the following schematic as an example:

Project Sentry Gun: Fritzing Schematic

Here's my current thought process, which is probably fundamentally flawed. The large Voltage source pumps current through the curcuit to drive the motors. That same current comes back through the servos and hits the ground wire. As it goes through the ground wire, how does it know to go back to the battery and not the ground on the Arduino chip?

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    \$\begingroup\$ Basically, every point on a conductor has the same potential (voltage), just like communicating vessels. It does not matter where you pour water (=electrons) into the system (=ground line), it will balance immediately. Also, remember that voltage is always the difference in potential between two points, and this is usually kept constant in an electronical circuit by the voltage source. So whether you pour more or less electrons into the ground line, the voltage between the ground line and e.g. the +5V rail will always be the same. \$\endgroup\$ – JimmyB Jun 20 '13 at 20:34
  • \$\begingroup\$ Thanks Hanno! Why not leave this as an answer? \$\endgroup\$ – Brian Wigginton Jun 20 '13 at 21:40
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    \$\begingroup\$ How does all the water in the bathtub find the drain hole? How does all the various air in a tire know where the tiny leak is? \$\endgroup\$ – Kaz Jun 21 '13 at 5:46
  • \$\begingroup\$ Thanks Kaz, a very simple analogy that I can understand. \$\endgroup\$ – Brian Wigginton Jun 21 '13 at 17:10
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how does it know to go back to the battery and not the ground on the Arduino chip

The most negative point on the system is where the current heads for just like a plug hole in a bath - this is where the water wants to go because of gravity. If there's a ridge in the tub (equivalent to one amongst several ground wires having slightly higher resistance), the water (or current) will tend to go round it to find the lowest point.

Your wiring schematic should also look like this so that there is no chance current taken by the motors are drawn along digital ground wires. It's called star-pointing: -

enter image description here

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  • \$\begingroup\$ Thanks Andy, this makes a lot of sense. I guess I was making it harder than it needed to be. \$\endgroup\$ – Brian Wigginton Jun 21 '13 at 17:10
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    \$\begingroup\$ The diagram is a bit confusing. I see that the arduino is connected to ground, but it is not connected to the power (no red wire). This circuit ahouldn't work. Am I correct? \$\endgroup\$ – user148298 Aug 25 '16 at 14:37
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As it goes through the ground wire, how does it know to go back to the battery and not the ground on the Arduino chip?

It doesn't, and it doesn't need to. The charge that was pushed out by one battery need not return to the same battery. Any charge will do. In fact, the motion of charge is very slow in most circuits, and it might not even make a complete lap around your circuit before you turn it off.

Imagine the circuit as pipes filled with water, and the batteries as pumps. How does the water know to which pump to return? It doesn't, but each bit of water will tend to flow to the point of lowest pressure.

This is what keeps the voltage equal at the ground of each of your batteries; if some charge were to make a "wrong turn", then there would be too much pressure there, and some other charge would flow the other way to balance it out.

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  • \$\begingroup\$ Thanks Phil. Also, thanks for providing the link about the slowness of currents in most circuits. \$\endgroup\$ – Brian Wigginton Jun 21 '13 at 17:12

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