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I am dividing voltage of a 9V battery to 3V for a Rasperry Pi GPIO. I have the ground connected to the end of the resistors, so it is back to 9V when I connect it to the Pi's ground pin. Is it ok to have the ground at 9V?

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ By definition, "Ground" is zero volts. Draw a diagram to show us exactly how you have connected and measured things. This site has a nice schematic editor you can access by hitting "Control-M". \$\endgroup\$ – Peter Bennett Feb 17 '15 at 5:24
  • \$\begingroup\$ First time ever drawing a diagram. Let me know if it makes no sense. \$\endgroup\$ – rys Feb 17 '15 at 5:39
  • \$\begingroup\$ I think I did that wrong. Is that really 3V there or not? I kind of guessed that 3 resistors divides 9V by 3. I was using a multimeter and testing between source power and between each resistor. Is that the right way to measure it? So 3V after first, 6V after second, all 9V after all 3 resistors? \$\endgroup\$ – rys Feb 17 '15 at 5:50
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    \$\begingroup\$ to get 3v, you should take voltage across R3. @rys \$\endgroup\$ – tollin jose Feb 17 '15 at 5:57
  • \$\begingroup\$ I hope you are using this 3V just to connect to a GPIO (to indicate that the abttery is present??), not to power the RaPi. \$\endgroup\$ – Wouter van Ooijen Feb 17 '15 at 8:12
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If what I understood from your question is correct, this circuit will help you. It is a voltage divider whose input is 9v and output is 3v (which can be given to your rasperry-pi)

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Ok, I must be using my multimeter wrong, so by the time it is at ground it is at 0V? \$\endgroup\$ – rys Feb 17 '15 at 5:59
  • \$\begingroup\$ Yes ground voltage is 0v. @rys \$\endgroup\$ – tollin jose Feb 17 '15 at 6:01
  • \$\begingroup\$ Node 3v can be connected to GPIO and the ground pin can be connected to raspberry-pi ground. \$\endgroup\$ – tollin jose Feb 17 '15 at 6:04
  • \$\begingroup\$ 0V is wherever you want it to be - voltage is relative not absolute. When using a multimeter 0V is where you place your black(-) probe. In many circuits voltages are measured with respect to "ground" but that's not a law just a convention. Sorry if this makes things more complicated but one day you might need to know :) \$\endgroup\$ – user1890202 Mar 3 '17 at 15:44

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