Dear ARDUINO community,
I have a question regarding my project, (i'm quite new with electronics by the way). I'm using an Arduino which should measure 4 individual voltages from 3 circuits. i.e. there is one circuit with 2 voltages that I want to know.
Now this circuit operates > 5V so I've purchased 2x voltage sensors (which is actually just a voltage divider iirc) and only one voltage divider works. The voltage divider gives the correct voltage where the other one gives out 0. Now I think that my problem is that the second voltage sensor is shorted to the arduino's ground but I just do not know. Maybe some of you guys can help me?
The circuit is in the attachements (fear my paint skills).
Sensor used:
The R_known is there so I can calculate the current from the voltage. The cell's resistance is unknown. The power is from a potentiostat, generating 0- 25V. The current measured is in the 10 mA range (with a simple multi-meter as measuring device).
TL;DR problem: 2 sensors, only one gives an response to the arduino, second one doesn't.
What I've done:
- Change the resistor to 1 ohm, 10, ohm, 330 ohm, 1k ohm (did not do anything).
- Try to do without the second sensor but measure directly in the arduino (<5V because of arduino pins cant handle >5V.) NO response, just a 0 as output, the first one DID work and gave the voltage that was supplied from the potentiostat.
Please help me I've been stuck on this for quite some time now 
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Yeah, you do need a common between all three (2 voltage sensors and the Arduino). If you would replace the "sensors" in the schematic for what they really are (aka voltage dividers) you would see it directly.
Simple fix, move the - connection of the Rcell over to the same point as the other sensor aka to battery -. Yes, now the sensor does not give you the voltage Rcell, but do you still know your basic electronics aka second law of Kirchhoff? If you know the battery voltage and you know the voltage across Rknown you can calculate Rcell, Vbattery - Vknown = Vcell 
septillion:
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Yeah, you do need a common between all three (2 voltage sensors and the Arduino). If you would replace the "sensors" in the schematic for what they really are (aka voltage dividers) you would see it directly.
Simple fix, move the - connection of the Rcell over to the same point as the other sensor aka to battery -. Yes, now the sensor does not give you the voltage Rcell, but do you still know your basic electronics aka second law of Kirchhoff? If you know the battery voltage and you know the voltage across Rknown you can calculate Rcell, Vbattery - Vknown = Vcell
I'm going to try this now! BRB 15 minutes.
Thanks in advance
So I've tried the set-up in the Attachment, and I still get only two signals, maybe I misunderstood? And maybe I misunderstood the second part, about Vcell = Vpower - Vknown. Do I only need a voltage sensor for the R_known?
Hi,
If you follow the gnd wires in your circuit, I have made them blue, you will see you are shorting out R_known.
This is because the voltage potential dividers have the gnd connected through the circuit, this is how a potential divider works.
You need to modify your circuit and do some maths in your code.
Tom...
Mm, that looks right. But two signals, that's fine isn't it? You measure Vbat via A0 and VRknown via A1.
Thank you all very much for all your replies! They helped a lot, and now it works! Now only to understand the science behind it haha
Why it works?
-Hardware: because as you can see in the images of TomGeorge, the new setup does not short things.
-Electrical: Second law of Kirchhoff aka the sum of all voltages in a loop is 0 (where a supply imposes a minus) So:
Vcell + Vknown - Vbattery/supply = 0V
