user41139: I'm new to this forum, so hello to everyone out there.
So I have an Uno with A0 connected to the positive end of a chain of LEDs (they light up when a motion sensor goes off, clunky, but it works), and the negative end is hooked up to the GND of the analog side of the Uno. It runs the following code:
void setup() 
{
  Serial.begin(115200);
  pinMode(A0, INPUT);
  pinMode(A1, INPUT);
}

void loop() 
{

  Serial.println(printVals(A0));
}

float printVals(uint8_t pin)
{
  int sensorValue = analogRead(pin);
  float voltage = sensorValue * (5.00 / 1023.00);
  return voltage;
}

And when I open the Serial Plotter, I get a weird square wave for the voltage (when there is 0V on the LEDs, I used a volt meter).
Out of curiosity, I added a jumper cable from A2 to +5V (the red wire in the diagram) and added plotting for A2, and this solved the problem - except now, the read on A0 is about 0.53.
Now, I'm more of a software guy, so I figure just subtract 0.53 and move on (varies only +- 0.03V), but I want to know: What in the world did that jumper cable do?

user41139: @Ferba, my chain of LEDs is actually part of a circuit board for a driveway monitor receiver. When it detects motion, the LEDs light up and the voltage measured by analogRead increases by about a volt. 
Also, as it turns out, the jumper to +5v was functioning as a pull-up wire... not a good idea. It burned out the chip in the driveway monitor by backfeeding an output. 
In the end, I added an "_PULLUP" after the INPUT in setup and that solved the problem. 

41139: <p>I'm new to this forum, so hello to everyone out there.</p>

<p>So I have an Uno with A0 connected to the positive end of a chain of LEDs (they light up when a motion sensor goes off, clunky, but it works), and the negative end is hooked up to the GND of the analog side of the Uno. It runs the following code:</p>

<pre><code>void setup() 
{
  Serial.begin(115200);
  pinMode(A0, INPUT);
  pinMode(A1, INPUT);
}

void loop() 
{

  Serial.println(printVals(A0));
}

float printVals(uint8_t pin)
{
  int sensorValue = analogRead(pin);
  float voltage = sensorValue * (5.00 / 1023.00);
  return voltage;
}
</code></pre>

<p>And when I open the Serial Plotter, I get a weird square wave for the voltage (when there is 0V on the LEDs, I used a volt meter).
Out of curiosity, I added a jumper cable from A2 to +5V (the red wire in the diagram) and added plotting for A2, and this solved the problem - except now, the read on A0 is about 0.53.</p>

<p>Now, I'm more of a software guy, so I figure just subtract 0.53 and move on (varies only +- 0.03V), but I want to know: What in the world did that jumper cable do?<img src="https://i.stack.imgur.com/Bc9XT.png" alt="enter image description here"><img src="https://i.stack.imgur.com/7u1nq.png" alt="enter image description here"><img src="https://i.stack.imgur.com/voQYE.png" alt="enter image description here"></p>
 41139: <p>@Ferba, my chain of LEDs is actually part of a circuit board for a driveway monitor receiver. When it detects motion, the LEDs light up and the voltage measured by analogRead increases by about a volt. </p>

<p>Also, as it turns out, the jumper to +5v was functioning as a pull-up wire... not a good idea. It burned out the chip in the driveway monitor by backfeeding an output. 
In the end, I added an "_PULLUP" after the INPUT in setup and that solved the problem. </p>
