Arduino Notes: Potentiometer

October 2009 -

The potentiometer has three leads: a,b,c.  A is on one end. C is on the other. The pot is a resister between A and C. The B lead is in the middle. When you twist the pot to one side it touches the A end; twist the other way and it touches the C end.

Lead A we attach to Vcc (5vdc). Lead C we attach to ground. The B lead is attached to an input on the Arduino. The Analog.read function measures the voltage between B and ground. When the knob is twisted so that B touches A then B is at 5v, when you twist it the other way and B touches C, then B is at 0.

What is the voltage on B when the potentiometer is in the middle? To figure this out we have to consider the potentiometer as two resistors: one from A to B and one from B to C. Current flows from A (Vcc) to B and then it splits. It flows from B to C (ground) and from B through the arduino input to ground. So we have two resistive segments.

Let's assume that the potentiometer is 1M ohms from A to C. Since the potentiometer in this example has set B in the middle, we know that A to B has a resistance of 500,000 ohms. The second segment has two resistors in parallel: B to C and B through the Arduino.

We calculate parallel resistance as 1/ (1/r1 + 1/r2). B to C is 500,000 ohms. The Arduino input to ground is 1M ohms. So the second segment has a resistance of 333,333 ohms.

The voltage at B is split between the two segments in the ratio of their resistance. So we have two segments:

     5vdc -----   500,000 ohms  ---- B ----- 333,333 ohms ------ ground

In this case B will be at 2 volts. Get that: the potentiometer is in the middle, but B is at LESS than half. That's not so good.

Let's consider 3 positions. The potentiometer at 75%, 50%, 25% of the dial. Now the B voltages are:

     5vdc -----   750,000 ohms  ---- B ----- 200,000 ohms ------ ground       B is at 1 volt

     5vdc -----   500,000 ohms  ---- B ----- 333,333 ohms ------ ground       B is at 2 volts

     5vdc -----   250,000 ohms  ---- B ----- 428,571 ohms ------ ground       B is at 3.1 volts

The output voltage is not linear as we turn the potentiometer.

Let's do it again with a 1k pot.

     5vdc -----   750 ohms  ---- B ----- 249.93 ohms ------ ground       B is at 1.25 volts

     5vdc -----   500 ohms  ---- B ----- 499.75 ohms ------ ground       B is at 2.5   volts

     5vdc -----   250 ohms  ---- B ----- 749.43 ohms ------ ground       B is at 3.75 volts

We see that with the low resistance potentiometer, its values dominate the calculation of resistance for the parallel segment of the circuit. As a result the voltage at B will vary in a linear fashion as the potentiometer is turned. This is a good thing.