1. What is a Potentiometer?

A potentiometer is a device that allows us to create a sliding-scale knob. The potentiometer pictured below has three pins in a triangular arrangement on the bottom and a knob on top:

When the potentiometer is connected in a circuit, as you turn the knob, the resistance through the potentiometer changes, which also changes the voltage potential across the device (thus the name "potentiometer").

If we connect an LED to the potentiometer, turning the knob allows us to change the voltage for the LED, which in turn causes the LED to grow brighter or dimmer (brighter for a higher voltage, dimmer for a lower voltage). 

2. Analog Devices vs. Digital Devices

Because a potentiometer allows for a continuous, sliding-scale voltage, it is an analog device (as opposed to a digital device, which operates with just two voltages, high and low).

3. Writing To and Reading From Analog Devices

We use the digitalWrite code block to control a digital device. For an analog device, we use the analogWrite block:

And to read the voltage on the potentiometer, we use the analogRead block:

analogWrite can be used on any Pulse-width modulation pin on the Arduino board (digital pins 3, 5, 6, 9, 10, and 11).

analogRead reads a voltage from the Arduino's analog pins only. When it reads a voltage between 0V and 5V, it will convert it to an integer value between 0 and 1023 (where 5V corresponds to 1023).

For a potentiometer, the middle pin is the pin we read from and write to.

4. Creating a Circuit with a Potentiometer to Control an LED's Brightness

We can use a breadboard to create a circuit with a potentiometer and LED. Turning the knob on the potentiometer will change the brightness of the LED's light. Here's what the circuit looks like:
 

Connections:

• The longer leg of the LED connects to digital (PWM) pin 3 of the Arduino.
• The shorter leg of the LED connects to ground through a 220-ohm resistor.
• The middle pin of the potentiometer connects to analog pin A0 of the Arduino.
• The other two pins of the potentiometer connect to 5V and ground, respectively (it doesn't matter which pin is which).

5. The Code to Control the Potentiometer

To write a program that uses a potentiometer to control the brightness of an LED, we first need to specify what pins the LED and potentiometer are connected to (digital pin 3 for the LED and analog pin A0 for the potentiometer). So we create a variable for each and set those values:

The pin mode for the LED needs to be set for output:

We also need to create variables to store the voltage values we read from the potentiometer and the the voltage value we write to the LED. We'll name them "potValue" and "LEDvalue" and set them both to 0 to start:

The main part of the program will do two things:

1. Read and store the voltage value of the potentiometer.
2. Set the voltage value of the LED to the voltage value gotten from the potentiometer.

The code to read the potentiometer value uses the analogRead block and stores the result in the variable "potValue":

The next step is to change the voltage of the LED based on the value gotten from the potentiometer. We have to do a little math first, however, because the values for the potentiometer voltage go from 0 to 1023, while the values for the LED voltage go from 0 to 255. In other words, a value of 1023 for the potentiometer (max voltage) corresponds to a value of 255 for the LED (max brightness). 

We therefore take the potentiometer value and divide it by 1023 to get the fraction of its max value. For example, if the potentiometer value is 512, then it's at half value. Or in numbers, 512/1023 is about 0.5.

Then we muliply this number by 255 to get the corresponding value for the LED. For example, 0.5 times 255 is about 128, or about half brightness.

Here's what it looks like in the code:

Then we change the LED voltage to this newly calculated LEDvalue by writing the value to the LED pin (pin 3):

Finally, we have to remember that we need to check the potentiometer continually, because as the user turns the knob on the potentiometer, the voltage will change. Therefore we need to put these last three code blocks into a repeat while loop:

This loop will run continuously, constantly reading the potentiometer's voltage value and changing the LED's voltage to a corresponding value. So as the user turns the knob on the potentiometer the brightness of the LED will change.

To start the program, click the  Run  button. To stop the loop, click the  Reset  button. 

Note: The program in Example 1 does not end on its own. As a result, this program cannot be submitted as an assignment.