Potentiometers

Another option for user input is to use the ADC to read the position of a potentiometer. Let's revisit some of the things you learned in Chapter 4, PWM and ADC Subsystems, to make our program a bit more interesting.

For this circuit, you will need:

• Breadboard

• 2x potentiometers

• 1x 5 mm LED

• 1x CdS photocell (also known as photoresistor / light-dependant resistor / LDR)

• 2x 10 kΩ resistor

• 1x 4.7 kΩ resistor

• 1x 68 Ω resistor

• 1x 2N3904 NPN transistor

• 1x LM358 (or similar rail-to-rail op-amp)

• Jumper wires

Put it all together on your breadboard, as shown in the figure:

This circuit might look a lot more complicated than what we've done so far, but really it's just a combination of a few of the earlier circuits. The only difference is the configuration of the op-amp.

Here it is being used to buffer the output of a 2:1 voltage divider across the 3.3 V rail, which creates a 1.65 V reference voltage. Let's take a look at the following diagram:

This 1.65 V reference is then being used by the two potentiometers...