Analog Input with Potentiometers

Analog Input

Plan

  • Analog vs. digital
  • Potentiometer
  • Voltage dividers
  • Reading analog input

Analog vs. Digital

analog clock digital clock

Analog vs. Digital

Analog Digital
analog clock digital clock
infinite variations / states discrete (or finite) states

Potentiometer (Pot)

potentiometers

  • Variable resistor with 3 pins
    • power, ground, wiper

Potentiometer (Pot)

potentiometer schematic symbo

  • Wiper pin connects to circuit
    • As wiper moves, resistance varies from 0 Ohms to max (e.g. our kit has 10K Ohms pots)

Where do we find pots?

Where do we find pots?

  • Also car stereo knobs, dimming light switches, etc.

How a Potentiometer Works

Schematic symbol for a potentiometer

How a Potentiometer Works

Schematic symbol for a potentiometer

  • Vout is the wiper (middle pin; connects to Photon 2 analog input)
  • Inside the potentiometer is a resistor
  • As the knob moves, the wiper divides the resistor, and the ratio of resistance between Vin-and-Wiper and Wiper-and-Gnd varies
  • This is called a variable voltage divider (more on this later)

Question

Schematic symbol for a potentiometer

  • With a 10K Ohm pot, if the wiper is exactly halfway, what is the resistance between Vout and Gnd?

Voltage Dividers

Schematic symbol for a potentiometer

  • Photon 2 input pins can measure voltage, not resistance
  • By connecting Vin (3.3v), Gnd, and Vout (wiper), we can now vary the voltage difference Vin-and-Wiper and Wiper-and-Gnd

Voltage Dividers

Schematic symbol for a potentiometer

  • If the wiper is exactly halfway, what is the voltage between Vout and Gnd?
  • If the wiper is all the way to the top, what is the voltage between Vout and Gnd?

Voltage Divider

  • As those resistances changes, so does the voltage difference between Vin-and-Vout and Vout-and-Gnd
  • This is known as a voltage divider bg right:50% fit
    bg fit

Setting up Analog Input

Syntax

const int PIN_POT = A0;		//input pin
int potValue = 0;			//value from read

void setup() {
    pinMode(PIN_POT, INPUT);
}
  • Pins A0-A5 are analog input
  • Technically, analog pins are input by default but I like to do this for clarity and consistency

Reading Analog Input

Syntax

void loop() {
    potValue = analogRead(PIN_POT);
}

Wiring Diagram

  • Using Vin=3.3v and a 10k potentiometer, what do you expect for the range of values of potValue?

Credit

Updated: