# 3-way switch and Arduino

luca 01/08/2014 2

With this small tutorial, I’m going to show you how to read the position of a 3-way switch using just one Arduino’s analog PIN.

A 3-way switch (sometimes called ON-OFF-ON or SPTT) has 3 PINs: the common (on the center) and the two outputs. When the lever is up or down, the common PIN is connected to one of the output ones; when the lever is in the central position, the common PIN is not connected (floating).

Using only two resistors, you can read the lever position with an analog PIN: To understand how the circuit above works, let’s inspect what happens in the three different positions of the lever:

• when the lever is in the central position (2), the AN0 PIN is connected to 5V through the R1 resistor (pull-up): the read value is about 1023
• when the lever is in the top position (3), the AN0 PIN is connected to a voltage divider (R1 and R2). Being the two resistors of the same value, the voltage at the AN0 PIN is about 2.5V and therefore the read value is 512
• when  the lever is in the bottom position (1), the AN0 PIN is connected to the ground; the read value is 0.

The sketch is very simple:

``` #define BUTTON_PIN A0 int previousState;   void setup() {   Serial.begin(9600); previousState = 0; }   void loop() {   int analogValue = analogRead(BUTTON_PIN);   int actualState; if(analogValue < 100) actualState = 1; else if(analogValue < 900) actualState = 3; else actualState = 2;   if(previousState != actualState) {   previousState = actualState; Serial.print("Button state: "); Serial.println(actualState); } } ```

#### Demo

The circuit is very simple, I built it on a prototype board:   1. Mark 18/10/2015 at 13:02 - Reply

Luca thanks for the code. I would like to add another 3-way switch so I would have two. But i’m not a coder so I don’t know how to do it. Can you help?

• luca 19/10/2015 at 08:25 - Reply

Hi Mark, simply use a different analog PIN (for example A1):
```#define BUTTON_PIN2 A1 int analogValue2 = analogRead(BUTTON_PIN2); ...```