3-way switch and Arduino

Posted by: Jia Qun , May 19, 2016

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:

3sbutton-1

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);
  }
}

I added a tolerance threshold to the read values.