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Topic: Button state change help (Read 1 time) previous topic - next topic


Dec 29, 2012, 04:17 am Last Edit: Dec 29, 2012, 04:18 am by Coding Badly Reason: 1
im trying to understand how to work with a button. i want an led to turn on after one press and remain one untill i press the button again.

here is a button state change code but im having issues understanding parts of it. could someone explain what buttonstate!+lastbuttonstate and

what these mean: buttonPushCounter++;
     Serial.print("number of button pushes:  ");

Code: [Select]
 State change detection (edge detection)
Often, you don't need to know the state of a digital input all the time,
but you just need to know when the input changes from one state to another.
For example, you want to know when a button goes from OFF to ON.  This is called
state change detection, or edge detection.

This example shows how to detect when a button or button changes from off to on
and on to off.
The circuit:
* pushbutton attached to pin 2 from +5V
* 10K resistor attached to pin 2 from ground
* LED attached from pin 13 to ground (or use the built-in LED on
  most Arduino boards)

created  27 Sep 2005
modified 30 Aug 2011
by Tom Igoe

This example code is in the public domain.


// this constant won't change:
const int  buttonPin = 2;    // the pin that the pushbutton is attached to
const int ledPin = 13;       // the pin that the LED is attached to

// Variables will change:
int buttonPushCounter = 0;   // counter for the number of button presses
int buttonState = 0;         // current state of the button
int lastButtonState = 0;     // previous state of the button

void setup() {
 // initialize the button pin as a input:
 pinMode(buttonPin, INPUT);
 // initialize the LED as an output:
 pinMode(ledPin, OUTPUT);
 // initialize serial communication:

void loop() {
 // read the pushbutton input pin:
 buttonState = digitalRead(buttonPin);

 // compare the buttonState to its previous state
 if (buttonState != lastButtonState) {
   // if the state has changed, increment the counter
   if (buttonState == HIGH) {
     // if the current state is HIGH then the button
     // wend from off to on:
     Serial.print("number of button pushes:  ");
   else {
     // if the current state is LOW then the button
     // wend from on to off:
 // save the current state as the last state,
 //for next time through the loop
 lastButtonState = buttonState;

 // turns on the LED every four button pushes by
 // checking the modulo of the button push counter.
 // the modulo function gives you the remainder of
 // the division of two numbers:
 if (buttonPushCounter % 4 == 0) {
   digitalWrite(ledPin, HIGH);
 } else {
  digitalWrite(ledPin, LOW);

Moderator edit: [code] [/code] tags added.


its pretty simple once you grasp it

you read a button using digital read at the start of each looop, which is happening millions of times per second

Code: [Select]
buttonState = digitalRead(buttonPin);

at the end of the loop (as far as the button is concerned) you set the variable lastButtonState to whatever the button read

so if the button is up at the beginning of the loop and at the end of the loop, nothing has changed right? when you press the button, the reading at the beginning of the loop says down, but last button says it was up, so you now know that the state of the button has changed.


im sorry i wrote that wrong i understand what button state normall means but what does if (buttonState != lastButtonState)



!= is a test for "not equal to", so if (buttonState != lastButtonState) is checking to see if the button's state has changed since the last time through loop()

(Read about it here)

Then, if it has changed, it checks to see if it's HIGH, which means it was pressed and so it increments the counter and reports that to the monitor. If it has changed and is LOW though, it means it was released and it reports the fact that it's now off

If it hasn't changed, then nothing happens....
Roy from ITCrowd: Have you tried turning it off an on again?
I'm on LinkedIn: http://www.linkedin.com/in/jimbrownza


then all this id just taking about the button counter? 
      Serial.print("number of button pushes:  ");

also it explains that this function is the remainer of the division of two numbers, could someone clarify this?  if (buttonPushCounter % 4 == 0)

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