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I had a Coursera Arduino homewhere wherein you create a simple pushbutton-controlled LED circuit.

I used the example "button.ino" script that's builtin to the Arduino IDE, without modification. I'll reproduce it at the end of this question.

Here's what I did (wrong):

My circuit went from 5V Power to the top pin of a push button. The bottom of the pushbutton was (correctly) wired to Pin 2 on one side and a resistor was used to wire the other bottom pin to the anode of LED. The cathode of the LED was wired to ground.

The reason I think this is wrong is that I forgot that I was supposed to wire the LED to a pin corresponding to ledPin in the example script. The example script uses Pin 13, which has the builtin LED. I was using an external LED. Yet, the circuit worked perfectly.

Why did it work? Doesn't that mean the 5V power output was getting turned off (I'd guess you call that being set to LOW) when Pin 13 was set to LOW? Why would that be the case?

/*
  Button

 Turns on and off a light emitting diode(LED) connected to digital  
 pin 13, when pressing a pushbutton attached to pin 2. 


 The circuit:
 * LED attached from pin 13 to ground 
 * pushbutton attached to pin 2 from +5V
 * 10K resistor attached to pin 2 from ground

 * Note: on most Arduinos there is already an LED on the board
 attached to pin 13.


 created 2005
 by DojoDave <http://www.0j0.org>
 modified 30 Aug 2011
 by Tom Igoe

 This example code is in the public domain.

 http://www.arduino.cc/en/Tutorial/Button
 */

// constants won't change. They're used here to 
// set pin numbers:
const int buttonPin = 2;     // the number of the pushbutton pin
const int ledPin =  13;      // the number of the LED pin

// variables will change:
int buttonState = 0;         // variable for reading the pushbutton status

void setup() {
  // initialize the LED pin as an output:
  pinMode(ledPin, OUTPUT);      
  // initialize the pushbutton pin as an input:
  pinMode(buttonPin, INPUT);     
}

void loop(){
  // read the state of the pushbutton value:
  buttonState = digitalRead(buttonPin);

  // check if the pushbutton is pressed.
  // if it is, the buttonState is HIGH:
  if (buttonState == HIGH) {     
    // turn LED on:    
    digitalWrite(ledPin, HIGH);  
  } 
  else {
    // turn LED off:
    digitalWrite(ledPin, LOW); 
  }
}
  • 1
    It worked because your button was controlling power to your led. The Arduino was doing nothing at all. – Majenko Aug 19 '17 at 20:52
  • @Majenko Thanks. Yes, they did say that the Arduino was only supposed to supply power and ground for this homework, so I guess that was as it should be after all. Incidentally, the on-board LED seemed to read the state of the pin correctly, which was something I didn't even think to look at until just now. As simple as this was, I think it's a good beginner exercise. – Hack-R Aug 19 '17 at 21:13
2

The LED wired to the push button was turned ON by the push button closing the circuit, without any intervention from the micro controller. That's a pretty simple circuit: a voltage source, a switch, an LED and a resistor, all in series. When you close the circuit the LED turns on. Simple.

The code on your microcontroller is watching the state change on the button and turning ON/OFF the on board LED: it has nothing to do with the LED in your circuit/breadboard.

2

Why did it work?

the button is shorting the led (turning it off) when it is closed. when the button is open, the led lights up from the resistor.

essentially your arduino is not involved with the led's operations.

  • Follow up question: what happens to the 5V when the button is open? It doesn't have a complete path so it just stays in the Arduino and the wire that leads up to the button? – Hack-R Aug 19 '17 at 23:40
1

The circuit you are describing seems to just have the button control whether the LED got the supply or not. Unless pin2 was grounded, you effectively supplied 5V to the resistor-LED as well as pin2 through the button. This essentially gives you the standard LED circuit. Have a great day!

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