Using only two digital pins for a button? (no GND, no VCC)

[alowhum]

If I only have two digital pins left, could they be used to support a button? Online searches have proven fruitless.

For example:

- D4 functions as output, set to permanently high.
>>
[ button ]
>>
- D5 functions as input, with pullup resistor enabled.

Would this work? Or would this fry the Arduino/pin?

[Yveaux]

@alowhum almost, if you set D4 permanently LOW.
Then you button on D5 toggles between HIGH (when open, due to pullup) and LOW (when closed, shorted to D4).

[alowhum]

@Yveaux Thanks!

// It works!

For anyone finding this:

In the Setup / Before function:

  // BUTTON
  pinMode(BUTTON_OUTPUT_PIN, OUTPUT);                          // Set a digital pin as power output
  digitalWrite(BUTTON_OUTPUT_PIN, LOW);
  pinMode(BUTTON_INPUT_PIN, INPUT_PULLUP);                     // Set another digital pin as power input

In the loop:

  wait(20);

  // ON DEVICE TOGGLE BUTTON
  if (digitalRead(BUTTON_INPUT_PIN) == LOW) {
    button_pressed = true;
    Serial.print(F("."));
  }

[dbemowsk]

Out of curiosity, would there be any practical reason for doing it this way? Maybe there is an advantage that I am not thinking of. It just seems though that it is a waste of a pin, not to mention that it is more code to get it working.

[alowhum]

@dbemowsk The Nano only has so many GND and VCC pins. This allows me to connect a relay, an LED and this button directly with dupont wires. No breadboard/expansion board/soldering required.

[zboblamont]

@alowhum Seems a bit wasteful of Nano pins for the sake of 2 lines of Veroboard and some header pins though, but if it works for you...

[alowhum]

@zboblamont It's not very wasteful if you're not using them for anything else anyway. I'd rather use some extra code than extra hardware.