Expected Behavior:

I am attempting to use && or nested if statements to achieve the effect of two switches being required to close prior to an LED light (i.e., flip both switches), the LED turns on for 1 second, and then turns off.

Encountered Behavior:

It appears that no matter how I write the statement, if the SECOND if is true, the LED lights up.

Additional information:

The switches connect directly to ground when closed.

I have tried multiple conditional statements:

  1. using &&: if (switch1 == LOW && switch2 == LOW) {LED code}
  2. Nested if: if (switch1 == LOW) { if (switch2 == LOW) {LED code} }

I have tried multiple variable declarations:

  1. int switch1 = 0; int switch2 = 1; (using integer representation of digispark pinout)
  2. #define switch1 PB1
#define switch1 PB1
#define switch2 PB0

void setup()
  pinMode(2, OUTPUT);
  pinMode(switch1, INPUT_PULLUP);
  pinMode(switch2, INPUT_PULLUP);

void loop() {
  if (!digitalRead(switch1)) {
    if (!digitalRead(switch2)) {
      digitalWrite(2, HIGH);
      // turn the LED on (HIGH is the voltage level)
      delay(1000); // wait for a second
      digitalWrite(2, LOW);

  else {
    digitalWrite(2, LOW);

My Digispark Attiny85 is attached to a breadboard, where it receives power through the 5V and ground pins on the Digispark board. PB0 and PB1 connect directly to ground when closed, otherwise I count on the pull-up resistors. PB2 is connected to an LED -> resistor -> ground. All components share the same 5V power supply and ground.

enter image description here

  • 1
    what happens if you change the second if statement to if (false){?
    – jsotola
    Commented Apr 14 at 16:33
  • @jsotola Nothing happens now when I connect to ground PB0, PB1, or both
    – jmarywien
    Commented Apr 14 at 17:16
  • @jsotola. One last issue. It is the SECOND if , that if true, turns LED on. That is, the first if is irrelevant for some reason and I cannot figure out why this is the case
    – jmarywien
    Commented Apr 14 at 17:26
  • 1
    make the first if (false) ... what happens then?
    – jsotola
    Commented Apr 14 at 17:30
  • @jsotola, I think I answered below. Can you verify my reasoning?
    – jmarywien
    Commented Apr 14 at 17:45

2 Answers 2


I am confident I solved the problem and think I know why it occurs.

PB1 on the digispark Attiny85 has a built in LED. This LED is still active when I run my code. The LED is dim, but consuming power. My hypothesis is that is is consuming enough current to drive that pin low when digital_read is called on PB1. Therefore, PB1 always evaluates to true, because of the internal LED (despite the fact that input_pullup should drive it false by default).

Here is how I tested this hypothesis:

  1. If I plug PB1 into 5v+, the internal LED gets brighter, and my project LED never lights up. i.e., my first if is now !true, so the conditional fails to execute.
  2. If I recode to exclude PB1 and instead map to PB3, my code runs as intended.

All of this is to say, I think that even when you specify PB1 as input_pullup, it does not behave as expected because of the internal LED.

@jsotola, thank you for your kindness and patience. What do you think?

  • 2
    makes sense ... remove the external LED and use the internal LED instead ... use PB0 and PB2 for the switches
    – jsotola
    Commented Apr 14 at 17:57
  • 1
    Good debugging effort. In the future, you can also just spam a bunch of Serial.print() in there to monitor if your switches work
    – towe
    Commented Apr 15 at 10:00
  • @towe I am not sure I can do that with digispark, but I am new to this and will let you know what I find out
    – jmarywien
    Commented Apr 15 at 12:20

You would appear to have the incorrect physical pins connected to the switches:

Pins 0 and 1 connect directly to ground when closed, otherwise I count on the pull-up resistors.


  • PB0 is physical pin 5
  • PB1 is physical pin 6

ATTiny85 pinout

  • I apologize for the sloppy language. The digispark has the pins labeled and what I meant to say is that I connected to the pins labeled PB0 and PB1 on the digispark. So, I have the pins labeled PB0 and PB1 on the digispark connected to ground when closed
    – jmarywien
    Commented Apr 14 at 16:09
  • 2
    @jmarywien please update your question with this new information
    – jsotola
    Commented Apr 14 at 16:11
  • 1
    @jsotola. Thanks updated language and added schematic
    – jmarywien
    Commented Apr 14 at 16:13

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