Arduino LED lighting up when signal is reading low on multimeter

Question

My code is designed to play a tone when ever pin 3 is high on my Arduino. When I am examine the pins voltage with a multimeter it is always reading below 1 volt. When I examined the voltage off of a osiliscope it is reading the voltage pretty steady at 0.2 to 0.3.

My multimeter reads at 0.26

:

My code is:

void setup() {
 // put your setup code here, to run once:
Serial.begin(9600);
}
void loop() {
 // put your main code here, to run repeatedly:
int input = digitalRead(3);
Serial.println(input);
if(input == 1){
 tone(A5,4000);
 }
}

My board looks like: enter image description here The bottom of the board looks like: enter image description here

• Nick Gammon: What pin are you measuring exactly? Pin 3 or pin A5? Is there a resistor in series with that LED? Is there a pull-up or pull-down resistor on pin 3? See Switches Tutorial.

Answer 1

You left pin 3 floating. The multimeter will lower the voltage, resulting the the led lighting. Try enabling the pull-up resistor in pin 3 by adding pinMode(3, INPUT_PULLUP).

PS you might want to add a resistor in series with that led, or it will burn out and/or damage pin 3 on the arduino.