Digital pins on a Uno

Projects Programming
1

Hi
I have noticed a problem on one of my UNO's Digital pins.

Let me explain....
I boot up the UNO, it has no connections on any of the pins.

If you put it into the main loop to do a digital read on pin 7 I get 0 all the time which is correct.

Now if I add a dupont jumper wire into D7 and not connected to anything on the other end and run the loop it just keeps flipping between 0 and 1 even though it's not connected to anything.
If I pull the jumper wire out of D7 it then sticks at 0.

Any ideas anyone why it keeps flipping between 0 and 1? Is the UNO on it's way out?
Thanks

int ledPin = 13;
int inputPin = 7;
int val = 0;

void setup() {
pinMode(ledPin, OUTPUT);
pinMode(inputPin, INPUT);
Serial.begin(9600);
}

void loop(){
val = digitalRead(inputPin);
Serial.println(val);
}

2

It is because the pin is floating at an unknown voltage, maybe HIGH, maybe LOW

Set the pinMode() of the input pin to INPUT_PULLUP to turn on the built in pullup resistor to hold the pin HIGH unless you expressly take it LOW by connecting it to GND

3

Because you just created a crude digital radio?

-jim lee

4

I don’t know why my previous answer to this question has disappeared but here is a link that explains it all.
http://www.thebox.myzen.co.uk/Tutorial/Inputs.html

6

Hi
I have noticed a problem on one of my UNO's Digital pins.

Let me explain....
I boot up the UNO, it has no connections on any of the pins.

If you put it into the main loop to do a digital read on pin 7 I get 0 all the time which is correct.

Now if I add a dupont jumper wire into D7 and not connected to anything on the other end and run the loop it just keeps flipping between 0 and 1 even though it's not connected to anything.
If I pull the jumper wire out of D7 it then sticks at 0.

Any ideas anyone why it keeps flipping between 0 and 1? Is the UNO on it's way out?
Thanks

int ledPin = 13;
int inputPin = 7;
int val = 0;

void setup() {
pinMode(ledPin, OUTPUT);
pinMode(inputPin, INPUT);
Serial.begin(9600);
}

void loop(){
val = digitalRead(inputPin);
Serial.println(val);
}

7

Change that to INPUT_PULLUP, then the pin will not be floating and any loose wire attached wil not act as an antenna picking up stray electronic fields.
It will return 1 until you take the pin low (ground) and then it will return 0.

1 Like
8

congratulation. you reinvented antenna.

9

You are not the first to think you have discovered something. Please read How inputs work

So the answer to your question is that nothing is wrong.

10

It's because your pin is floating so it's state will be true and false randomly changing.

More on floating inputs:

11

Thanks for the replies. I am at work at the moment so will try when I get home.

The reason for putting in the jumper wire was that I had D7 wired up to a PIR sensor following a tutorial and I say geting flipping 0 or 1, so I thought the PIR may of been faulty so plugged in another and still got the flipping 0 or 1. I then noticed that it all stopped when I pulled the jumper out of D7 and then plugging the jumper in it then went 0 or 1 again. This is why I thought D7 might be faulty

When I get home I shall take all your suggestions onboard and thank you all.

12

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