Analog Pins with mystery voltage in Mega 2560

Development Tools IDE 1.x
1

Arduino Mega 2560 reads a voltage in the analog in pins while nothing is connected. I noticed the problem while working the bugs out of a photo-resistor/IR sensor project. Finally I wrote a sketch to read the analog pins to the serial monitor and they read out a number that starts just shy of 400, over the span of a few seconds they drop to about 300.

Is this a defective board? Am I missing something? Nothing is plugged in except the usb... code and serial monitor info is provided below.

void setup(){
Serial.begin(9600);
}

void loop(){

Serial.println(analogRead(1));
}

2รท393
393
394
395
397
398

a few seconds down the line...

286
286
287
286
286
287
286

Thanks for any input you have!

2

Floating pins (aka unconnected pins) pick up stray energy, creating a voltage on them.

It is sort of why the following note is on the analogRead() reference page:
"If the analog input pin is not connected to anything, the value returned by analogRead() will fluctuate based on a number of factors (e.g. the values of the other analog inputs, how close your hand is to the board, etc.)."

3

The pins, if not connected to anything, are "floating". Neither high nor low.
Use a 10K resistor and connect them high or low.

Or, you may be able to use pinMode and set them as inputs, with digitalWrite (pinX, HIGH); to enable the internal pullup resistors.

pinMode (pinX, HIGH);
digitalWrite (pinX, HIGH);

Then this
int pinXvalue = analogRead (pinX);
should yield close to 1023.

4

James, I have not yet looked at the reference pages yet. I bought the Arduino book from Radio Shack and have been working through that. I just looked at the reference page and found a wealth of information. Thank you!

Cross Roads, Thanks for your input. My electronics is a bit lacking. The issue was a lack of grounding... please don't tell anyone. Now my Night Light Motion Detector works like Ford from the 60's (great!).

5

Missing grounding occurs a lot, you are faaaaar from the first 8)