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Topic: Analog Pins with mystery voltage in Mega 2560 (Read 958 times) previous topic - next topic

fedusenko

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.

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

void loop(){
 
Serial.println(analogRead(1));
}


Quote
2รท393
393
394
395
397
398

a few seconds down the line...
Quote

286
286
287
286
286
287
286



Thanks for any input you have!

cmiyc

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.)."
Capacitor Expert By Day, Enginerd by night.  ||  Personal Blog: www.baldengineer.com  || Electronics Tutorials for Beginners:  www.addohms.com

CrossRoads

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.
Code: [Select]

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


Then this
int pinXvalue = analogRead (pinX);
should yield close to 1023.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

fedusenko

#3
Apr 16, 2012, 06:30 am Last Edit: Apr 16, 2012, 06:34 am by fedusenko Reason: 1
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!).

CrossRoads

Missing grounding occurs a lot, you are faaaaar from the first 8)
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

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