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Topic: weird stuff happening with sensor program o_0 (Read 2455 times) previous topic - next topic


I'm really confused. I have a program that's supposed to take sensor readings (it's a Sharp long-distance IR sensor). It's supposed to print them out on the serial monitor in values ranging from 0 to 1023. It worked just fine before, and then it suddenly started acting strangely; now the values seldom go any higher than 200, and they are totally inconsistent. I'll place my hand directly in front of the sensor and start to move it away, and the numbers start to grow larger, which is what it's supposed to do…but then they fluctuate  like crazy, even when my hand is barely moving! What have I done? Here is my code:

int sensorvar = 0;
int led = 9;
int outputValue = 0;

void setup(){
  pinMode(led, OUTPUT);

void loop(){

  sensorvar = analogRead(1);
  outputValue = map(sensorvar, 0, 1023, 0, 255);
  analogWrite(led, outputValue);
  Serial.print("sensor value:");




Rob Tillaart

Nederlandse sectie - http://arduino.cc/forum/index.php/board,77.0.html -
(Please do not PM for private consultancy)


I don't think it's the wiring; I have the signal wire connected to analog pin 1, and the power and ground wires are connected to the power and ground rails on the breadboard. Also, I doubt that it's dirt, because I tried using my other Sharp sensor in case this one is defective, and that one produced the same results.


steps to test (I understood that it partly works and the problems are fluctuations, but nevertheless):

  • remove for a moment the analogWrite(led, outputValue); and see if it becomes stable

  • Connect analog 1 to gnd, does it read 0

  • Connect analog 1 to VCC or also to 3.3, does it read 1023 (or for 3.3V around 675 give and take a bit)

  • Connected something odd to AREF?

  • Tried another pin?

  • if you put the sensor and a free wire to analog 1, does it fluctuate when you touch the wire?

  • is it analog 1 or the 1st analog in?

  • recheck the wiring!


scjurgen, thank you so much for your helpful debugging tips. It reads 1023 when I connect the pin to power and 0 when I connect it to ground. I also tried a different analog pin. I even tried snipping off the edge of the wire a bit (it's the stranded kind, so it tends to split easily). I revised my program so that it looks like this:

int analogPin = 4;     // potentiometer wiper (middle terminal) connected to analog pin 3
                       // outside leads to ground and +5V
int val = 0;           // variable to store the value read

void setup()
  Serial.begin(9600);          //  setup serial

void loop()
  val = analogRead(analogPin);    // read the input pin
  Serial.println(val);             // debug value

Still, no results… :(
I'm wondering if it's a problem with the sensor itself.


I don't have anything connected to AREF, either. What do you mean by putting a free wire at the analog pin?


Oh wow, now it mysteriously works just fine. I tried connecting a wire from analog pin 4 (I switched from 1 to 4) to the breadboard and then connecting the sensor to the same node on the breadboard. I had it wired that way before, but then while I was in the process of debugging, I tried connecting the wire directly to the pin. Go figure. o.O

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