LDR weird behavior.

Projects Programming
1 
const int ledPin = 13; // LED connected to digital pin 13
const int sensorPin = 0; // connect sensor to analog input 0
// the next two lines set the min and max delay between blinks
const int minDuration = 100; // minimum wait between blinks
const int maxDuration = 1000; // maximum wait between blinks
void setup()
{
pinMode(ledPin, OUTPUT); // enable output on the led pin
Serial.begin(9600); // initialize Serial
}
void loop()
{
int rate = analogRead(sensorPin); // read the analog input
// the next line scales the blink rate between the min and max values
rate = map(rate, 200,800,minDuration, maxDuration); // convert to blink rate
Serial.println(rate); // print rate to serial monitor
digitalWrite(ledPin, HIGH); // set the LED on
delay(rate); // wait duration dependent on light level
digitalWrite(ledPin, LOW); // set the LED off
delay(rate);
}

Hello All,
When running this code with LDR everything seems to be fine, but when I cover the LDR it freeze up on serial with a negative value. Can anyone explain why this is happening please.

2

Can anyone explain why this is happening please.

Because analogRead returns a value below 200?

3

Yes you right playing around with that value seems to help. I just dont understand why it will go into the negative range :o . I changed it from 200 to 0 and now it works ok. But still it bothers me that it will go till the negative range. I thought that it will only have a value between 0 and 1023.

4

But still it bothers me that it will go till the negative range. I thought that it will only have a value between 0 and 1023.

The value from analogueRead will have that range, but then this rate = map(rate, 200,800,minDuration, maxDuration); happens.

5

It sounds like you want to constrain the value to passed to map() to the range 200-800. Or the output to a range minDuration to maxDuration.

Could be done manually like this too (where x, a, and b are same as in constrain)
(x>b?b:(x<a,a,x))

6

Hi,
The map function basically allows you to rescale and offset your input variable.
It does not restrict the output value to the output parameters you have used to rescale.

rate = map(rate, 200,800,minDuration, maxDuration)

So if rate input is below 200, then rate output will be below minDuration.
The same if you are larger then 800, output will be larger than maxDuration.

Tom.... :slight_smile: