Comparing the current value vs previous value of LDR

Using Arduino Project Guidance
1

Hi guys, now I using LDR to measure the light and read the value using Arduino. How can I comparing the current measure value with the previous measure value? For example, the first value read is 555 and delay for 5 seconds. After 5 seconds, the second value read is 520. I wanted to find the difference of the current value(second value) and the previous value(first value), how can I achieve this?

2

Use a static or global variable to store the older reading.

3

(deleted)

4

AWOL:
Use a static or global variable to store the older reading.

Since the measuring is continuously, so can the static variable update every time?

5

You can't update a static variable,
but you can use a current and a previous variable and calculate the difference every x seconds.

currentValue = analogRead(ldrPin);
difference = currentValue - previousValue;
Serial.println(difference);
previousValue = currentValue;
delay(5000);

Leo..

6

Wawa:
You can't update a static variable,
but you can use a current and a previous variable and calculate the difference every x seconds.

currentValue = analogRead(ldrPin);

difference = currentValue - previousValue;
Serial.println(difference);
previousValue = currentValue;
delay(5000);



Leo..

I found a solution using millis()! Thanks for the reply!

7

Wawa:
You can't update a static variable,

That is not correct. It is a CONST that you cannot update.

"static" is used to tell the compiler that a local variable must be preserved between calls to a function. (And, before you say it, I agree that "static" was poor word to choose for the purpose).

...R