Arduino: LCD won't turn off - arduino

Question

a few days ago I started working with the Arduino. I've set up a small project with a DHT22 to read the temperature and humidity and write it to an LCD. That works without a problem. Now I want to only turn on the backlight of the LCD when I press a button. That mostly works too:
void loop() {
buttonState = digitalRead(BUTTONPIN);
currentMillisScreen = millis();
if (buttonState == HIGH) {
screenOn = true;
lcd.backlight();
}
// DHT22 related code in here
if (currentMillisScreen - previousMillisScreen >= SCREEN_ON_TIME) {
previousMillisScreen = currentMillisScreen;
screenOn = false;
lcd.noBacklight();
}
}
The problem is that with this code is that the Backlight won't always stay on for exactly 5 seconds. I thought putting the currentMillisScreen = millis() in the following if-Statement would fix it:
if (buttonState == HIGH) {
currentMillisScreen = millis();
screenOn = true;
lcd.backlight();
}
But if I do that, the Backlight won't turn off again and I don't understand why.

Answer 1

You are not updating currentMillisScreen in the loop and that is your problem. You just need to find different between currentTime (equal to millis()) and previous time that light turned on and if it reaches above the threshold then turn it off. Something like this:
#define SCREEN_ON_TIME 5000
bool screenOn = false;
void setup()
{
//setup
}
void loop()
{
buttonState = digitalRead(BUTTONPIN);
if (buttonState == HIGH)
{
previousMillisScreen = millis();
lcd.backlight();
screenOn = true;
}
// DHT22 related code in here
// if time passed above SCREEN_ON_TIME after we turned on light
if ((screenOn) && (millis() - previousMillisScreen >= SCREEN_ON_TIME))
{
lcd.noBacklight();
screenOn = false;
}
}