Triggering a solenoid EVERY FEW DAYS?

Using Arduino Programming Questions
1

I need to write a little arduino program that is going to trigger a solenoid every few days. I’m a newbie programmer. I’m told that millis() will overflow and reset after awhile. If I just do a super long delay is that ok? Will a long delay somehow be affected by millis() resetting? Or be affected by some other clock thing resetting after awhile?

void setup() {     
  pinMode(3, OUTPUT);
}

int count = 96;

void loop() {
  if(count < 96){
    count++;
  }
  else if(count == 96){
    digitalWrite(3,HIGH);
    delay(15000);
    digitalWrite(3,LOW);
    count = 0;
  }
  delay(3600000);
}
2

"I’m told that millis() will overflow and reset after awhile."
You are correct. With proper calculations (current time - earlier time) the overflow will not matter (when it occurs in 49+ days).

Try it: FFFF FFE0 - 0000 0020 = 0000 0040, the proper result.

"If I just do a super long delay is that ok? "
That's one way that works.
Use 3600000UL as 360000 is more than fits in an INT (I believe this is th default type if not defined).

"Will a long delay somehow be affected by millis() resetting?"
No.

"Or be affected by some other clock thing resetting after awhile?"
Only loss of power.

You could add an 8-pin real time clock chip, and watch for the next time you want.

3

millis () overflows every 50 days or so, but this doesn't matter if you test by doing subtraction.

eg.

unsigned long start_time = millis ();

...

if (millis () - start_time >= (3UL * 24UL * 60UL * 60UL * 1000UL))
  {
  // time is up

  }

(The ULs are for Unsigned Long constants). This would test for 3 days to pass (24 hours * 3).

This code will work even over the wrap-around period.

4

Thank you very much!

5

have fun waiting for the code to loop to make sure it works...it'll take a day or two :smiley: