Arduino controlled relay for certain duration of time

Question

I am using a relay on my Arduino Uno and i want it to be on LOW for a calculated interval of time, let's say ftime=7 seconds, and after that i need it to be set as HIGH (no power through relay). My code below makes no changes to the relay state. How would that be possible? I tried to use "delay()" too (which i know that is generally a bad idea), but had no success.

 unsigned int interval = ftime*1000; //ftime in ms
do
  {
  digitalWrite(Relay1, LOW);
  }
  while(timeElapsed < interval);
digitalWrite(Relay1, HIGH);

Also, if i try without:

digitalWrite(Relay1, HIGH);

relay is LOW (motor running) but it never returns to HIGH obviously.

Answer 1

you need to update the timeElapsed variable inside the while loop:

 unsigned int interval = ftime*1000; //ftime in ms
 unsigned long startTime = millis();
do
  {
      digitalWrite(Relay1, LOW);
      timeElapsed = millis() - startTime;
  }
  while(timeElapsed < interval);
digitalWrite(Relay1, HIGH);

Another way is to create a state machine make startTime a global and update the logical state (and the pin) when the time is elapsed.

unsigned long startTime;
bool relayLOW = false;

void loop(){

    if(relayLOW && millis() - startTime > interval){
        digitalWrite(Relay1, HIGH);
        relayLOW = false;
    }
    if(!relayLOW){
        bool closeRelay= //decide if relay needs to be triggered
        if(closeRelay){
            startTime = millis();
            digitalWrite(Relay1, LOW);
            relayLOW = true;
        }
    }
}