This is a "water level alarm/display" sketch using and Arduino and ultrasonic sensor. The buzzer gets on when the water level hits 100% or more. But most of the time the water level will stay @ 100%, and I dont want the buzzer to run for long time. How can I set the buzzer to make sound only for 15 seconds ? It should turn off after 15 seconds, even if the water level is @ 100%

#define trigPin 8
#define echoPin 9
#include <LiquidCrystal.h>

LiquidCrystal lcd(12, 11, 5, 4, 3, 2);
int BUZZER = 10 ;

void setup(){
      int duration,distance,percentage,heightTank;
      Serial.begin (9600);

void loop(){
      int duration,distance,percentage,heightTank,deviation;
      //You'll probably want to change the next 2 lines.
      // The first one is the max. level of the water.
      // The next one is how high the sensor is above that max. level.

      lcd.print("Current tank");
      //lcd.print(" cm");
      lcd.print("level: ");
      lcd.print(" %");

  if (percentage > 99) {digitalWrite(BUZZER,HIGH); delay(100); digitalWrite(BUZZER,LOW); delay(100);}
  else  {digitalWrite(BUZZER,LOW);}

Try this at the end of loop

static unsigned long lastBuzzer = 0;

if(percentage > 99 && lastBuzzer == 0) {
    lastBuzzer = millis();

if(percentage > 99) {
    if(millis() - lastBuzzer < 15000L) {
} else  {
    lastBuzzer = 0;

lastBuzzer is the last buzzer activation time (in ms). It's use to control that buzzing stop after 15 seconds. It's zero when the percentage is < 99; it's not zero when percentage >= 100.

(Not tested). (Two if's instead of one for clarity).

  • 2
    May be nitpicking but... 0 is a perfectly valid return value for millis(), once every 49.7 days. Dec 9 '17 at 10:30
  • @EdgarBonet. True, but I can't see how it affects operation. If millis is zero in lastBuzzer = millis();, then millis() - lastBuzzer (both unsigned long) still will be < 15000L.
    – user31481
    Dec 9 '17 at 12:53

For this you will have to implement a finite state machine. This is a very common paradigm in the embedded world. I recommend you start by studying this tutorial: The Finite State Machine, by Majenko.

Four your particular problem, I would implement a machine with three states: IDLE, ARMED and ALARM, the ALARM state being the only one in which the buzzer is on. The transitions would be:

  • ARMED → ALARM when the level hits 100%
  • ALARM → IDLE after 15 seconds spent in the ALARM state
  • IDLE → ARMED when the level gets below 90%

Note that the only difference between IDLE and ARMED is that the IDLE state will not go into ALARM when the level reads 100%, as you have to go through the ARMED state before. The threshold at 90% percent is meant to add some hysteresis. You can tweak the threshold value to your taste.

I leave the implementation to you. It should be easy one you have studied Majenko's tutorial.

Edit: Just to illustrate the generality of the concept, I would like to point out that the code in Look Alterno's answer can be thought of as a finite state machine where the state is implicitly encoded into the variables lastBuzzer and percentage. The implicit state is:

  • ARMED when lastBuzzer == 0
  • ALARM when percentage > 99 && millis() - lastBuzzer < 15000L
  • IDLE when percentage > 99 && millis() - lastBuzzer >= 15000L

The line lastBuzzer = 0 establishes the ARMED state as soon as percentage ≤ 99. Note that this lacks hysteresis: if the reading fluctuates between 99% and 100%, the alarm will sound again and again.

I personally prefer using an explicit state variable, as it makes the code clearer to me. But then, the best approach is whichever makes the code clearer to you.

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