Behaviour of tone() not as expected within while loop

Question

I have a smoke detector with a piezo buzzer. I want to operate the arduino in low power mode so that it will last on a 9V battery. I've attached an interrupt, and when the digital output of the MQ-2 sensor goes high, it switches to a monitor function.

In the while loop, I check to see if the analog sensor has reached the threshold value, and I output to serial to see if I am in the conditional. If I use:

digitalWrite(buzzer, HIGH);

then the piezo sounds, however if I use

tone(buzzer, 1000,200);
delay(200);

it does not. I'm confused as to why this would be the case.

Full code as follows:

#include "LowPower.h"

int redLed = 12;
int greenLed = 11;
int buzzer = 10;
int smokeA0 = A5;
// Your threshold value
int sensorThres = 50;
int smokeD0 = 2;

void setup() {
  pinMode(redLed, OUTPUT);
  digitalWrite(redLed, LOW);
  pinMode(greenLed, OUTPUT);
  digitalWrite(greenLed, HIGH);
  pinMode(buzzer, OUTPUT);
  pinMode(smokeA0, INPUT);
  pinMode(smokeD0, INPUT);


}
void switchToMonitor() {
  Serial.begin(9600);
  int digitalSensor = digitalRead(smokeD0);
  if (digitalRead(smokeD0) == HIGH)
  {
    digitalWrite(redLed, HIGH);
    digitalWrite(greenLed, LOW);
  }
  else {
    digitalWrite(redLed, LOW);
    digitalWrite(greenLed, HIGH);
  }
  //This will be updated later to use the analog input
  //Using digital to make debugging easier
  while (digitalSensor == HIGH) {
    int analogSensor = analogRead(smokeA0);
    Serial.print("Pin A0: ");
    Serial.println(analogSensor);
    Serial.print("Pin D0: ");
    Serial.println(digitalSensor);
    // Checks if it has reached the threshold value
    if (analogSensor > sensorThres) {
      digitalWrite(buzzer, HIGH);
      //tone(buzzer, 1000,200);
      Serial.println("here");
      delay(1000);
    }
    else {
//      digitalWrite(redLed, LOW);
//      digitalWrite(greenLed, HIGH);
      noTone(buzzer);
      Serial.println("also here?");
    }
    //delay(100);
    digitalSensor = digitalRead(smokeD0);
  }
  noTone(buzzer); 
}

void loop() {
  //delay(100);
  //LowPower.standby();
  attachInterrupt(0, switchToMonitor, CHANGE);
  LowPower.powerDown(SLEEP_FOREVER, ADC_ON, BOD_ON); 
  detachInterrupt(0); 
}

Answer 1

You can't use delay() or timer based tone() function in interrupt handler. Set a flag in interrupt and run the code in loop() conditionally based on the flag state. Do not forget to have the flag variable volatile.