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); 
}

• David Bekker: @Juraj The tone function works when used within void loop() and not switching between low power and operational. It's only not working in the code example given
• jsotola: connect the buzzer to a battery ... does it make a sound?

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.

• David Bekker: Took it out of the interrupt handler and it worked. Thanks