Is a 9v battery appropriate for a piezo and 1 LED?

Question

I'm making a basic "door open" detector that, when a door is opened, it turns on an LED and then sounds an alarm via a piezo.

This is controlled via a 5v Adafruit Trinket.

Is a 9v battery the correct thing to power this with, if the goal is to change batteries as infrequently as possible?

Here's the code, incase that's relevant.

#include <elapsedMillis.h>
elapsedMillis timeElapsed;

const int switchPin = 2;
const int ledPin = 1;
const int piezoPin = 0;
const uint32_t openLength = 5 * 1000L; // X seconds
int notes[] = {862,294,730};

void setup() {
  pinMode(switchPin, INPUT);
  pinMode(ledPin, OUTPUT);
  pinMode(piezoPin, OUTPUT);
  digitalWrite(switchPin, HIGH);
}

void loop() {
  if(digitalRead(switchPin) == LOW){    
    digitalWrite(ledPin, LOW);
    timeElapsed = 0;
  } else {
    digitalWrite(ledPin, HIGH);

    if (timeElapsed > openLength){
      for (int n = 0; n < 200 ; n++){
        beep(notes[random(0,2)]);

        if(digitalRead(switchPin) == LOW){ break; }
      }
    }
  }
}

void beep(int delayAmount)
{
  for (uint16_t t = 0; t < 30*1000/2; t += delayAmount)
  {  
    digitalWrite(piezoPin,HIGH);
    delayMicroseconds(delayAmount);
    digitalWrite(piezoPin,LOW);
    delayMicroseconds(delayAmount);
  }  
}

UPDATE: I had a 9V plugged in to this for about 3 days before it used all the power in 9V. Is that an issue w/ voltage on the battery (i.e. I should be using AA's instead of a 9V)? Or does that point largely to some other inefficiency w/ the code or hardware?

Answer 1

A couple things to note:

• Get rid of the PWR LED on the Trinket so it's not always drawing current.
• Sleep the MCU whenever possible, only waking on interrupts.
• Converting 9V to 5V with an LDO (in this case, the MIC5225) implies a 55% efficiency, max, which isn't great. But at least the LDO has a low ground current.

Also, ask yourself if you really need a micro for this task.

If you want a ballpark for runtime, find the capacity of your battery, estimate your current draw, and see what your upper bound on runtime is.

Answer 2

You can power the Trinket (ATTiny85) from as little as 2.7 volt. So you could power it by connecting 2 (or 3) AA-batteries directly to the 5V pin.

Using a 9V battery means you would "throw away" 4 of the 9 Volts to get 5Volt. So you'd waste almost half of the capacity of the battery.

Also a 9V battery only has around 500mAh, where as an AA battery has 2500mAh.