How to make battery last for months?

Question

I have a simple circuit: detect that a person is walking towards the circuit (with HC-SR04), measure distance, light diodes (a wave effect) according to that distance – the closer, the faster the led wave.

When no person detected, sleep longer:

LowPower.powerDown(SLEEP_250MS, ADC_OFF, BOD_OFF);

When person detected, sleep shorter (so that diodes can be often turned on/off):

LowPower.powerDown(SLEEP_30MS, ADC_OFF, BOD_OFF);

Without LowPower use – delay() used instead – the current taken is 15 mA. This is with diodes off. After using LowPower – it's still ~15 mA. This is very disappointing. I've expected the battery to last at least for 3 months, but it (9V) was drained within hours. Am I missing something?

Also a curiosity: the execution was fully blocking itself on LowPower calls, I've had to add short delay() before the calls, then they don't block execution.

#include <LowPower.h>
#include <NewPing.h>

#define trigPin 13
#define echoPin 12

NewPing sonar(trigPin, echoPin, 250);

// Defines speed of LEDs traversal
int delayTime = 40;

// Current led
int currentLED = 0;

// Direction of travel over LEDs
int dir = 1;

// Stores the last time LEDs were changed
unsigned long timeChanged = 0;

// Stores the last time sensor was read
unsigned long timeOfQuery = 0;

// LEDs
byte ledPin[] = { 4, 5, 6, 7, 8, 9, 10, 11 };

// Stores if any diodes are on
int leds_used = 1;

// Stores if wave is active
int off = 1;

// 5 last distances, circular buffer;
long distances[7] = { 400, 400, 400, 400, 400, 400, 400 }, currentDistance = 0;

// Last chosen distance
long distance = 400;

void setup() {
  Serial.begin (9600);
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
  for (int x = 0; x < 8; x++) {
    pinMode(ledPin[x], OUTPUT);
  }
  timeChanged = millis();
}

void loop() {
  long duration, cur_time, cm;

  cur_time = millis();

  /* Read distance when off, or when some time passed */
  if (off || (!off && (cur_time - timeOfQuery) > 100)) {
    timeOfQuery = cur_time;

    cm = sonar.ping_cm();
    if ( cm == 0 ) {
      cm = 400;
    }


    distances[(++currentDistance) % 7] = cm;
    // At least 1 proper distance?
    off = 1;
    for (int i = 0; i < 7; i++) {
      if (distances[i] <= 200) {
        off = 0;
        // Remember minimum distance
        if (distances[i] < distance) {
          distance = distances[i];

        }
      }
    }
    delayTime = distance;
  }

  if (!off) {
    // Check whether it has been long enough
    if ((cur_time - timeChanged) > delayTime) {

      // Turn off all of the LEDs
      for (int x = 0; x < 8; x++) {
        digitalWrite(ledPin[x], LOW);
      }

      // Turn on the current LED
      digitalWrite(ledPin[currentLED], HIGH);

      // Mark that diodes are being used
      leds_used = 1;

      // Increment by the direction value
      currentLED += dir;

      // If we are at the end of a row, change direction
      if (currentLED == 7) {
        dir = -1;
      }
      if (currentLED == 0) {
        dir = 1;
      }

      // Store the current time as the time we last changed LEDs
      timeChanged = cur_time;
    }

    // Shorter delay when sleeping
    delay(5);
    LowPower.powerDown(SLEEP_30MS, ADC_OFF, BOD_OFF);
    //delay(50);
  } else {
    if (leds_used) {
      // Turn off all of the LEDs
      for (int x = 0; x < 8; x++) {
        digitalWrite(ledPin[x], LOW);
      }
      currentLED = 0;
      dir = 1;
      leds_used = 0;

      // Store the current time as the time we last changed LEDs
      timeChanged = cur_time;
    }

    // Longer delay when sleeping
    delay(10);
    LowPower.powerDown(SLEEP_250MS, ADC_OFF, BOD_OFF);
    //delay(200);
  }

  /*
    if (distance >= 200 || distance <= 0) {
      Serial.print(distance);
      Serial.println(" Out of range");
    }
    else {
      Serial.print(distance);
      Serial.println(" cm");
    }
  */
}

Update: The board is Arduino UNO.

Answer 1

The Arduino UNO always consumes current mainly because:

• it has 2 MCU, the ATmega328 which you can power down, and an ATmega16u2 for USB communication which is never powered down.
• in addition, the voltage regulator is permanently leaking some current

To be honest, I am surprised your UNO consumes only 15mA, on my boards, it is rather near 40-50mA.

Also, a 9V battery is probably the battery with the smallest charge capacity.

I would suggest you:

1. work on a breadboard with just the ATmega328,
2. power it with two 1.5V batteries (AA or AAA)
3. and run @8MHz

That should already reduce current consumption.

You may even try smaller MCU (e.g. ATtiny84A) which consumes less current than the ATmega328 in the same conditions.

Finally, I highly recommend Nick Gammon's blog on that topic. It includes many good tricks to help you with current consumption.