47. Deep Sleep and Power Management

Project: Battery pulse logger

Field nodes often wake briefly, take one reading, log or publish, then sleep for minutes. You will build that pulse pattern with a potentiometer as the sensor stand-in.

Sketch 1 proves timer wake + RTC boot count. Sketch 2 is the logger: wake, read GPIO 34, print Boot/ADC/wake cause, sleep 5 s again.

Deep sleep and wake sources

Deep sleep loses normal SRAM. Timer wake is the logger clock. EXT0/EXT1 and touch are other wake sources for later (button to wake). Always print esp_sleep_get_wakeup_cause() while learning.

RTC memory versus NVS

RTC_DATA_ATTR keeps bootCount across sleeps until USB power is removed. Credentials and durable settings belong in Preferences / NVS (lesson 45 ideas), not RTC alone.

Honest current measurement

DevKit USB-UART chips and LEDs often dominate a USB current meter. Expect a qualitative drop in class, not a datasheet microamp figure.

Design pattern for a battery node

Wake -> sense quickly -> connect WiFi only if publishing -> sleep. This lesson stops before WiFi so the sleep/sample rhythm stays clear. Adding a publish step is a natural challenge after the logger works.

Wiring and safe build sequence

1. Sketch 1: USB only
2. Sketch 2: pot 3V3 -> one end, GND -> other end, wiper -> GPIO 34
3. Serial Monitor 115200 before reset to catch boot lines

What this sketch is for: Prove deep sleep: every ~5 s the board restarts setup(), bootCount rises, and wake cause becomes timer.

#include <esp_sleep.h>

RTC_DATA_ATTR int bootCount = 0;

void printWakeCause() {
  esp_sleep_wakeup_cause_t cause = esp_sleep_get_wakeup_cause();
  switch (cause) {
    case ESP_SLEEP_WAKEUP_TIMER:
      Serial.println("Wake cause: timer");
      break;
    case ESP_SLEEP_WAKEUP_EXT0:
      Serial.println("Wake cause: EXT0");
      break;
    case ESP_SLEEP_WAKEUP_EXT1:
      Serial.println("Wake cause: EXT1");
      break;
    default:
      Serial.println("Wake cause: power-on / reset / other");
      break;
  }
}

void setup() {
  Serial.begin(115200);
  delay(300);

  bootCount++;
  Serial.print("Boot #");
  Serial.println(bootCount);
  printWakeCause();
  Serial.println("Sleeping 5 seconds...");

  esp_sleep_enable_timer_wakeup(5ULL * 1000000ULL);
  esp_deep_sleep_start();
}

void loop() {
}

What this sketch is for: Useful logger behaviour: each wake reads the pot on GPIO 34, prints Boot, ADC and wake cause, then sleeps again. Turn the pot between wakes to see changing samples. Match the breadboard layout below before upload.

#include <esp_sleep.h>

RTC_DATA_ATTR int bootCount = 0;
const int sensePin = 34;

void printWakeCause() {
  esp_sleep_wakeup_cause_t cause = esp_sleep_get_wakeup_cause();
  if (cause == ESP_SLEEP_WAKEUP_TIMER) Serial.println("Wake cause: timer");
  else Serial.println("Wake cause: power-on / reset / other");
}

void setup() {
  Serial.begin(115200);
  delay(300);

  bootCount++;
  int sample = analogRead(sensePin);

  Serial.print("Boot #");
  Serial.print(bootCount);
  Serial.print("  ADC ");
  Serial.println(sample);
  printWakeCause();
  Serial.println("Pulse logged - sleeping 5 s");

  esp_sleep_enable_timer_wakeup(5ULL * 1000000ULL);
  esp_deep_sleep_start();
}

void loop() {
}