Build a battery-powered mailbox notifier with an ESP32

Get a phone notification the moment the mail arrives, from a sensor that runs on a small battery for a year or more. The trick is deep sleep: the ESP32 does nothing at all until the lid opens.

How it works

Most of the time the ESP32 is in deep sleep, drawing about 10 µA on a good low-power board. A magnetic reed switch on the lid is wired to a wake-up pin. When the lid opens, the switch closes, the ESP32 wakes, joins Wi-Fi, tells Home Assistant the mailbox was opened, and goes back to sleep after 20 seconds.

Because the board only ever wakes when the lid opens, every wake-up means the mailbox was opened. Fitting a fresh battery also counts once, so expect one false alert then.

Check your Wi-Fi first

Stand at the mailbox with your phone and look at the Wi-Fi signal. If it's weak or drops out, fix that first, for example with an outdoor access point or a mesh node in a window facing the street. A metal mailbox blocks Wi-Fi, so on a metal box mount the electronics in a plastic box under or beside it, with only the reed switch inside.

Parts

• A low-power ESP32 board, such as the DFRobot FireBeetle ESP32. Ordinary DevKits waste several milliamps in sleep through their USB chip and regulator, which flattens a battery in days. Tests of the FireBeetle measured about 10 µA in deep sleep.
• A 3.7 V LiPo battery, 1000–2000 mAh, with a JST-PH plug to fit the FireBeetle's battery socket and charger.
• A reed switch that's open while the magnet is next to it, sold as normally closed (NC) or "Form B", plus a magnet. With this type, no current flows while the lid is shut.
• Two 100 kΩ resistors to measure the battery (skip these if your board has a built-in battery divider), and one 1 MΩ resistor.
• A small weatherproof plastic enclosure.

• FireBeetle ESP32 (low-power board)
• 3.7 V LiPo battery, JST-PH plug
• 
  NC reed switch (open with magnet)Photo: André Karwath, CC BY-SA 2.5
• 
  Resistor assortmentView on AmazonPhoto: Afrank99, CC BY-SA 2.5
• Plastic project enclosure

Wiring

Mount the magnet on the lid and the reed switch on the box, so they line up within a centimeter when the lid is shut. The wake-up pin must be an RTC pin: on a classic ESP32 that's GPIO0, 2, 4, 12–15, 25–27 or 32–39. GPIO14 is a good choice.

Build this config in the generator Pick your board, adjust pins, and copy a ready-to-flash config with fresh keys.

The ESPHome config

esphome:
  on_boot:
    priority: -100
    then:
      # The board only wakes when the lid opens, so every boot means mail
      - wait_until:
          condition:
            api.connected:
          timeout: 15s
      - binary_sensor.template.publish:
          id: mail_event
          state: ON
      - delay: 3s
      - binary_sensor.template.publish:
          id: mail_event
          state: OFF

deep_sleep:
  id: mailbox_sleep
  wakeup_pin:
    number: GPIO14
    mode:
      input: true
      pulldown: false
  wakeup_pin_mode: KEEP_AWAKE
  run_duration: 20s

binary_sensor:
  - platform: template
    id: mail_event
    name: "Mailbox opened"
    device_class: opening

sensor:
  - platform: adc
    pin: GPIO35
    id: battery_v
    name: "Battery voltage"
    attenuation: 12db
    update_interval: 60s
    filters:
      - multiply: 2.0

KEEP_AWAKE handles the lid being left open: instead of waking straight back up in a loop, the board stays awake until the lid closes, then sleeps.

Battery life

Asleep at 10 µA, the board uses about 0.24 mAh a day. Each wake-up costs roughly 0.5 mAh, mostly from Wi-Fi. At four openings a day that's about 2.5 mAh a day, so a 2000 mAh battery lasts well over a year on paper. Cold weather and the battery's own self-discharge cut into that, so expect around a year in practice. LiPo batteries mustn't be charged below freezing, so bring it indoors to recharge in winter.

Home Assistant automations

"You've got mail", once

The mailbox opens twice a day: once for the delivery and once when you collect it. Track which is which with a helper:

1. Create a toggle helper called Mail waiting.
2. When Mailbox opened turns on and Mail waiting is off, send "You've got mail" and turn Mail waiting on.
3. When Mailbox opened turns on and Mail waiting is on, it's you collecting it, so turn Mail waiting off.

Show Mail waiting on a dashboard or a light near the door.

Low battery

Notify when Battery voltage drops below 3.5 V, which leaves a few weeks to recharge.