Food Temperature Sensor — DIY Build Guide¶

A step-by-step guide to building a freezer-grade temperature monitor for the garage fridge and freezers using an ESPHome microcontroller + waterproof DS18B20 probes. One small board monitors all three appliances, runs on USB power (no batteries to die in the cold), stays fully local, and drops straight into the existing Food Temperature alerts, dashboard tiles, and Grafana panel.

How it works¶

   GARAGE WALL OUTLET                      APPLIANCE
   ┌──────────────┐                ┌───────────────────────┐
   │  5V USB       │   USB cable    │  (cold interior)      │
   │  charger      ├───────────────►│                       │
   └──────────────┘   ┌─────────┐   │   ┌───────────────┐   │
                      │ ESP32 / │   │   │ DS18B20 probe │   │
                      │ D1 mini ├───┼──►│ in water/glycol│  │
                      │(ESPHome)│   │   │ bottle (reads  │   │
                      └────┬────┘   │   │ food, not air) │   │
                           │        │   └───────────────┘   │
                    WiFi (local API)│   cable out through    │
                           │        │   the door gasket      │
                           ▼        └───────────────────────┘
                   Home Assistant
              (ESPHome integration, auto-discovered)

• The DS18B20 is a 1-Wire digital sensor — several probes share one data pin, each with a unique 64-bit address. So one board monitors the fridge + fridge-freezer + deep freezer.
• ESPHome talks to Home Assistant over its local encrypted API — no cloud, sub-minute updates.
• USB-powered → no battery to die in the cold and nothing to maintain.

Bill of materials¶

Total ≈ $20–30 for all three appliances. A solder-free DS18B20 adapter module (resistor pre-mounted) is the easiest route.

Before you start¶

ESPHome is already installed in this setup (it runs the garage door openers), so no new add-on is needed.

Step 1 — Wire it up¶

DS18B20 waterproof probe leads (colors vary by vendor — verify before connecting):

• Place one 4.7 kΩ resistor between the Data line and 3.3 V (the bus pull-up). Just one, even with three probes.
• For multiple probes, join all Red → 3.3 V, all Black → GND, all Yellow → GPIO4 (shared 1-Wire bus):

 3.3V ──┬───────────────┬──────────── red (all probes)
        │              [4.7kΩ]
        │               │
 GPIO4 ─┴───────────────┴──────────── yellow/data (all probes)

 GND ───────────────────────────────── black (all probes)

Power the board from the 5 V USB port; the probes run off the board's 3.3 V pin.

Step 2 — Create the device and discover probe addresses¶

In Home Assistant → ESPHome → New Device, name it garage-food-temp. Start with this config (addresses left out so the probes can be discovered):

substitutions:
  name: garage-food-temp
  friendly_name: Garage Food Temp

esphome:
  name: ${name}
  friendly_name: ${friendly_name}

esp32:
  board: esp32dev          # for a Wemos D1 mini, replace this block with:
  framework:               #   esp8266:
    type: esp-idf          #     board: d1_mini

logger:                    # required to read probe addresses on first boot

api:
  encryption:
    key: !secret garage_food_temp_api_key

ota:
  - platform: esphome
    password: !secret ota_password

wifi:
  ssid: !secret wifi_ssid
  password: !secret wifi_password
  ap:
    ssid: "Garage-Food-Temp Fallback"

captive_portal:

one_wire:
  - platform: gpio
    pin: GPIO4

Flash it (USB the first time, OTA after), then open the device logs. ESPHome lists every probe it finds:

[one_wire]: Found sensors:
[one_wire]:   0x1c00000abcdef028
[one_wire]:   0x3b00000123456710
[one_wire]:   0x7a000009876543aa

Copy those addresses.

Step 3 — Final config (3 probes, °F output, health signal)¶

Keep the esphome / esp32 / logger / api / ota / wifi / captive_portal blocks from Step 2, then replace the one_wire section with:

one_wire:
  - platform: gpio
    pin: GPIO4

sensor:
  - platform: dallas_temp
    address: 0x1c00000abcdef028     # deep freezer probe
    name: "Deep Freezer"
    unit_of_measurement: "°F"
    accuracy_decimals: 1
    update_interval: 60s
    filters:
      - lambda: return x * 9.0 / 5.0 + 32.0;   # DS18B20 reads °C → output °F

  - platform: dallas_temp
    address: 0x3b00000123456710     # fridge-freezer probe
    name: "Fridge Freezer"
    unit_of_measurement: "°F"
    accuracy_decimals: 1
    update_interval: 60s
    filters:
      - lambda: return x * 9.0 / 5.0 + 32.0;

  - platform: dallas_temp
    address: 0x7a000009876543aa     # refrigerator probe
    name: "Refrigerator"
    unit_of_measurement: "°F"
    accuracy_decimals: 1
    update_interval: 60s
    filters:
      - lambda: return x * 9.0 / 5.0 + 32.0;

  - platform: wifi_signal
    name: "Garage Food Temp WiFi"
    update_interval: 120s

binary_sensor:
  - platform: status
    name: "Garage Food Temp Status"   # online/offline health signal

Outputting °F natively (the lambda filter) makes the new entities match the existing sensors exactly, so the alert thresholds and dashboard tiles need no changes.

Step 4 — Calibrate (optional)¶

DS18B20s are usually accurate out of the box, but you can verify with an ice-water bath: fill a cup with crushed ice and a little water, stir, submerge the probe for ~30 s. It should read 32 °F / 0 °C. To correct any offset, add to that probe's filters:

    filters:
      - lambda: return x * 9.0 / 5.0 + 32.0;
      - offset: 0.4        # °F — nudge until it reads 32.0 in ice water

Step 5 — Physical install¶

• Cable routing: run the thin cable out through the door gasket — fridge/freezer seals close around a thin wire without breaking the seal.
• Mount the board on top of or behind the appliance, and add strain relief so the cable can't pull the probe out of the bottle.

Step 6 — Cut over in Home Assistant¶

The Food Temperature alerts, the Climate-tab tiles, and the Grafana Garage Food Temperature panel all key off these entity IDs:

• sensor.freezer_temperature_sensor_temperature — deep freezer
• sensor.refrigerator_freezer_temperature_sensor_temperature — fridge-freezer
• sensor.refrigerator_temperature_sensor_temperature — fridge

To swap with zero edits to automations or tiles, give the new ESPHome entities those same IDs:

1. Confirm the new entities report sane values (e.g. sensor.garage_food_temp_deep_freezer).
2. Remove or disable the old ThirdReality device in Settings → Devices (this frees up the old entity IDs).
3. For each new entity: Settings → Entities → (entity) → ⚙ → Entity ID → rename it to the matching ID above.
4. Refresh the test fixture so CI stays green:

   make snapshot     # regenerates tests/fixtures/entities.txt (needs HA_URL + HA_TOKEN)
   git add tests/fixtures/entities.txt
   

   If you pair the board before renaming, temporarily list the new IDs in tests/fixtures/entities_allow.txt.

Troubleshooting¶

Summary¶

• ≈ $20–30, about 30–45 minutes of work (less with a solder-free DS18B20 module).
• No batteries, fully local, freezer-accurate, and one board covers all three appliances.

Related pages: Climate & Comfort · Automations Reference · Grafana Analytics