An ESP32 temperature sensor in an afternoon

A temperature sensor is the best first project in smart-home electronics. The parts are cheap, the wiring is simple and you learn the whole path from a sensor to a dashboard.

Here is how I would build one with ESPHome. It turns a short YAML file into firmware for the ESP32, so you don’t need to write any C++.

• an ESP32 development board (any common one will do)
• a DS18B20 temperature sensor (the waterproof probe is handy)
• a 4.7 kΩ resistor, unless your sensor board already has one
• a USB cable and ESPHome, either as the Home Assistant add-on or on your computer

The DS18B20 has three wires:

1. Red to 3.3 V
2. Black to GND
3. Data (usually yellow or white) to GPIO4

Then fit the 4.7 kΩ resistor between the data wire and 3.3 V. This pull-up keeps the data line high while nothing is talking. Without it you get no readings, or strange ones.

In ESPHome, choose New device, give it a name, for example Room sensor, and pick ESP32. ESPHome writes a starter file for you. It already has the board, your Wi-Fi details, an encryption key for Home Assistant and a password for updates over Wi-Fi. Keep all of that and add these lines at the end:

one_wire:
  - platform: gpio
    pin: GPIO4

sensor:
  - platform: dallas_temp
    name: Temperature
    update_interval: 60s

The one_wire part tells the ESP32 which pin the data wire is on. The dallas_temp sensor reads the DS18B20 on that wire once a minute. With one sensor on the wire, ESPHome finds its address by itself. With several, give each one an address.

The first upload goes over USB. Plug in the board, choose Install and pick the USB option. After that, every update can go over Wi-Fi.

Open the logs. Once a minute you should see a line with the temperature. Hold the probe in your hand and watch the number climb.

Home Assistant usually finds the new device on its own. Open Settings → Devices & services, where it waits under Discovered. Add it and, if you are asked, paste the encryption key from the device file (it sits under api:). The reading then appears as sensor.room_sensor_temperature.

Now it’s an entity like any other. Put it on a dashboard, draw its history or use it in an automation.

• Keep the probe away from the board. The ESP32 gets warm and will push the reading up.
• Once a minute is plenty for a room. Faster updates only fill the history.
• A DS18B20 is accurate to about ±0.5 °C. If you want humidity and air pressure too, a BME280 is a good next step.

That is the whole path: a sensor, a small computer, a network and a dashboard. Once it works, every other sensor is a variation on the same idea.