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Bathroom humidity fan controller

Switches a fan on when the air gets damp, using dew point instead of a fixed humidity level. IntermediateWi-FiESP32ESPHome

Updated November 2026

Illustration of what you build. See the maker’s guide for photos of the real device.

Can I build this?

Estimated cost€15–40 (indicative)

Build timeNot stated

DifficultyIntermediate

ToolsComputer to flash ESPHome; multimeter recommended

SolderingNot stated

3D printingNot stated

What you need

ESP32 board
Two BME temperature and humidity sensors (one inside, one outside; a DHT22 also works)
5 V relay
12 V 2 A power supply
12 V to 5 V step-down converter
12 V PC fan

How it works

Board: ESP32 (ESP32-WROOM)
Connects via: Wi-Fi with ESPHome
Made by: Julius (jwmw), Home Assistant Community

Good to know

It compares the dew point inside and outside, so the fan only runs when it actually helps dry the room.
The build uses a 12 V fan. To switch a built-in mains extractor fan, use a certified relay module and have it installed by a qualified electrician.

Build itBuild post on the Home Assistant CommunityFull instructions, wiring and firmware from the maker →

This page summarises the maker’s project; the maker’s own guide is leading. Missing a tool? See In the toolbox. Need a better casing? See 3D printing & enclosures. Keep DIY builds to low voltage (USB or battery).

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