To be able to switch certain relays in Home Assistant via an ESP32, I tested a relay board and integrated it via ESP-Home.

Necessary hardware

Besides an appropriate hardware for Home Assistant, I married an ESP32 with a 4-channel relay board: 

• ESP32 Dev Kit C V4 NodeMCU WLAN WiFi Development Board
  
  on amazon.com:
  
  HiLetgo ESP-WROOM-32 ESP32 ESP-32S Devel...
  

  Availability: Now
  Price: $9.99
  as of: 2025-05-09 03:50 Details

  [Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on amazon.com at the time of purchase will apply to the purchase of this product.]
• 4-Channel Relay Board
  on amazon.com:
  
  SunFounder Lab 4 Relay Module 5V 4 Chann...
  

  Availability: Now
  Price: $7.99
  as of: 2025-05-09 00:52 Details

  [Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on amazon.com at the time of purchase will apply to the purchase of this product.]

Wiring

As GPIO-PINs I used the PINS G25,26,32 and G33 for testing:

ESP-Home

If you already have the ESP32 connected to Home Assistant, you can add the relay board with the following lines:

...
switch:
  - platform: gpio
    pin: GPIO32
    name: Relay1
    id: relay1
  - platform: gpio
    pin: GPIO33
    name: Relay2
    id: relay2
  - platform: gpio
    pin: GPIO25
    name: Relay3
    id: relay3
  - platform: gpio
    pin: GPIO26
    name: Relay4
    id: relay4

This will make the relays show up as "controls" in Home Assistant.

Use in practice: Heating control

Control heating with Home Assistant