Apple HomeKit Temperature and Humidity Sensor

Building an Apple HomeKit integration for the DHT22 Temperature and Humidity sensor and a Raspberry Pi using Go. Connects over Wifi - no bridge required.

Description

There aren't many reliable thermometers/hygrometers (humidity sensor) available for HomeKit which do not require their own bridge. This project shows how to build one based on a Raspberry Pi and the DHT22 sensor which is commonly available. Since the Raspberry Pi is powered via DC, a wall converter or battery can power the system.

A simple transistor switch is used to power down the sensor when not in use to conserve power. The system first turns on the sensor and then waits to take a reading. Once a clean reading is taken, the sensor is powered off for an interval.

References

This project would not be possible without the prior work done in these fantastic Go packages:

go-dht - reading the DHT22 sensor

go-rpio - basic handling of GPIO pins

hap - development of HomeKit accessories

Configuration

A summary of the program flow:

• Set up the thermometer HomeKit accessory
• Set up the hygrometer HomeKit accessory
• Start a loop to retrieve the temperature and humdity from the sensor
  • Set the control pin to high, turning on the sensor
  • Wait 3 seconds for the sensor to stabilize
  • Read the sensor data until a valid reading is obtained
  • Set the control pin to low, turning off the sensor
  • Update the HomeKit accessories with the sensor data
  • Wait 5 minutes for the next reading
• Start the HomeKit server

Adding to HomeKit:

• Ensure the hktemphum service is running
• Ensure the Raspberry Pi is connected to Wifi
• Open Home on an iPhone or iPad
• Click the + and Add Accessory
• Choose More options...
• Select the thermometer1 accessory
• Input the pin defined in main.go
• Add the thermometer and hygrometer

Parts List

• 10k ohm resistor
• 2N2222 transistor
• DHT22 sensor package (pull down resistor included)
• Raspberry Pi Zero 2 W
• Blank PCB / Wire / Soldering Kit

Wiring Diagram

Raspberry Pi GPIO pinouts: https://pinout.xyz

Build

It is suggested to build the binary executable and then copy to the Raspberry Pi. For example, to build for Raspberry Pi on Apple silicon:

env GOOS=linux GOARCH=arm go build

Linux Service

After building, copy hktemphum to /opt/hktemphum

Edit /lib/systemd/system/hktemphum.service

[Unit]
Description=Temperature and Humidity HomeKit Integration
[Service]
Type=simple
Restart=always
RestartSec=60s
WorkingDirectory=/opt/hktemphum
ExecStart=/opt/hktemphum/hktemphum
[Install]
WantedBy=multi-user.target

Enable the service systemctl enable hktemphum

Start the service systemctl start hktemphum

Service status systemctl status hktemphum