htu21

htu21 Small HTTP service targeting the Raspberry Pi that reads temperature and humidity from an GY-21/HTU21 board with an SHT21 sensor over I²C and makes it available on the network by returning JSON to a simple HTTP API.

Default port: 273 (configurable via HTU21_PORT environment variable).

• Endpoints:
  • GET / → {"temperature_c":..,"humidity_percent":..,"timestamp_utc":".."}
  • GET /health → {"status":"ok"}

Prerequisites

• Raspberry Pi with I²C enabled: (sudo raspi-config → Interface Options → I2C → Enable)
• Check your user is in the i2c group.
• Wiring:
  • VCC → 3.3V (pin 1)
  • GND → GND (pin 9)
  • SDA → GPIO2 (pin 3)
  • SCL → GPIO3 (pin 5)

---

Quick run (no install)

1. Download and extract the release tarball:

   tar xzf htu21-linux-arm.tar.gz
   cd htu21-linux-arm

2. Run:

   sudo ./htu21             # listens on port 273 by default

   Optional: allow binding to ports <1024 without sudo

   sudo setcap 'cap_net_bind_service=+ep' ./htu21
   ./htu21

   Or change the port:

   HTU21_PORT=8080 ./htu21

3. Test:

   curl http://localhost:273/
   curl http://localhost:273/health

---

Install as a systemd service

The service will be installed to /opt/htu21 and run as a dedicated htu21 system user.

cd /tmp
tar xzf htu21-linux-arm.tar.gz
cd htu21-linux-arm
./install.sh

The installer:

• Creates /opt/htu21/ directory with the binary
• Adds htu21 system user for running the service
• Adds systemd service at /etc/systemd/system/htu21.service

Check status and logs:

systemctl status htu21
journalctl -u htu21 -f

---

Change the port

1. Edit the service (recommended)

sudo nano /etc/systemd/system/htu21.service
# in the [Service] section, set:
Environment=HTU21_PORT=273

Reload and restart:

sudo systemctl daemon-reload
sudo systemctl restart htu21

---

Uninstall

cd htu21-linux-arm
./uninstall.sh

---

JSON examples

curl -s http://localhost:273/ | jq .
# {
#   "sensor": "HTU21/SHT21",
#   "temperature_c": 21.37,
#   "humidity_percent": 48.12,
#   "timestamp_utc": "2025-09-20T00:00:00.0000000Z"
# }

curl -s http://localhost:273/health
# {"status":"ok"}