izar2mqtt

Read Diehl IZAR RC 868 water meters with a cheap RTL-SDR and publish them to Home Assistant via MQTT auto-discovery — including a derived flow-rate (L/min) sensor. No cloud, no meter key required.

IZAR uses Diehl's PRIOS protocol with LFSR obfuscation (not AES), so meters can be decoded with NOKEY — no per-meter key from your utility needed.

This is a thin bridge on top of the excellent wmbusmeters and rtl-wmbus projects — see Credits. It does not reimplement any decoding; it turns wmbusmeters' JSON into Home Assistant MQTT-discovery entities and adds a computed flow sensor.

How it works

 RTL-SDR (868 MHz)                 this host (SDR)                 Home Assistant
┌────────────┐   I/Q   ┌───────────────────────────────┐  MQTT  ┌──────────────┐
│  dongle    │────────▶│ rtl_sdr | rtl_wmbus            │───────▶│ auto-discovery│
│            │         │   → wmbusmeters (JSON, stdout) │        │  devices +   │
└────────────┘         │   → wmbus-ha-bridge.py         │        │  sensors     │
                       └───────────────────────────────┘        └──────────────┘

Why MQTT and not a native HA integration? In the typical setup the RTL-SDR lives on a Linux box while Home Assistant runs as a separate HAOS appliance — so the radio data has to cross machines. MQTT auto-discovery is the natural, low-maintenance decoupling layer, and the entities still look fully native in HA. (If your dongle is on the HA host itself, look at the wmbusmeters HAOS add-on instead.)

Entities per meter

One HA device per received meter id, with sensors:

Sensor	Unit	Notes
Total consumption	m³	device_class: water, total_increasing (Energy/water dashboard ready)
Flow	L/min	derived, see below
Last month total	m³	meter's stored month boundary value
Alarms	—	diagnostic
Signal	dBm	diagnostic
Battery remaining	a (years)	diagnostic
Transmit period	s	diagnostic (8 = R3 mode)

The flow sensor (honest caveats)

The meter only transmits a cumulative total at ~1 litre resolution, so flow is computed by the bridge as Δlitres / Δtime between two telegrams:

• Quantised. With 1 L resolution and an ~8 s transmit interval, a running tap shows up in ~7.5 L/min steps (1 L per 8 s). Good for "is water flowing and roughly how much", not for precise instantaneous readings.
• Duplicate telegrams (same frame received twice, Δt≈0) are filtered.
• Flow is sent in every state update (falls back to 0.0 within ~8 s when the meter stops counting) so the sensor never flaps to unavailable.

If you want it smoother, add a Home Assistant filter sensor (moving average) on top.

Hardware

• Any RTL2832U RTL-SDR dongle. A meter transmits on 868.95 MHz (IZAR "R3", ~8 s) and/or 868.30 MHz ("R4", ~15 min).
• Tuner note: weak tuners such as the Fitipower FC0012 (max gain ~19 dB) receive 868 MHz poorly. The big lever is running rtl_wmbus without -a (accurate atan demodulation) and with -o (DC-offset removal) — that took the decode rate from ~0% to ~50% on an FC0012 here. A R820T2 tuner is much stronger and works with default flags and higher gain.

Install

git clone https://github.com/deviant-aut/izar2mqtt.git
cd izar2mqtt
sudo ./install.sh

install.sh installs dependencies, builds rtl_wmbus and wmbusmeters from source (neither is packaged in apt), and installs the bridge, example config and a systemd service. Then:

1. Edit /etc/wmbusmeters.conf — RTL device string, frequency, gain, tuner flags.
2. Edit the MQTT_* lines in /etc/systemd/system/wmbusmeters-izar2mqtt.service.
3. Check the dongle: rtl_test -t
4. Start it:

   sudo systemctl daemon-reload
   sudo systemctl enable --now wmbusmeters-izar2mqtt.service
   sudo journalctl -u wmbusmeters-izar2mqtt -f

Meters appear in Settings → Devices → MQTT in Home Assistant. Any IZAR meter in range (including neighbours') is picked up automatically via the id=* wildcard — just delete the ones you don't want in HA.

Configuration (environment variables)

Set in the systemd unit:

Variable	Default	Meaning
MQTT_HOST / MQTT_PORT	localhost / 1883	MQTT broker
MQTT_USER / MQTT_PASS	mqtt / changeme	broker credentials
HA_DISCOVERY_PREFIX	homeassistant	HA discovery prefix
WMBUS_LABELS	{}	JSON map id → name suffix, e.g. {"12345678":"home"} → device watermeter 12345678 (home)

MQTT topics

• Discovery: homeassistant/sensor/wmbus_izar_<id>/<key>/config (retained)
• State: wmbusmeters/izar/<id> (retained JSON, all fields + flow_l_min)

Credits

This project stands entirely on the shoulders of others:

• wmbusmeters by Fredrik Öhrström — the wM-Bus decoder incl. the IZAR/PRIOS driver (GPLv3).
• rtl-wmbus by xaelsouth — the software T1/C1/S1 receiver that turns RTL-SDR I/Q into wM-Bus frames.
• rtl-sdr / librtlsdr by Osmocom — the SDR driver and rtl_sdr / rtl_test tools.
• IZAR PRIOS / LFSR reverse engineering by Erwan Martin — the write-up that made keyless IZAR decoding possible.
• ha-watermeter by zibous — a reference for RTL-SDR + wmbusmeters + Home Assistant setups.
• Home Assistant MQTT discovery.

"IZAR" and "PRIOS" are trademarks of Diehl Metering. This is an independent, non-commercial project and is not affiliated with or endorsed by Diehl; the names are used only to describe the compatible meters.

Only the glue in this repository (wmbus-ha-bridge.py, config, service, installer) is original work.

License

MIT for the code in this repository. The upstream tools it builds on have their own licenses (wmbusmeters is GPLv3) — they are fetched/built by install.sh, not redistributed here.