Tutorial ISM sensors

Tutorial: ISM sensors

What it is. Weather stations, thermometers, tyre pressure sensors (TPMS), remote controls, doorbells and energy, water and gas meters transmit short packets on the licence-free bands at 433, 868 and 915 MHz. A software-defined radio can read many of them; this is the idea of the well-known rtl_433 tool. OnAir lists the devices around you with their readings and history, and adds views for Z-Wave and Sigfox.

Read this first. These are other people's devices, and your neighbours' weather station or car tyre sensor can identify them. OnAir shows only what a device broadcasts openly (model, the ID it sends, its readings). Do not publish IDs or readings that identify a person's home or car. Receive only; Z-Wave S0 and S2 traffic stays encrypted and is never decrypted. Check your local rules. See Legal and Privacy.

What you need

• A radio at 2 Msps (works from 1 Msps). An RTL-SDR is the classic choice. The mode table covers 300 to 930 MHz, channel width 1.6 MHz.
• An antenna for the band: a quarter-wave at 433 MHz or 868/915 MHz works well.
• Frequencies from the database: 433.92 MHz (start; sensors, remotes), 868.3 MHz (EU 868.0 to 868.6 MHz sensor band), 868.95 MHz (wireless M-Bus mode T and C meters), 915 MHz (North America and Australia), 315 MHz (North America and Asia: garage doors, car keys, TPMS), 345 MHz (security sensors) and 169.4125 MHz (wM-Bus N-mode).

Before you start

Sensors transmit rarely, some only once a minute or when something changes. Use a moderate gain; strong remote controls can swamp a cheap front end. A good location (window, outdoors) matters more than gain.

Step by step

1. In the mode list open IoT & sensors and choose Sensors, 433 / 868 MHz.
2. Pick the radio and start the receiver.
3. Under Band press 433.92, 868.3, 868.95 or 915. The tooltips name typical devices for each.
4. For Z-Wave use the Z-Wave menu (frequencies by region, for example Europe 868.40 and 869.85 MHz, USA / Canada 908.40 MHz). For Sigfox use the Sigfox menu: RC1 Europe 868.130, RC2 USA 902.200 and RC3 / RC4 920.800 MHz.
5. Use the kind filter to show All, Weather, TPMS, Remote, Energy, Meter, Sensor or Z-Wave devices.
6. Watch the Devices tab.

What you should see

Devices appear as model plus ID with a history of their readings (temperature, humidity, wind, pressure and so on), the signal level and when they were last heard.

Reading the screen

• Devices: the device table with history.
• Log: a live list of decoded messages.
• Unknown: bursts that no protocol accepted, with their pulses. The line code is a guess from pulse and gap lengths. Copy pulses copies them, which helps if you want to report a new device.
• Supported: the devices the decoder knows. The formats follow rtl_433 (GPL-2.0); each decoder names the rtl_433 device file it was written from.
• Z-Wave: frames grouped by Home ID (ITU-T G.9959 R1 9.6 kbit/s, R2 40 kbit/s, R3 100 kbit/s) with nodes and command classes; passive only, S0 and S2 payloads stay encrypted.
• Sigfox: uplinks (ultra-narrowband DBPSK, 100 bit/s) within 192 kHz of the dial; passive only, payloads are not decoded.

Options and settings

Clear forgets the devices. Copy copies the table.

Troubleshooting

• Nothing appears. Wait several minutes; many sensors send every 30 to 60 seconds or less often. Check the band button.
• Only Unknown bursts. The device is not supported yet; copy the pulses and report it.
• Z-Wave quiet. Tune to your region's channel first.
• Same device twice with different IDs. Some devices reset their ID on battery change.
• Strong interference. Lower the gain.

Tips

Use the kind filter to find your own sensors first, so you know what a good decode looks like.

Legal and privacy notes

Passive only. The project does not decrypt Z-Wave or Sigfox payloads. Respect neighbours' privacy.

Under the hood

Pulse-width, pulse-position and Manchester OOK/FSK decoders for many models; wireless M-Bus headers (EN 13757-4); Z-Wave per ITU-T G.9959.

Related

Mode-ISM-sensors, Tutorial-LoRaWAN, Tutorial-LoRa-raw, Tutorial-ANTplus-sensors, Tutorial-Zigbee-802-15-4, Modes, Main-window, Supported-radios, Troubleshooting, FAQ, Legal.