The Raspberry Pi companion: decodes sensors, follows trunked radio and names who's transmitting

2 Nov 2026 · 11 min read · Shop · Full Spectrum · Raspberry Pi · RTL-SDR · Trunking

Spotter extends the phone's own radios. The Pi companion adds radios the phone doesn't have. An RTL-SDR dongle tunes anything from 24 MHz to 1.7 GHz, which covers the bands where cars, buildings and two-way radios actually talk. The Raspberry Pi runs the software that turns those samples into named devices and audio, and hands it all to Wavelens over Bluetooth.

What the Pi arrives with

No longer sold on its own: the Pi comes in Full Spectrum, with its two RTL-SDRs and a Spotter.

• A Raspberry Pi 4 (4 GB), USB-C powered, in a vented case, with a 32 GB endurance microSD card already imaged with all the Wavelens software.
• Updates are free for life. The Pi talks to your phone over Bluetooth; join its Wi-Fi to your home network once and update.sh pulls the latest bridge in one command.
• Raspberry Pi OS Lite, the Wavelens bridge as a service that starts at boot, rtl_433, rtl_power, rtl_fm, and dsd-fme built from source for digital voice. Building dsd-fme is the slowest part of a home install, about forty minutes on a Pi 4; it's done.
• Each dongle's role pinned by serial number, so it survives a reboot in a different USB order.
• The local pack for the area you gave at checkout installed in /etc/wavelens/packs/, and tested with the phone-side pack.
• A USB-C power lead. You supply 5 V 3 A: a decent car adapter, or the buck converter described in the car install guide.

Two 6-inch USB extension cables are in the box, one per dongle, so each RTL-SDR sits clear of the Pi. Full Spectrum includes two RTL-SDRs, each pre-configured and with the antenna for its job. For a Pi you built yourself, they're $40 each.

What each dongle does

The bridge gives each dongle a role. With one dongle you choose; with two you get both at once.

Dongle A: sensors

rtl_433 parks on 315, 433 and 915 MHz and decodes a few hundred device types by name: Schrader and Continental tyre-pressure sensors, key fobs, garage and gate remotes, Acurite and Oregon weather stations, door and window sensors, utility meters, LoRa telemetry. Each one has its own ID, so a car whose tyre sensors keep turning up near you is noticed the same way a phone that follows you is. It ships with a 17 cm whip tuned for 433 MHz on a small mag base; sensors are near-field, so it can sit on the dash.

Dongle B: the scanner

This is the one that does the Scanner tab. It needs a TCXO (temperature-stable crystal), because P25 wants the tuned frequency within a kilohertz and a cheap crystal drifts that far with the car's temperature swing; the dongles we ship have one. Its antenna is a wideband mobile scanner whip, 25–1300 MHz, on a mag mount for the roof. The stock stub that comes with a dongle is 15–20 dB down at 850 MHz, and that difference is whether trunking works.

With dongle B the bridge does, in turn or together:

• Sweep. rtl_power passes over VHF, UHF and 700/800 MHz and reports every carrier that stands out of the noise, labelled by band from the allocation table: "radio keying up nearby", "FRS/GMRS walkie-talkie", "marine VHF", "406 MHz distress beacon".
• Listen. Tap a carrier and rtl_fm demodulates it; the audio goes to the phone as ADPCM over Bluetooth LE, 40 ms frames. Analog FM, P25 Phase 1 and 2 and DMR all work, the digital ones through dsd-fme.
• Follow. Point it at a P25 or DMR control channel and it follows the system: each group grant switches the dongle to the voice channel for the call and back. Motorola Type II (SmartNet/SmartZone) has its own decoder in the bridge, smartnet.py, reading the 3600-baud control channel; no dsd-fme needed. Encrypted and private calls are never tuned.
• Names. With a RadioReference export loaded, every talkgroup gets its name. With the FCC licence pack for your county, a licensed carrier gets the licensee's name and the distance to their site: "county DOT, 4 km" instead of "UHF land mobile". How the exports work.

A third dongle: aircraft

Add a third ($40) and the bridge runs dump1090 on it: every aircraft overhead with callsign, altitude and position in the detail view. Unclassified on purpose; an airliner is not a drone.

What the phone sees

Everything the Pi decodes arrives in Wavelens as if the phone heard it: the same rules, confidence levels, alerts, history, map and Rule hits. A tyre sensor that keeps pace with you through turns gets the riding with you verdict. A carrier on the Scanner tab shows its band, its label and, when known, its owner; a tap listens. The companion screen shows each dongle's state and hit rate, and the phone can tell the bridge which band to park on or which pack to use.

In a car

The Pi wants clean power and air. Hard-wire a 5 V 3 A buck converter to an ignition-switched circuit via an add-a-fuse tap; cheap lighter plugs sag over bumps and reset it. Two dongles and Bluetooth is at the limit of what a Pi 4's own USB ports source, so a powered USB hub is in the car install guide and the full kit. Put each dongle on its own USB extension so it sits by its antenna and away from the Pi and each other; dongles run hot and pick up each other's oscillator noise side by side. The wiring diagram shows the whole thing.

The case we ship is vented. A Pi 4 throttles at 80 °C and a closed glovebox in a Florida July reaches 60 °C ambient; the $25 case with heatsinks and a fan at checkout is for that car.

What it doesn't do

An RTL-SDR tops out at 1.7 GHz, so no 2.4 GHz Wi-Fi, no Bluetooth and no 5.8 GHz video from the Pi itself; that's what pairing a Spotter as the second companion is for, and what the full kit includes. LTR trunking is unsupported (no control channel). Everything is receive-only: the bridge never transmits. The only link out is Bluetooth LE to your phone, and nothing it hears leaves the phone.

Or build it

The bridge is one Python script, open on GitHub, and the step-by-step Pi setup gets you the same thing with a Pi you own. Full Spectrum is for people who'd rather have the dsd-fme build, the serial pinning and the pack already done, with the dongles and a Spotter tested alongside.

Get Full Spectrum, $499

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