Build a $25 analog scanner from an ESP32 and an SA818 module

31 Oct 2026 · 7 min read · ESP32 · SA818 · analog · companion

The Raspberry Pi companion is the one for digital trunking. But a lot of what's interesting on the air is plain narrow FM — marine channel 16, 2 m and 70 cm repeaters, business itinerants, fire-ground tactical channels in many counties, railroad — and for that a Pi is overkill. An ESP32 with an SA818 radio module does it on a fraction of the power, from a battery, in a box the size of a deck of cards.

Parts

• ESP32 dev board (any; the firmware is tested on a WROOM-32 and an S3).
• SA818-U (400–480 MHz) or SA818-V (134–174 MHz). It's a complete FM transceiver on a 1 W module, controlled by AT commands over serial. We only ever receive; the PTT pin stays high.
• Optional: an SX1276 or CC1101 module, which gives you an RSSI spectrum strip and sub-GHz sensor decoding (rtl_433-style, via rtl_433_ESP).
• An antenna for the band. A 70 cm whip on an SMA edge connector, or a mag-mount.

Wiring

SA818  TXD  → ESP32 GPIO16  (Serial1 RX)
SA818  RXD  → ESP32 GPIO17  (Serial1 TX)
SA818  SQ   → ESP32 GPIO4   (squelch open = low)
SA818  AF   → ESP32 GPIO36  (ADC1_CH0 / VP, through a 1 µF cap + 10k/10k bias to 1.65 V)
SA818  PTT  → 3.3 V         (never key up)
SA818  PD   → 3.3 V
SA818  VCC  → 3.3 V, 500 mA capable; GND common

The AF line is line-level audio centred near 1.5 V; the ESP32's ADC samples it at 8 kHz, encodes IMA ADPCM, and ships 40 ms frames over BLE — identical to the Pi, so the app doesn't care which it's talking to.

Firmware

It's companion/esp32/wavelens_companion/ in the repo, built with PlatformIO (pip install platformio; the platformio.ini is next to the code). A bare board with no module is pio run -e esp32 -t upload — the Wi-Fi/BLE sniffer, and the place to start. For the SA818, pick the esp32-scanner environment and set your module's pins if they differ from the defaults:

[env:esp32-scanner]
build_flags = -DHAS_SA818_U            ; pins default to TX 17, RX 16, SQ 4, AF 36 — override with -DSA818U_TX=… if yours differ
pio run -e esp32-scanner -t upload

Three things that bit us on the first flash, all of which the build now avoids: the sketch has to be compiled as a .cpp (the Arduino .ino prototype pass mangles the module structs), the Wi-Fi + NimBLE image is 1.4 MB so it needs the huge_app partition table, and on Windows the board only appears as a COM port once the CP210x/CH340 driver is in. If upload picks COM1, that's the motherboard, not the board.

On boot it probes the module with AT+DMOCONNECT, advertises as Wavelens-XXXX, and its hello tells the phone exactly what it found — "sa818u":"up", or "fitted" if the module didn't answer, so a wiring mistake shows up on the Companion screen instead of as silence.

Using it

Pair it like any companion (More → Companion radio). In the app's Companion screen, the Scan list editor takes the channels you want — 156.800 marine 16, 462.5625 FRS 1, your local repeater outputs — and the ESP32 steps through them, stopping when the squelch opens. Tap a frequency in the Scanner tab to listen directly. With the SX1276 fitted, the Spectrum strip shows a live RSSI sweep of the module's band and the companion reports carriers it finds, so Follow can tune to something you didn't list.

What it won't do: digital. No P25, no DMR, no trunking — the SA818 is an FM discriminator, and the app knows (the hello carries listen and scan but not trunk). For that, the Pi. The two coexist happily; the app pairs with one companion at a time but remembers both.

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