AtomS3 Lite + ENV IV: ESPHome BLE proxy and room sensors in one box
I wanted a small Bluetooth proxy near two Vent-Axia Svara extractors (Pax BLE fans) and a cheap way to read temperature, humidity, and pressure in the same spot. M5Stack’s AtomS3 Lite (~£9) (ESP32-S3) plus the Unit ENV IV (~£6; EOL from M5Stack, still findable from resellers) (SHT40 + BMP280 on Grove I2C) turned out to be one ESPHome device that does both — about £15 of hardware in total.
This post covers what the hardware is, what it reports to Home Assistant, how much RAM the firmware uses, and the sanitized YAML I run.
The two parts (one firmware)
M5Stack AtomS3 Lite (~£9)
Tiny ESP32-S3 board with Wi-Fi, Bluetooth 5 LE, a button, and four RGB LEDs under the cap.
From esptool when the board is connected over USB:
Chip type: ESP32-S3 (QFN56) (revision v0.2)
Features: Wi-Fi, BT 5 (LE), Dual Core + LP Core, 240MHz, Embedded Flash 8MB (GD)
Crystal frequency: 40MHz
In practice that means: enough flash for ESP-IDF + BLE proxy + sensors, dual-core headroom for Wi-Fi/BLE coexistence, and native USB-CDC for first flash (no separate USB-UART chip).
M5Stack Unit ENV IV (~£6)
Grove/STEMMA QT breakout with:
- Sensirion SHT40 — temperature and relative humidity (0x44 on I2C)
- Bosch BMP280 — barometric pressure (0x76 on I2C)
On the AtomS3 Lite the Grove port maps to SDA = GPIO2, SCL = GPIO1. A short Grove cable is enough; if I2C scan finds nothing, swap the yellow/white data lines — AtomS3 Grove pin order is not always the cable default.
What it does in Home Assistant
Primary job: ESPHome Bluetooth proxy so Home Assistant can reach Pax fans that only speak BLE GATT, not Wi-Fi. I use Erik Nordby’s community integration ha-pax_ble — actively maintained, responsive on issues/PRs, and the right layer for Vent-Axia Svara hardware without a official HA integration.
Secondary job: publish room environment every five minutes (I kept env reads slow so the chip stays cool and I2C traffic stays out of the way of BLE).
After OTA to firmware 1.1.1 (ESPHome 2026.7.3), a typical live snapshot:
| Check | Status |
|---|---|
| ESPHome integration | Loaded — AtomS3 Lite Temp 5e5b70 |
| Firmware | 1.1.1 (ESPHome 2026.7.3) |
| Connectivity (Status) | On |
| Uptime | ~5 min (post-OTA) |
| Wi-Fi signal | -53 dBm (IoT VLAN) |
| Temperature | 25.0 °C |
| Humidity | 47.7 % |
| Pressure | 1017 hPa |
| Absolute humidity | 11.0 g/m³ |
| Dew point | 13.2 °C |
| Altitude (barometric) | ~-32 m vs ISA 1013.25 hPa |
| LED | Blue when Wi-Fi up; green/red 10 s flash after each env publish |
Pax fans via the proxy
With the Atom on the IoT VLAN and the proxy active, both Svara units stay reachable:
| Fan | State | RPM | Temperature |
|---|---|---|---|
top_bathroom_extractor |
Trickle ventilation | 1000 | 25.4 °C |
tom_bedroom_ensuite_extractor |
Trickle ventilation | 1641 | 22.9 °C |
Home Assistant sees full Pax entities (flow, humidity sensor on the unit, boost mode, silent hours, etc.) through ha-pax_ble. The Atom does not run the Pax logic — it forwards BLE so the integration on the HA host can connect.
RAM and flash budget
Compiled with ESPHome 2026.7.3, ESP-IDF 5.5.5, project version 1.1.1:
RAM: [==== ] 42.6% (used 145515 bytes from 341760 bytes)
Flash: [======== ] 75.1% (used 1377767 bytes from 1835008 bytes)
That leaves comfortable headroom for three BLE connection slots (Pax may hold active GATT sessions), Wi-Fi, captive portal fallback, and the LED/env automation scripts. I stayed on three slots rather than four — each slot costs roughly 1 KB RAM and Wi-Fi proxies rarely need more unless you are saturating connections.
LED feedback (v1.1.1)
The four WS2812 LEDs under the button are status-only:
- Blue — Wi-Fi connected to HA
- Red pulse — fallback AP / Wi-Fi down
- Green 10 s — env reading published and HA API reachable
- Red 10 s (solid) — reading OK but Wi-Fi or API unreachable
- Short press — toggle LED; long press ~1 s — reboot
Handy when the puck is on a USB charger away from the desk and you want to know it is still talking to HA.
Sanitized ESPHome config
Secrets (wifi_ssid, wifi_password, api_encryption_key, fallback AP password) live in secrets.yaml on the HA host — not in git. Example placeholders:
wifi_ssid: "your-ssid"
wifi_password: "your-wifi-password"
api_encryption_key: "generate-with-esphome secrets create"
wifi_ap_password: "fallback-ap-password"
Full public YAML (no credentials):
atoms3-lite-temp.yaml (click to expand)
```yaml
# Public/sanitized ESPHome config for AtomS3 Lite + Unit ENV IV (SHT40 + BMP280).
# Secrets: copy secrets.yaml.example → secrets.yaml and fill in locally.
# BLE proxy baseline: https://github.com/esphome/bluetooth-proxies/tree/main/m5stack
substitutions:
name: atoms3-lite-temp
friendly_name: AtomS3 Lite Temp
sea_level_pressure_hpa: "1013.25"
wifi_ap_ssid: "AtomS3-Lite-Fallback"
esphome:
name: ${name}
friendly_name: ${friendly_name}
min_version: 2025.8.0
name_add_mac_suffix: true
project:
name: homelab.atoms3-ble-proxy
version: "1.1.1"
esp32:
variant: esp32s3
framework:
type: esp-idf
logger:
baud_rate: 0
level: INFO
api:
encryption:
key: !secret api_encryption_key
ota:
- platform: esphome
id: ota_esphome
wifi:
ssid: !secret wifi_ssid
password: !secret wifi_password
ap:
ssid: ${wifi_ap_ssid}
password: !secret wifi_ap_password
on_connect:
- script.execute: led_wifi_connected
on_disconnect:
- script.execute: led_wifi_ap
captive_portal:
esp32_ble_tracker:
bluetooth_proxy:
active: true
cache_services: true
connection_slots: 3
i2c:
sda: GPIO2
scl: GPIO1
scan: true
id: bus_grove
light:
- platform: esp32_rmt_led_strip
rgb_order: GRB
pin: GPIO35
num_leds: 4
chipset: ws2812
id: atom_led
name: "${friendly_name} LED"
restore_mode: RESTORE_DEFAULT_OFF
effects:
- pulse:
name: Pulse
transition_length: 1s
script:
- id: led_wifi_connected
then:
- light.turn_on:
id: atom_led
brightness: 30%
red: 0%
green: 0%
blue: 100%
- id: led_wifi_ap
then:
- light.turn_on:
id: atom_led
brightness: 30%
red: 100%
green: 40%
blue: 0%
effect: Pulse
- id: led_update_sent
mode: restart
then:
- light.turn_on:
id: atom_led
brightness: 30%
red: 0%
green: 100%
blue: 0%
- delay: 10s
- if:
condition:
wifi.connected:
then:
- script.execute: led_wifi_connected
else:
- script.execute: led_wifi_ap
- id: led_update_failed
mode: restart
then:
- light.turn_on:
id: atom_led
brightness: 30%
red: 100%
green: 0%
blue: 0%
- delay: 10s
- if:
condition:
wifi.connected:
then:
- script.execute: led_wifi_connected
else:
- script.execute: led_wifi_ap
sensor:
- platform: sht4x
i2c_id: bus_grove
address: 0x44
update_interval: 300s
precision: High
heater_max_duty: 0.0
temperature:
name: "${friendly_name} Temperature"
id: sht40_temperature
humidity:
name: "${friendly_name} Humidity"
id: sht40_humidity
- platform: bmp280_i2c
i2c_id: bus_grove
address: 0x76
update_interval: 300s
iir_filter: 4x
temperature:
id: bmp280_temperature
internal: true
pressure:
name: "${friendly_name} Pressure"
id: bmp280_pressure
on_value:
then:
- if:
condition:
and:
- wifi.connected:
- api.connected:
then:
- script.execute: led_update_sent
else:
- script.execute: led_update_failed
- platform: absolute_humidity
name: "${friendly_name} Absolute Humidity"
temperature: sht40_temperature
humidity: sht40_humidity
- platform: template
name: "${friendly_name} Dew Point"
unit_of_measurement: "°C"
device_class: temperature
icon: mdi:thermometer-alert
update_interval: 300s
lambda: |-
return (243.5 * (log(id(sht40_humidity).state / 100.0) +
((17.67 * id(sht40_temperature).state) /
(243.5 + id(sht40_temperature).state)))) /
(17.67 - log(id(sht40_humidity).state / 100.0) -
((17.67 * id(sht40_temperature).state) /
(243.5 + id(sht40_temperature).state)));
- platform: template
name: "${friendly_name} Altitude"
unit_of_measurement: "m"
icon: mdi:image-filter-hdr
update_interval: 300s
lambda: |-
const float sea_level = ${sea_level_pressure_hpa};
return ((id(sht40_temperature).state + 273.15) / 0.0065) *
(powf(sea_level / id(bmp280_pressure).state, 0.190234) - 1);
- platform: wifi_signal
name: "${friendly_name} WiFi Signal"
update_interval: 60s
- platform: uptime
name: "${friendly_name} Uptime"
update_interval: 60s
binary_sensor:
- platform: status
name: "${friendly_name} Status"
- platform: gpio
name: "${friendly_name} Button"
id: atom_button
pin:
number: GPIO41
inverted: true
mode:
input: true
pullup: true
filters:
- delayed_on_off: 50ms
on_click:
- min_length: 50ms
max_length: 800ms
then:
- light.toggle: atom_led
- min_length: 1000ms
max_length: 5000ms
then:
- button.press: device_restart
button:
- platform: restart
name: "${friendly_name} Restart"
id: device_restart
internal: true
- platform: safe_mode
name: "${friendly_name} Safe Mode Boot"
id: button_safe_mode
- platform: factory_reset
name: "${friendly_name} Factory Reset"
id: factory_reset_btn
```
Broad structure:
- ESP-IDF on ESP32-S3 (not Arduino — lower RAM use for BLE + Wi-Fi)
bluetooth_proxywithactive: true,cache_services: true,connection_slots: 3- SHT4x + BMP280 on Grove I2C, 300 s update interval
- Derived absolute humidity, dew point, altitude templates
- Wi-Fi signal and uptime sensors for placement debugging
- Captive portal + named fallback AP for recovery
Official BLE proxy starting point: esphome/bluetooth-proxies — M5Stack Atom S3.
Links
- Pax BLE for Home Assistant: github.com/eriknn/ha-pax_ble
- ESPHome Bluetooth proxies: github.com/esphome/bluetooth-proxies
- M5Stack AtomS3 Lite: docs.m5stack.com — AtomS3 Lite
- M5Stack Unit ENV IV: docs.m5stack.com — Unit ENV IV
If you are running Vent-Axia fans and HA, ha-pax_ble is worth a star.