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+# ESP8266 Lua/NodeMCU module for Sensirion SCD40/SCD41 CO₂ sensor
+
+This repository contains a Lua module (`scd4x.lua`) as well as
+MQTT-based HomeAssistant integration (`init.lua`) for **Sensirion SCD40/SCD41**
+CO₂ sensors.
+
+## Dependencies
+
+scd4x.lua has been tested with Lua 5.1 on NodeMCU firmware 3.0.1 (Release
+202112300746, integer build). It requires the following modules.
+
+* i2c
+
+The MQTT HomeAssistant integration in init.lua additionally needs the following
+modules.
+
+* gpio
+* mqtt
+* node
+* tmr
+* uart
+* wifi
+
+## Usage
+
+Copy **scd4x.lua** to your NodeMCU board and set it up as follows.
+
+```lua
+scd4x = require("scd4x")
+i2c.setup(0, sda_pin, scl_pin, i2c.SLOW)
+scd4x.start()
+
+-- can be called with up to 1 Hz
+function some_timer_callback()
+	local co2, raw_temp, raw_humi = scd4x.read()
+	if co2 == nil then
+		print("SCD4x error")
+	else
+		-- CO₂[ppm] == co2, Temperature[°c] == raw_temp/2¹⁶ - 45, Humidity[%] == raw_humi/2¹⁶
+	end
+end
+```
+
+See **init.lua** for an example. To use it, you need to create a **config.lua** file with WiFI and MQTT settings:
+
+```lua
+station_cfg.ssid = "..."
+station_cfg.pwd = "..."
+mqtt_host = "..."
+```