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diff --git a/.gitignore b/.gitignore
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+config.lua
diff --git a/README.md b/README.md
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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 = "..."
+```
diff --git a/init.lua b/init.lua
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+station_cfg = {}
+dofile("config.lua")
+
+delayed_restart = tmr.create()
+push_timer = tmr.create()
+chip_id = node.chipid()
+device_id = "esp8266_" .. chip_id
+mqtt_prefix = "sensor/" .. device_id
+mqttclient = mqtt.Client(device_id, 120)
+
+
+print("ESP8266 " .. chip_id)
+
+ledpin = 4
+gpio.mode(ledpin, gpio.OUTPUT)
+gpio.write(ledpin, 0)
+
+scd4x = require("scd4x")
+i2c.setup(0, 2, 1, i2c.SLOW)
+
+function log_restart()
+	print("Network error " .. wifi.sta.status() .. ". Restarting in 20 seconds.")
+	delayed_restart:start()
+end
+
+function setup_client()
+	print("Connected")
+	gpio.write(ledpin, 1)
+	if not scd4x.start() then
+		print("SCD4x initialization error")
+	end
+	publishing = true
+	mqttclient:publish(mqtt_prefix .. "/state", "online", 0, 1, function(client)
+		publishing = false
+		push_data()
+	end)
+end
+
+function connect_mqtt()
+	print("IP address: " .. wifi.sta.getip())
+	print("Connecting to MQTT " .. mqtt_host)
+	delayed_restart:stop()
+	mqttclient:on("connect", hass_register)
+	mqttclient:on("offline", log_restart)
+	mqttclient:lwt(mqtt_prefix .. "/state", "offline", 0, 1)
+	mqttclient:connect(mqtt_host)
+end
+
+function connect_wifi()
+	print("WiFi MAC: " .. wifi.sta.getmac())
+	print("Connecting to ESSID " .. station_cfg.ssid)
+	wifi.eventmon.register(wifi.eventmon.STA_GOT_IP, connect_mqtt)
+	wifi.eventmon.register(wifi.eventmon.STA_DHCP_TIMEOUT, log_restart)
+	wifi.eventmon.register(wifi.eventmon.STA_DISCONNECTED, log_restart)
+	wifi.setmode(wifi.STATION)
+	wifi.sta.config(station_cfg)
+	wifi.sta.connect()
+end
+
+function push_data()
+	local co2, raw_temp, raw_humi = scd4x.read()
+	if co2 == nil then
+		print("SCD4x error")
+	else
+		local json_str = string.format('{"co2_ppm": %d, "temperature_degc": %d.%d, "humidity_relpercent": %d.%d, "rssi_dbm": %d}', co2, raw_temp/65536 - 45, (raw_temp%65536)/6554, raw_humi/65536, (raw_humi%65536)/6554, wifi.sta.getrssi())
+		if not publishing then
+			publishing = true
+			gpio.write(ledpin, 0)
+			mqttclient:publish(mqtt_prefix .. "/data", json_str, 0, 0, function(client)
+				publishing = false
+				gpio.write(ledpin, 1)
+				collectgarbage()
+			end)
+		end
+	end
+	push_timer:start()
+end
+
+function hass_register()
+	local hass_device = string.format('{"connections":[["mac","%s"]],"identifiers":["%s"],"model":"ESP8266","name":"ESP8266 with SCD4x","manufacturer":"DIY"}', wifi.sta.getmac(), device_id)
+	local hass_entity_base = string.format('"device":%s,"state_topic":"%s/data","expire_after":600', hass_device, mqtt_prefix)
+	local hass_co2 = string.format('{%s,"name":"CO₂","object_id":"%s_co2","unique_id":"%s_co2","device_class":"carbon_dioxide","unit_of_measurement":"ppm","value_template":"{{value_json.co2_ppm}}"}', hass_entity_base, device_id, device_id)
+	local hass_temp = string.format('{%s,"name":"Temperature","object_id":"%s_temperature","unique_id":"%s_temperature","device_class":"temperature","unit_of_measurement":"°c","value_template":"{{value_json.temperature_degc}}"}', hass_entity_base, device_id, device_id)
+	local hass_humi = string.format('{%s,"name":"Humidity","object_id":"%s_humidity","unique_id":"%s_humidity","device_class":"humidity","unit_of_measurement":"%%","value_template":"{{value_json.humidity_relpercent}}"}', hass_entity_base, device_id, device_id)
+	local hass_rssi = string.format('{%s,"name":"RSSI","object_id":"%s_rssi","unique_id":"%s_rssi","device_class":"signal_strength","unit_of_measurement":"dBm","value_template":"{{value_json.rssi_dbm}}","entity_category":"diagnostic"}', hass_entity_base, device_id, device_id)
+
+	mqttclient:publish("homeassistant/sensor/" .. device_id .. "/co2/config", hass_co2, 0, 1, function(client)
+		mqttclient:publish("homeassistant/sensor/" .. device_id .. "/temperature/config", hass_temp, 0, 1, function(client)
+			mqttclient:publish("homeassistant/sensor/" .. device_id .. "/humidity/config", hass_humi, 0, 1, function(client)
+				mqttclient:publish("homeassistant/sensor/" .. device_id .. "/rssi/config", hass_rssi, 0, 1, function(client)
+					collectgarbage()
+					setup_client()
+				end)
+			end)
+		end)
+	end)
+end
+
+delayed_restart:register(20 * 1000, tmr.ALARM_SINGLE, node.restart)
+push_timer:register(20 * 1000, tmr.ALARM_SEMI, push_data)
+
+connect_wifi()
diff --git a/scd4x.lua b/scd4x.lua
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+local scd4x = {}
+local device_address = 0x62
+
+scd4x.bus_id = 0
+
+function scd4x.start()
+	i2c.start(scd4x.bus_id)
+	if not i2c.address(scd4x.bus_id, device_address, i2c.TRANSMITTER) then
+		return false
+	end
+	i2c.write(scd4x.bus_id, {0x21, 0xb1})
+	i2c.stop(scd4x.bus_id)
+	return true
+end
+
+function scd4x.stop()
+	i2c.start(scd4x.bus_id)
+	if not i2c.address(scd4x.bus_id, device_address, i2c.TRANSMITTER) then
+		return false
+	end
+	i2c.write(scd4x.bus_id, {0x3f, 0x86})
+	i2c.stop(scd4x.bus_id)
+	return true
+end
+
+function scd4x.read()
+	i2c.start(scd4x.bus_id)
+	if not i2c.address(scd4x.bus_id, device_address, i2c.TRANSMITTER) then
+		return nil
+	end
+	i2c.write(scd4x.bus_id, {0xec, 0x05})
+	i2c.stop(scd4x.bus_id)
+	i2c.start(scd4x.bus_id)
+	if not i2c.address(scd4x.bus_id, device_address, i2c.RECEIVER) then
+		return nil
+	end
+	local data = i2c.read(scd4x.bus_id, 9)
+	i2c.stop(scd4x.bus_id)
+	local co2 = string.byte(data, 1) * 256 + string.byte(data, 2)
+	local temp_raw = 175 * (string.byte(data, 4) * 256 + string.byte(data, 5))
+	local humi_raw = 100 * (string.byte(data, 7) * 256 + string.byte(data, 8))
+	return co2, temp_raw, humi_raw
+end
+
+return scd4x