ESP8266 Lua/NodeMCU MQTT to UART bridge

esp8266-nodemcu-uart-bridge provides an ESP8266 NodeMCU Lua application (init.lua) that mirrors incoming MQTT messages to the ESP8266's UART and can also transmit messages received by UART to an MQTT broker or to InfluxDB. This way, it can add simple MQTT/InfluxDB connectivity to devices that do not have a WiFi / ethernet connection.

Dependencies

init.lua has been tested with Lua 5.1 on NodeMCU firmware 1.5.4.1(39cb9a32) (commit 81ec3665cb5fe68eb8596612485cc206b65659c9, integer build). This allows it to run on old ESP8266-01 boards with just 512kB of Flash. It requires the following modules.

• http
• mqtt
• node
• tmr
• wifi

Usage

Startup

Once connected, the ESP8266 will output the following lines on its TX line:

• WiFi:Rdy IP-address
• MQTT:Rdy MQTT-prefix

The default MQTT prefix is uart/esp8266_Chip-ID, e.g. uart/esp8266_82B45C. Additionally, the ESP8266 will publish a retained "online" message under MQTT-prefix/state and subscribe to MQTT-prefix/in.

MQTT to UART

The ESP8266 prints each received message with an "RX:" prefix on its TX line, like so:

RX:message

UART to MQTT

When the ESP8266 receives publish_mqtt(topic, payload, retain) on its RX line, it will publish the specified payload under the specified topic. The retain flag defaults to false. For instance, the following input will publish "hello" under uart/esp8266_Chip-ID/out:

publish_mqtt(mqtt_prefix .. "/out", "hello")

If the message has been sent out successfully, it will reply with MQTT:OK or > MQTT:OK. Otherwise, it will reply with MQTT:Err or > MQTT:Err

Watchdog

If none of the following four events happen for a time span of 90 seconds, the ESP8266 will reset itself, causing a re-connection to WiFi and MQTT server.

• The ESP8266 receives a message via MQTT
• The ESP8266 receives a publish_mqtt command via RX and the command completes successfully
• The ESP8266 receives a publish_influx command via RX and the command completes successfully
• The ESP8266 receiveds a wdr() command via RX.

Error Handling

The following messages on the ESP8266 TX line indicate errors. Note that some errors can have several reasons.

• MQTT:Err or > MQTT:Err – Failed to publish MQTT message (another message was still being processed)
• MQTT:Err or > MQTT:Err – Connection to MQTT broker has been lost
• WiFi:Err or > WiFi:Err – Unable to establish WiFi connection
• WiFi:Err or > WiFi:Err – WiFi connection has been lost
• Watchdog:Err or > Watchdog: Err – watchdog timeout, the device will reset (see above)

Configuration

To use this application, you need to create a config.lua file with WiFI and MQTT settings:

station_cfg = {ssid = "...", pwd = "..."}
mqtt_host = "..."

To use InfluxDB, configure URL and (optional) header:

influx_url = "..."
influx_header = "..."

Resources

Mirrors of this repository are maintained at the following locations:

• Chaosdorf
• git.finalrewind.org
• GitHub