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water-level-receiver-software.md

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The WaterLevelReceiver software

Operation

Every second, the program sends a single "?" character to the Serial2 port, at 115200 Baud. Each "?" acts as a request for a new measurement to the connected transmitter. If all's well, the transmitter will reply with a JSON output as document in the transmitter documentation.

The receiver will decode this JSON payload and can publish it to several destinations. The supported destinations, selectable at compile time:

  • the serial port accessible through the USB connection (at 115200 Baud);
  • a character-based LCD display using the hd44780 chip;
  • a locally hosted webpage
  • MQTT, in this case to a topic "garden/waterlevel"

In addition, the program watches button input. Currently, there is only one button defined:

  • a button connected between GPIO12 and ground: controls LCD backlight (button closed = backlight on).

Serial port output

Serial port output consists of:

  • the temperature in numerical form;
  • the volume in liters in numerical form;
  • the volume as percentage in numerical form;
  • a LOW indicator, only written if the level is low.

LCD output

LCD output consist of:

  • a representation of the volume as percentage in a bar graph;
  • the volume as percentage in numerical form;
  • the volume in liters in numerical form;
  • a LOW indicator, only displayed if the level is low;
  • a heartbeat indicator, changing between '*' and ' '.

lcd-output

HTTP output

HTTP output can consist of all information available in the serial port output.

Currently, a simple HTML representation shows:

  • volume in liters, in a HTML meter element and as percentage (the LOW indicator is represented with red volume values)
  • temperature in °C (red in case of frost)

An example:

html-representation

MQTT output

An MQTT server is accessed via WiFi using the Arduino library 'EspMQTTClient' and the output is published on a single topic "garden/waterlevel" once per minute.

The payload is a JSON structure consisting of the fields:

  • "t_C": the temperature;
  • "vol_l": the volume in liters;
  • "vol_percent": the volume as percentage;
  • "low: the LOW indicator (true/false).

Possibilities with MQTT

What follows is a non limiting list of examples.

Display data on Node-RED user interface elements

The Node-RED package node-red-dashboard has interesting nodes such as a gauge, allowing to display output as shown below.

nodered-output

An example flow for this can be imported here.

Export data to Influxdb for possible display using Grafana

The Node-RED contributed package node-red-contrib-influxdb has an influxdb out node, allowing to output data to Influxdb.

These data can then be read by Grafana, a great tool to visualise data.

grafana-output

An example flow can be imported here. It assumes local installations of Influxdb and Grafana.

Connect to ThingSpeak

Use an external tool to subscribe to the MQTT topic and forward the data to ThingSpeak from there. An example for Node-RED is available for import here. In addition to standard nodes, it depends on ThingSpeak42.

For illustration, in earlier versions, the ThingSpeak API was accessed directly fom the board via WiFi using the Arduino library 'ThingSpeak' and the output was directed to a specific channel, to be created by the user.

Output to the ThingSpeak channel was reduced to one write per minute.

The output in the channel consisted of:

  • field 1: the volume in liters;
  • field 2: the volume as percentage;
  • field 3: the temperature;
  • field 4: the LOW indicator (0 = OK; 1 = LOW).

thingspeak-output

Testing

Some DEBUG... preprocessor definitions are available to assist debugging the code. They provide possibilities to output debug information and to provide fake input.