What

Monitoring Soil Moisture with a Wemos D1 Mini, MCP3008 and cheap soil moisture probe. Some years ago I bought roughly a dozen cheap plant soil moisture probes. In the following I’ll describe how to connect them to a Wemos D1 Mini Dev-Board with an mcp3008 Analog-Digital-Conveter and how to send messages with the readings to your Telegram chat client.

Contents

When

Situation

It took me some years to get around to actually doing this. The project is more useful than it is interesting.

I’ve used a very similar approach to monitor the in- and outlet temperature of our heating system and the temperature of an aging refrigerator.

Why

Motivation

We have quite a few plants though only the most robust ones surive. Each plant would ideally be watered based on its individual needs, but all too often we just see the leaves on one plant go brown after not watering them for weeks and then we dump a couple of liters into all plants.

The result is that the most robust plants do surivive, but not all of them look particularly healthy.

Depending on the plant it would be ideal to water them at recommended amounts and intervals per type of plant.

In order to achieve this it would be helpful if the plants could “talk” to us and remind us when they need more water, at what intervals and how much.

One could maintain equal intervals from experience for summer and winter, but the amount of water the plants require depends heavily on the temperature in the room and it changes dramatically depending on whether there is a lot of direct sun light or not.

The best way I came up to enhance this is to use the soil moisture sensors and program an IoT Wifi-Chip to send us a message on the Telegram messaging App.

Background

Hardware

For the refridgerator and heating system monitoring I’ve used 1-Wire ds18b20 temperatur probes that I had from my DIY 1-Wire Smart Home from ca. 2010 long before smart home consumer products were common. The sensors can be connected directly to the Wemos D1 chip with an Arduino library.

This time around we replace the 1-Wire temperature sensor with an mcp3008 Analog Digital Converter and the Soil Moisture Sensors.

Parts

In total we need:

Schematic

Wiring is straight-forward: Just connect the SPI lines.

We can connect a total of eight Soil Moisture Sensors to the analog input pins of the mcp3008.

How

Wiring

That schematic was first wired on a breadboard.

And later soldered to a perfboard.

Power

For power the Wemos D1 Mini can be connected to a simple 5V smartphone charger with Mini-USB.

For setups that are not near a wall plug we can use small smartphone power banks that usually contain 3.6V 18650-cells. These can also be built from 16850 cells from old laptop battery packs. There are voltage regulators that convert from 3.6V to 5V which effectively yields the same as a smartphone power bank.

To increase batterylife we could use the Wemos D1 Mini Deep-Sleep mode and only wake it up every hour or so, connect and send the data.

Enclosure

To protect from water and to protect from short circuit fires I like to throw these kind of projects into simple junction boxes.

Software

For the software I’ve taken the same code I had for previous projects.

Getting Data out

There are multiple approaches to getting data from an IoT device onto your phone or computer.

You could

I’ve done it all, but for me, in order to not have to maintain and pay for a server running 24/7 I like to use Telegram-Bots.

That way IoT devices send me chat messages directly to my phone and I don’t have to deal with the server setup and hosting fees.

Set up a bot in Telegram

Contact Botfather as per guide from Telegram to create a Telegram Bot and get a Bot ID.

Write to “@get_id_bot” to get your Chat ID that we can hard-code below to prevent unauthorized access.

Deploying code to Arduinos

Over the years I’ve used scripts instead of the Arduino IDE so I can quickly setup additional boards and libraries

With the below code saved as “software/software.ino” and the Wemos D1 Mini connected via USB, the following shell script will download all required Arduino libraries, compile and deploy the code to the Wemos D1 Mini.

#!/bin/bash
set -e

ACLI=<path-to-arduino-cli>/arduino-cli

# -- set up board manager url
echo "board_manager:" > arduino-cli.yaml
echo "  additional_urls:" >> arduino-cli.yaml
echo "  - http://arduino.esp8266.com/stable/package_esp8266com_index.json" >> arduino-cli.yaml
echo "  - https://adafruit.github.io/arduino-board-index/package_adafruit_index.json" >> arduino-cli.yaml

# -- install wemos d1 board support
$ACLI core update-index --additional-urls http://arduino.esp8266.com/stable/package_esp8266com_index.json
$ACLI core install esp8266:esp8266

# -- add telegram bot library
$ACLI lib install "UniversalTelegramBot"

# -- add mcp3008 library
$ACLI lib install "Adafruit MCP3008"

# -- compile and upload code, then show serial monitor
$ACLI compile --fqbn esp8266:esp8266:d1_mini software/software.ino
$ACLI upload --fqbn esp8266:esp8266:d1_mini --port /dev/ttyUSB0 software/software.ino
$ACLI monitor -p /dev/ttyUSB0 -c baudrate=115200

Arduino Libraries

The deployment script automatically installs these libraries. For the code I’m using the following Arduino Libraries.

Writing the code

Create a file in software/software.ino.

We need the following headers for the Wemos D1 Mini, Wifi, the Telegram Bot and the mcp3008 Analog-Digital converter.

#ifdef ESP32
  #include <WiFi.h>
#else
  #include <ESP8266WiFi.h>
#endif
#include <WiFiClientSecure.h>
#include <UniversalTelegramBot.h>
#include <Adafruit_MCP3008.h>

Some configuration fields for you to fill out with your Wifi credentials, the Telegram Bot ID and your Telegram User ID.

// -- configuration
const char* ssid = "<Wifi SSID>";
const char* password = "<Wifi Password>";
#define BOT_TOKEN "<token from telegram botfather, number:letters>"
#define YOUR_CHAT_ID "<your-id-from-@get_id_bot>"

With the following configuration the Bot will check for new messages once per second. If you’ve sent “/state” it will send the readings immediatly.

Otherwise it will automatically send readings every 30 minutes. This is useful to see if the bot is still running after a longer time unattended.

const unsigned long CHECK_MESSAGE_DELAY = 1000;
const unsigned long AUTO_SEND_DELAY = 30*60;

Some global variables for tracking state

WiFiClientSecure client;
UniversalTelegramBot bot(BOT_TOKEN, client);
Adafruit_MCP3008 adc;
int counter = AUTO_SEND_DELAY;
unsigned long lastRunTime = 0;

to send and receive Telegram messages in a semi-colon separated format so we can copy it to a spreadsheet for analysis easily.

I like to add the physical Address of the Wifi-Chip in order to be able distinguish multiple Wemos D1 Mini from each other.

void sendValues() {
    String mac_addr = WiFi.macAddress();
    mac_addr.replace(":","");
    String postData = "mac_addr=" + mac_addr + ";";
   
    for(int i=0; i < 8; i++) {
      postData += "sensor_" + String(i);
      postData += "=" + String(adc.readADC(i)) + ";";
    }
    bot.sendMessage(YOUR_CHAT_ID, postData, "");
}

void handleNewMessages(int numNewMessages) {
  for (int i=0; i<numNewMessages; i++) {
    if (String(bot.messages[i].chat_id) != YOUR_CHAT_ID) {
      bot.sendMessage(YOUR_CHAT_ID, "Unauthorized user", "");
      continue;
    }
    if (bot.messages[i].text == "/state") {
      sendValues();
    }
  }
}

The setup and loop functions connect to the Wifi, get time from the NTP time server and routinely check for messages on Telegram. If a message with the /state command is received, the Bot will respond with the current readings. It also auto-sends readings every half hour.

void setup() {
  Serial.begin(115200);
  
  // -- ignore cerificates
  client.setInsecure();
  
  // -- set time server
  #ifdef ESP8266
    configTime(0, 0, "pool.ntp.org");
  #endif
  
  // -- WiFi
  WiFi.mode(WIFI_STA);
  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED) {
    delay(1000);
    Serial.println("Connecting to WiFi..");
  }

  // -- print IP
  Serial.println(WiFi.localIP());
  adc.begin();
  
  counter = AUTO_SEND_DELAY;
  lastRunTime = millis() - CHECK_MESSAGE_DELAY;
}

void loop() {
  if (millis() > lastRunTime + CHECK_MESSAGE_DELAY) {
    Serial.print(".");
    
    // -- receive messages
    int numNewMessages = bot.getUpdates(bot.last_message_received + 1);
    while(numNewMessages) {
      handleNewMessages(numNewMessages);
      numNewMessages = bot.getUpdates(bot.last_message_received + 1);
    }
    
    // -- automatic send values
    if(counter == 0) {
      counter = AUTO_SEND_DELAY;
      sendValues();
    }
    counter--;

    lastRunTime = millis();
  }
}

Progress

As you can see the messages are delivered to my phone. Mission accomplished.

From trying it out I know what the values approximately mean:

In order to further improve we could customize the messages to hold more informational value.

Currently I’m using these Blumat suction things.

They automatically pull water from a bottle, but are hard to properly adjust. The soil moisture sensor helps adjusting them and seeing how the plants react.

A further improvement could be to add a small drip pump controlled by relay instead to water the plants.

Update 2024-04-21

The readings were way off and so I investigated. Seems the probes on the sensors have completly disintegrated due to corossion. The connection pins break off and the traces on the boards are practically gone.

Seems the companies behind these probes don’t seem to expect anyone would seriously use them in projects. Many DIYs order things, myself included - these probes are probably 3 years old - and never get around to using them for a long time or ever. And so the company either had no idea what they were doing or thought they can get away with using coroding material on a plant sensor.

The part is very basic. I’ll probably just use something that doesn’t corrode like copper wires or aluminium rods with some 3D printed part to keep them at an equal distance and will connect the sensor boards to that.

Update 2024-07-16

In search for a simple and cheap fix I came up with this: Just solder regular household single-core electical wiring to the wires directly and glue it in place with hot glue. It looks bad, but since it’s in the soil it doesn’t matter. The corrosion issues should go away. I just hope I don’t poison my plants with the copper, but until now this works fine.