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A WiFi Enabled Soil Moisture Sensor

Added to IoTplaybook or last updated on: 04/05/2021
A WiFi Enabled Soil Moisture Sensor

Story

The Origin

A few months ago, I got my hands on one of these interesting capacitive soil moisture sensors.

These sensors output is an analog value that is proportional to how moist the soil around is. This voltage is based on how the capacitance between two large PCB traces varies, depending on the amount of water in the ground. This great post by wemakethings.net explains their inner working. And I also wrote a little bit about it here.

Things used in this project

Hardware components

 
Espressif ESP32-WROOM-32D
 
× 1

Expressif

 
LIR2450 Li-Ion coin cell
 
× 1

Farnell

 
XB3303A Battery Protection IC
 
× 1

LCSC Electronics

 
LIR2450 Battery Holder
 
× 1

LCSC Electronics

1N4148 – General Purpose Fast Switching
1N4148 – General Purpose Fast Switching
 
× 1

Newark

 
MMBT3904 NPN Transistor
 
× 2

DigiKey

Software apps and online services

PlatformIO IDE
PlatformIO IDE
 
 

PlatformIO

The First Stab

To visualize and store the soil readings, I wanted to try and connect these sensors to MQTT. I put together a little custom board with an ESP32 and a beefy 18650 Li-Ion battery.

Version 1 and 3d printed case

1 / 2 • Version 1 and 3d printed case


2 / 2 Version 1 and 3rd printed case

This worked okay. The ESP32 sleeps for most of the time and sends an MQTT message via WiFi every few minutes. Here's an example 7-day period of sensor data:

The two bumps at the bottom correspond to when these plants were watered

The two bumps at the bottom correspond to when these plants were watered

Shrinking it Down

While a few of these boards have been working for a few months, I wasn't super happy with the clunkiness of the PCB, sensor and battery assembly. At this point, I came across Andreas Spiess' video on running an ESP8266 on this tiny LIR2450 coin cell. It seemed like an interesting challenge.

To get rid of all these wires and make a small, all-in-one board, the other challenge was to implement the capacitive sensor itself on a custom board, without relying on the separate sensor module. Again, thanks to the excellent wemakethings.net post, I felt like taking a stab at it.

Sensor, battery and ESP32 on a single board

1 / 2 • Sensor, battery and ESP32 on a single board


2 / 2 • Sensor, battery and ESP32 on a single board

It took a couple of iterations, but the moisture sensor works. Under the hood, the original sensor module uses a 555 IC to repeatedly charge and discharge the parasitic "capacitor" (the two thick copper traces). Since we have a whole microcontroller there, I use a PWM signal to the same end.

All the design files and PlatformIO code is on GitHub.

On Battery Life

Besides implementing the moisture sensor itself, getting by with a little LIR2450 battery cell has been the most interesting challenge. These coin cells have only around 120mAh in them, and can only source peaks of 200mA. For reference, the ESP32 datasheet mentions peaks of 500mA.

In deep sleep, the ESP32 and the on board voltage regulator (HT7333 LDO) consume around 15uA. This is on the high side of sleeping battery devices, but still manageable. When active, the current is orders of magnitude higher, averaging around 150mA. The name of the game is trying to be as quick as possible and going right back to sleep. Currently, the active time is around 500ms.

WiFi is also relatively power hungry for IoT devices, and simply establishing a connection to access point takes around 300ms on a good day. An interesting alternative is using Bluetooth low energy (BLE) for broadcasting sensor data. On the other hand, we need an additional moving part for piping these BLE packets to MQTT. This is something I want to explore moving forward.

The board can be powered with the tiny LIR2450 cell or a conventional LiPo/Li-Ion battery

he board can be powered with the tiny LIR2450 cell or a conventional LiPo/Li-Ion battery

TI put together this spreadsheet for estimating the battery life given the active/sleep cycle and battery capacity. Here are some example scenarios:

  • LIR2450 cell (120mAh), transmitting every 30 minutes: around 80-90 days.
  • 1200mAh LiPo, transmitting every 10 minutes: over a year;
  • 18650 Li-Ion (e.g.: 2700mAh), transmitting every 5 minutes: over a year;

In Its Natural Habitat


1 / 3

 


2 / 3


3 / 3

Next Steps

I want to experiment with lower power chips such as Nordic's nrf52840. These are BLE-only SoCs and are incredibly energy efficient. They consume only a few microamps during sleep and only around 6mA when transmitting.

A nice goal is to be able to power the whole board with common CR2032 button cells for many months, which I think is doable with these chips. Additionally, we can drop the LDO regulator, since these chips can be powered directly by the whole range of the CR2032 voltage, which also increases the battery life. I will document the progress on my Twitter.

Thanks for reading!

Schematics

GitHub  - This is the repository that contains all the code and schematics for the board.

rbaron / w-parasite

A WiFi-enabled soil moisture sensor — Read More

Latest commit to the master branch on 3-13-2021 - Download as zip

Schematic

Credits

rbaron

rbaron

I'm a software developer who likes to build stuff.

 

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This article was originally published at Hackster.io. It was added to IoTplaybook or last modified on 04/05/2021.