If you are interested in connecting your sensors to LoRa WAN network (like TheThingNetwork) check out my I2C/RS485 LoRa Node

Each order contains 1 device.

This is a contact-less capacitive soil moisture sensor, a continuation of my I2C soil moisture sensor. This particular version of the sensor is suited for more professional installations where long cable runs is a requirement. It adds a local voltage regulator to mitigate the voltage drop and RS485 transceiver to accommodate long wires, also reverse polarity and an additional ESD protection level is added.

This is the easiest and the most robust way to read soil moisture and temperature values into Raspberry Pi. I have written a tutorial how to do it.

The main features:

• Capacitive soil moisture sensing
• Temperature sensing
• Good stability with temperature
• Good stability with power variation
• Suitable for long cable runs
• Uses industrial Modbus protocol
• Low current consumption in deep sleep mode - 11 uA

Technical data

• Supply voltage: 5V
• Bus can be run on both 3V and 5V
• Default serial settings: 19200, 8-bit frame, no parity, 2 stop bits
• Default Modbus address: 1
• Current consumption:
  • Active: 9 mA,
  • Deep sleep: 11 uA

Python library

A python library is available via pip. Type pip install chirp_modbus to install it. Check the source code for documentation. Usage example:

> from chirp_modbus import SoilMoistureSensor

> s = SoilMoistureSensor(1, '/dev/ttyUSB5')

> print(s.getTemperature())

> print(s.getMoisture())

Also check out this tutorial.

Arduino example

This is an example sketch that I use to communicate to the sensor from Arduino. This is an actual script I use to test every sensor and record their data to SD card: Arduino rs485 example

Principle of operation

Sensor makes periodic measurements (by default, once every 500ms), delays between measurements can be updated by writing to one of the Modbus registers. The results of the measurements are kept in specific Modbus registers and can be read by master at any time unless the sensor is in deep sleep.

Deep sleep is entered by writing a number of seconds to sleep into a special sleep register. The sensor will put the transceiver and all the periphery into the lowest possible power consumption mode and sleep for the specified amount of time. Sensor will not respond when in deep sleep. After the sleep period is over the sensor will wake up, start taking measurements and will be available on the bus.

Serial communication parameters as baud rate, parity and address can be updated by writing to related registers. The sensor will reset and boot with the new parameters if any of those registers are written.

Input (read only) registers

Holding (read/write) registers

Supported baudrates

Supported parity settings

Note: most of the cheap ebay USB to RS485 dongles and/or their drivers do not support parity setting properly! You will have to use a proper parity supporting adapter to unbrick the sensor if you set this to something that your dongle does not support! Luckily Adruino with ATmega 16u2 can be used as USB to Serial adapter to solve this.

How to interpret the readings

Moisture values are relative. Meaning, more moisture will give you higher reading, less moisture - lower reading. It's up to you to calibrate it to amount of water in the soil as readings will be different for different types of soils.

Moisture is somewhat linear. I test all sensors before shipping and they give about 211 in free air. Variation of this baseline figure between sensors is within range of 10 LSB, with a standard deviation of 2.

Temperature is measured by the thermistor on the body of the sensor. Calculated absolute measurement accuracy is better than 2%. The returned value is in tenths of degrees Celsius. I.e. value 252 would mean 25.2°C.

Controller support

The sensor will work fine with any controller supporting serial communication provided you have an RS485 transceiver. There are a lot of Modbus libraries readily available for a variety of micro-processor families. Thea easiest way to try the sensor is to use one of the abundant and cheap USB to RS485 dongles and connect it to a Raspberry pi or even directly to your computer.

I have written some basic test code in python that is available on github page.

Waterproofing

The sensor comes coated with PRF202 - a moisture resistant varnish for electronics. It's ok for play around in a flower pot but not enough for outdoor use. You must add an additional protection to the whole sensor after soldering cable to it! Some suggestions on making the sensor more robust after attaching the cable:

• Polyester or epoxy resin - this method is the most bullet proof as the resin is totally resistant to the water. On the par side, note that sensitivity of the sensor will decrease depending on how thick the layer you are going to apply. Also applying the resin in uniform manner presents some challenge.
• PlastiDip - some of my customers have tried this method - easy to apply by spraying and not a lot of loss of sensitivity.
• Rubber balloon - yes, just roll a long balloon over the sensor :)

Be sure to coat the whole thing - the sensor part, the electronics and the cable connection itself so no bare copper or solder is accessible to the water.

Rugged version

Pre-coated version is available. The sensor is coated in epoxy resin, cured and additionally protected by adhesive-lined heat shrink. The 1m (3 feet) long cable is pre-soldered.

The pinout of the rugged version (with the white cable):

• RED - VCC
• BLACK - GND
• BLUE or GREEN - RS485-B
• YELLOW - RS485-A

USB to RS485 dongle

The dongle I sell is sourced form Taobao. It is pretty decently designed in a sense that there is some effort put into adding input protection, also it has case, provides power on the connector and does not cost an arm and a leg. It works out of the box under Linux with kernel 4.9 or newer.

My Raspberry worked fine after I applied all the updates. Run uname -a to test what your kernel version is, run sudo apt-get update && sudo apt-get upgrade to update your system.

I did not have a chance to test it under Mac. Let me know if you can do that, I will throw one in for you for free with your order.

Windows drivers can be downloaded here: http://www.sah.co.rs/lx08h.html