Adafruit STEMMA Soil Sensor - I2C

Hi there!

I am kind of new to nerves and electronics and was hoping that someone here could give me some pointers.
I struggle with reading the soil sensor connected to a raspberry pi zero.

I2C.detect_devices gives:
Devices on I2C bus “i2c-1”:

  • 54

The documentation says that the default address for the sensor is 0x36.
When I try to read from the sensor all I get is <<255>>?

I2C.read(ref, 0x36, 11) gives:
{:ok, <<255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255>>}

If anyone has any experience with this sensor or could explain whats going on here I would really appreciate it!

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Hi klingberg!

For I2C, it’s normal to send a command to the device and then do the read. For example, a command could be to return the current soil moisture and then you’d read a few bytes back that would be the soil moisture.

The API for talking to devices is normally found in something called a datasheet that you can download from the manufacturer. I took a quick look at the soil sensor link. From what I can tell, Adafruit has a microcontroller on the soil sensor board and they programmed it themselves with something called the seesaw firmware.

I could not find a datasheet or API guide that lists commands from Adafruit, but they do have Python code that “documents” the interface. See the moisture_read function in seesaw.py.

Reading the code, it looks like the function sends something related to _TOUCH_BASE and _TOUCH_CHANNEL_OFFSET and then reads into buf (which is 2 bytes):

self.read(_TOUCH_BASE, _TOUCH_CHANNEL_OFFSET, buf, .005)

Going to the read function, it looks like it writes _TOUCH_BASE, _TOUCH_CHANNEL_OFFSET to the device, sleeps 0.005 seconds and then reads 2 bytes.

Translating this to Elixir, I’d try:

I2C.write(ref, 0x36, <<0x0f, 0x10>>)
Process.sleep(5)
{:ok, <<value::little-size(16)>> = I2C.read(ref, 0x36, 2)

_TOUCH_BASE = 0x0f and _TOUCH_CHANNEL_OFFSET = 0x10

Based on Adafruit’s code, it looks like they retry if value > 4095.

Hope this helps!

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Great! This is very helpful, thank you very much!!

I will give this a try as soon as I can!
Will let you know how it goes!

Thanks again!

1 Like