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I want to get some measurements from a soil using a simple soil moisture sensor like this one: enter image description here

It has an analog output proportional to soil moisture.

The goal is to send the measurements through Sub-1GHz to a gateway.

In order to avoid an overcrowded band due to number of transmitters-sensors I thought that would be better to connect over than one sensor to a single transmitter. But, it may be a case that a sensor would be more than 20m away.

And the questions is: What is the proper wired communication protocol to send data in that distance? Just analog value (0-3.3V)? 4-20mA? I2C?

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    \$\begingroup\$ Just a note: Only set the VCC high when you are actually measuring, turn off afterwards. These sensors will corrode within days in a flower pot if they are always supplied with 5V! \$\endgroup\$
    – idkfa
    Apr 27, 2018 at 21:29
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    \$\begingroup\$ Onewire is suitable for these distances, it enumerates the bus automatically and all chips have unique IDs by factory. Try the DS2450 ADC, or the DS2438 battery management IC, it also has an ADC. \$\endgroup\$
    – Janka
    Apr 27, 2018 at 23:05
  • \$\begingroup\$ Onewire also has tradeoffs in long distances especially when the bus is loaded with many devices. It's the most suitable among others thought. \$\endgroup\$
    – MrBit
    Apr 28, 2018 at 6:52
  • \$\begingroup\$ DS2450 costs a fortune. It's not affordable at all in my case. I could use just another transmitter instead \$\endgroup\$
    – MrBit
    Apr 28, 2018 at 7:15

4 Answers 4

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The output impedance of the module's analog output is in the hundreds of K ohms (it's a voltage divider between a 510K resistor and the electrodes), so it's a relatively high impedance. You could buffer it (at the source) with an op-amp (eg. TSV912) as a voltage follower and add some series resistance (a few hundred ohms is fine) to keep the op-amp stable. So, two parts per module. You're really only using the 510K resistor in the module and a couple of capacitors if you ignore the DO.

That crude module puts DC (albeit limited to < 10uA with a 3.3V or 5V supply) through the electrodes, which isn't great. Higher moisture = lower voltage out. I wouldn't put too much faith in voltages read from this particular piece of kit.

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If you're willing to try wireless, BluTooth transmitters could be the way to go. I was going to answer off experience, but I found this article which explained better than I could have. Maximum I2C Bus Length?

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    \$\begingroup\$ I2C bus is very susceptible to noise, mostly when it's working at low voltage levels. I didn't use or see it work at higher voltages but the troubles I've seen in it makes me don't like it \$\endgroup\$
    – MrBit
    Apr 28, 2018 at 7:50
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In my experience you should not be afraid of going "full-IoT" on this one.

Sub-ghz channels should employ "1% duty cycle" which is that each device can talk 36s per hour so that overcrowding is mitigated. If they all have "listen-before-talk" enabled this will leave you with plenty of room to wirelessly communicate. Also you will probably not send a lot of information so you might only need 100ms per device per hour.

Since your goal is to go wireless, I would just go for individual moisture-sensors communicating directly to your gateway.

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The illustration appears identical to a board I possess. If this is the case, the IC is a LM393, set up as a comparator between the probe resistance and on-board components. The pot adjusts the trigger point. The output (labelled DO) is the comparator output, and is thus a GO/NOGO signal.

In other words, this does NOT produce an analogue signal. I think it is intended as a rapid indicator of insufficient moisture in the soil.

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