Please use contact form at https://pino-tech.eu/contact/ Your sensor doesn't work as it should. It may be faulty.
You can find detailed instruction on how to calibrate the sensor here: https://pino-tech.eu/forum/soilwatch-10/soil-specific-calibration/
Show what you have done so far and we will sort this out. Paste your sketch here.
If you use Arduino Uno then just connect to 9-12V to jack. Arduino has an onboard 5V regulator so it will make required 5V. For Arduino Nano connect 9-12V to VIN and it has 5V onboard regulator as well.
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