Updated README.md

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marcel 22 hours ago
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## Abstract
A weather station build around a SparkFun Weather Meter Kit (SEN-15901). The temperature, humidity and pressure are measured with I2C sensors housed in an RS1 Passive Radiation Shield from Garni. The data is available via an RS485 ModBus RTU interface. The main processor is an Arduino Pro Mini (ATmega328P 5V@16MHz)
A weather station build around a SparkFun Weather Meter Kit (SEN-15901). The temperature, humidity, ambient lightmeter and pressure are measured with I2C sensors housed in an RS1 Passive Radiation Shield from Garni. The data is available via an RS485 ModBus RTU interface. The main processor is an Arduino Pro Mini (ATmega328P 5V@16MHz)
![Block diagram of weather station](./build-doc/images/block_diagram.svg "Block diagram of weather station")
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## Sensors
- SparkFun SEN-15901 Weather Station (wind speed, wind direction and rain fall)
- Silicon Labs Si7021 (humidity and temperature)
- Innovative Sensor Technology IST AG HYT 221 (humidity and temperature)
- Bosch BMP280 (pressure)
- DFRobot SEN0562 (ambient light)
## Measurements
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- Temperature
- Humidity
- Atmospheric pressure
- Ambient light
## ModBus
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Libraries are included with the source code of this project
## Known bugs
## Known issues
There is a bug in the Si7021 chip: when the sensor is exposed to extreme environmental conditions for a longer period (very high or very low humidity), the humidity register will wrap around and give wrong values. (https://community.silabs.com/s/question/0D51M00007xeOo9SAE/si7021-relative-humidity-reading-underflowing-?language=en_US).
Workaround: let's assume the humidity will never go below 15%. In the Netherlands a humidity below 20% is rare, but can occur and the minimum value ever measured in De Bilt was 6% (on 1 april 1965). At several other sites in the Netherlands the humidity went as low as 10%. But as this was a one time event and the measurements are doubtful, this event was not registered in the original database. (https://nl.wikipedia.org/wiki/Relatieve_luchtvochtigheid)
In rare cases, the humidity value could creep up above 15% (when the actual humidity is 100%) and still give wrong values.
- Pressure sensor (BMP280) failed after one year outside. Probably due to the long period of fog we had in the winter.
- Rain fall meter is sensitive for mechanical shock. Mount it on a sturdy pole to prevent false readings.
## License
Copyright (C) 2023, 2024 M.T. Konstapel
Copyright (C) 2023-2025 M.T. Konstapel
### Software

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