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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
In addition to the other great suggestions here I wanted to point out that you can practice and learn a lot for free using an online simulator such as wokwi.com and tinkercad.com (among others)! And you don't have to buy an Arduino or any parts to get started!
NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a more popular project.