You might think, that as long as you are careful to to exceed the 3V input voltage limit, then you can measure any low voltage whatever the source. Perhaps a photoresistor in a voltage divider arrangement with a fixed resistor.
While this is basically true, if the source of the voltage to be measured has a high output impedance, at some point the voltage measured by the micro:bit will diverge from reality as the impedance of the voltage source being measured increases.
The micro:bit does pretty well up until a value of R of 10kΩ after which it goes down hill pretty rapidly.
If you want to use your micro:bit to measure a voltage, try and keep the output impedance of the voltage source lower than 10kΩ.
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