Your sketch works on a Nano (clone) that I have
I am using a high-brightness white LED with a 10kΩ resistor to bring the brightness down to a bearable level, so the current drawn is about (5v-2.2v)/10kΩ = 0.28mA. This is a low level of current and should be safe for poking around with.
I can see the embedded LED blink if I program the Pin 13, but no impact ...
The ADC measures a voltage ratio. The conversion result is the rounded
value of 1024×V/Vref, where V is the voltage at the input and
Vref is the voltage reference.
In the default Arduino configuration, the reference is connected to Vcc.
This will not fit your use case, as the voltage you want to read does
not vary proportionately to Vcc. You could use an ...