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I have an arduino mega that i am using as a clock. I would like it to detect if it is connected to a USB or external power supply (e.g. a battery). I then want to set a variable to different values depending on the result, so i can display it on an OLED.

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Use a simple transistor where the Vin (external power) is connected to the base, which switches the transistor. Than connect the collector to +5V, and the emitter to a GPIO input pin and read the status with the Arduino. If it receives a HIGH you know external power (via Vin) is used.

See the useful 'good' answer as comment of Majenko below ... I will leave the answer above as is, to keep his remarks clear.

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    Slightly more involved than that - you want to have the transistor (NPN or N-channel FET) between the GPIO and Ground, not +5V, and a pullup resistor (or enable the internal pullup resistor of the GPIO pin) and look to see if it's reading LOW for "External Power Applied". Also a resistor in the base of an NPN is needed.
    – Majenko
    Sep 19 '17 at 9:04

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