How to detect led is on

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hi. i'm trying detect changes on power suply. have uncontinuous power supply , has tree leds indicating state of charge, green 1 when ac power supply on, yellow 1 when charging, , red 1 when ac power supply off.
i don't know how connec tto arduino , grove base shield. leds connected positive born , +5v. ground of each led selected on/off internal ups relay. can tell me how connect arduino , detect charge / led state changes ??
thanks in advance

connect arduino ground power supply ground.

connect 1 of arduino digital i/o pins switched side of led.

enable internal pull-up resistor on i/o (input) pin.

when led off, input read high (pulled-high internal resistor , leakage through led).   

when relay closes pulling led pin ground , turning on, input pin pulled ground , read low.


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