'Reading' LED state from independent circuit

I’m trying to use my Arduino to interact with a cheapo HDMI switch. It has 3 LED’s indicating which of the 3 HDMI inputs is active. I would like to ‘read’ the state of each of these LED’s on the digital input pins.

With my trial and error attempts the pin reads high whenever it is connected, regardless of weather the LED is on or not.

Can anyone point me in the right direction?

Attached photo to show the HDMI switch circuit and the grey and purple wires I’m using to attempt to read from.

With my trial and error attempts the pin reads high whenever it is connected, regardless of weather the LED is on or not.

Are those wires part of your attempt to read some values?
If so you need four wires, one for each signal and the other as a common ground.

What voltage do you measure when the LEDs are on and off?

Indeed they are! I was just focusing one the left one to see if I could read that before soldering up the rest.

Using my multimeter direct on those two contacts I get 3.3v when the input is active and 0.4v when its not.

However when i connect the arduino digital input pin to the positive and join the grounds I get 3.3v all the time.

However when i connect the arduino digital input pin to the positive and join the grounds I get 3.3v all the time.

That means you have not wired it up like you think you have.

Not sure how I could make a mistake on that?

Grey positive wire to digital input 40. (Tried other pins)
Purple ground wire to arduino ground. (Tried multiple ground pins)

Should that work?

Not sure how I could make a mistake on that?

No neither am I, but what you say only makes sense if you have misidentified pins on your hub and so wired up the wrong thing to the wrong thing. Those voltages will work with a 5V Arduino. Could you have made some sort of error identifying the ground on the hub?

A picture that actually shows both ends at the same time would help.

Appreciate your help.

Not easy to get a clear photo but best attempt attached.

Tried with a NodeMCU and got the same result (the device I hoped to use but generally easier to test on the Mega)

I remeasured more accurately on the multimeter. 3.3v when led is ‘on’. 3v when led is ‘off’ on the mcu, 2.8v when low on the Mega. Both still read high with the led on or off.

I just noticed on the Mega it reads HIGH with nothing attached, whereas the NodeMCU reads LOW. I assumed this would be some kind of pull up / down resistor kind of problem but my guesses of resistors in various places doesn’t seem to have helped.

I can't see where you have got the ground connection on your hub in that last photo, you seem to have lost it from the first photo.
On the first photo it dosn't look like that is the ground, use one of the connections of the sockets shield.

Hi,
Suggestion why you do not use an Opto-isolator. That will give isolation between the Arduino and the HDMI. Since they are using and led your are replacing the HDMI led with the Opto-isolator led.