Read the state from an LED in a MCP23017
As mentioned by Peter, you measuring inputs are all connected to ground.
You could do something like this







simulate this circuit &ndash; Schematic created using CircuitLab

This way you measure 660mV when the active LED is draining 20mA. By the way, 33R seems to be quite small, you may wanna doublecheck the effective forward voltage of your LEDs.