I am creating a little man in the middle circuit to hijack an LED in an appliance.
I am wondering if I have the circuit right-
I have simplified the appliance circuit down to the LED circuit to focus the clarity im looking for and put together a proposed circuit below it.

The cathode of the LED is switched to power the LED in normal operation.
Am I right in thinking I will need a diode infront of each MCU pin so while the appliance pin is HIGH(LED OFF state), I can light the LED with the new MCU with a LOW state without current flowing from the appliance MCU pin? and vice versa?

I have also added a 10k resistor to stop the LED floating but im certain the appliance has such a resistor already so perhaps this is unnecessary?

Does this all seem correct?

 

What voltages are VDD and VCC with respect to GND?

 

An LED does not need that resistor to avoid floating. It will conduct when the voltage increases in the forward direction, but not enough to produce noticable light, except maybe in total darkness.

 

Looks like your LED is shown backwards, other than that and no need for the pull resistor your circuit looks fine as a typical diode OR gated LED.

 

I have also added a 10k resistor to stop the LED floating but im certain the appliance has such a resistor already so perhaps this is unnecessary?

Once you turn the LED around, there will always be a small current in it. Whether that will be enough for it to appear to be on depends on the LED.

 

Thanks for all the help everyone!