On the left of the schematic is the main system. It has a MCU that turns on Q1 that provides ground to the on board LED1 and also provides ground to 1 of 2 display panels to indicate that Q1 is ON.

Everything works fine, Q1 conducting or not conducting, when TO_DISPLAY is connected to DISPLAY_2
However, when TO_DISPLAY is connected to DISPLAY_1 and Q1 is NOT conducting, the LED illuminates dim because (I believe) of the voltage differential between the +12v on the anode of LED1 and the +5v on the Pullup at the Display MCU1_INPUT_PIN.

I can correct the Issue by adding a Diode to the Right of R3 and before the Drain of Q1 but I'm not sure that is the correct Methode. What would be the correct method so I can use either display panel without this issue?

Thanks

 

I would try a diode to the left of R4.
Or try using +5 V to the LED instead of +12 V.

 

I would try a diode to the left of R4.
Or try using +5 V to the LED instead of +12 V.

Thanks,
I think a Diode at R4 would do the same as a diode ahead of R3. I could change the LED voltage to a +5v source at the LED but I think then I would have a similar issue when DISPLAY_2 is connected, just in the opposite direction and the Opto will conduct.

I've been using a diode right of R3 as described above, so maybe I will just continue with that.

 

Some LEDs will glow with a very low current, but with enough voltage. A plain diode in series will drop the voltage by at least 0.6 volts and stop the glow. It will also provide reverse voltage protection for the LED.

 

Some LEDs will glow with a very low current, but with enough voltage. A plain diode in series will drop the voltage by at least 0.6 volts and stop the glow. It will also provide reverse voltage protection for the LED.

Yep. This is how I have been doing it, so I guess I'll keep it that way. (note addition of D1)