I have three black boxes each controlling an LED output. Each black box takes an input voltage and provides an output voltage (ranging from 0-12V) to illuminate the LED.

Is it possible to alternate between a selected black box and have a common LED? Would I need a component such as a diode where there is the dotted line to prevent current entering the other two controllers?

I would appreciate some help with this. Many thanks.

![Block Diagram of intended circuit](https://i.stack.imgur.com/AjxXq.png)