I'd like to control 3 status LEDs with two different PCs:

  • an OrangePI-PC, using the GPIO pins as power source (same pinout of a RPi2);
  • regular desktop PC, using the parallel port as power source.

Both PCs can be powered at the same time and shall share the same LEDs (there is no need to enforce a priority between them in case of conflict).

This is the wiring scheme i am thinking to use (sorry if it is a bit confusing, this is the first one i've made with fritzing):


My ideal solution should rely only on discrete component (diodes, resistors, etc.). I'd like to avoid using ICs, Arduinos, etc.

UPDATE: i've made the circuit and it works pretty well. I've also experimented with other interfaces, but in the end i've settled with the GPIO pins and the LPT port. Here it is the driving script in Python.

  • \$\begingroup\$ Looks ok except Blue LED Vf is 1V higher so lower current. \$\endgroup\$ Commented Feb 27, 2017 at 23:15
  • \$\begingroup\$ there is no need to have diodes in series with the LEDs cathodes too? \$\endgroup\$
    – eadmaster
    Commented Feb 27, 2017 at 23:19
  • 1
    \$\begingroup\$ The diodes ensure that one controller holding a pin low/high isn't feeding that straight through to the other holding it the other way. There is no such requirement for the cathode as long as you can safely assume that the two can be grounded together. \$\endgroup\$ Commented Feb 27, 2017 at 23:23
  • 2
    \$\begingroup\$ We call that Diode OR logic \$\endgroup\$ Commented Feb 27, 2017 at 23:24

1 Answer 1


As mentioned, this is a simple diode OR bridge. It allows two sources to power a single circuit without back feeding each other. Prevents driving an output high when its own supply is off. Keep in mind that whichever has the higher voltage will be the actual source, but you already started that this is not an issue.

As you know, there must be a common ground between the two devices for this to work.

  • \$\begingroup\$ i've also tried by accident having a single ground connected and it worked as well. Not sure if it safe btw. \$\endgroup\$
    – eadmaster
    Commented Apr 30, 2017 at 22:32
  • \$\begingroup\$ You may have connected the grounds in a different way, possibly through a power strip \$\endgroup\$
    – Passerby
    Commented May 1, 2017 at 0:09

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