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


closed as unclear what you're asking by Passerby, PeterJ, Matt Young, Chetan Bhargava, Leon Heller May 4 '14 at 5:55

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Assuming each already has a resistor at the output that limits the current through the led to 5~20mA, a transistor at each output will work.


simulate this circuit – Schematic created using CircuitLab

If any of the outputs is high, the led will be on.

  • \$\begingroup\$ What about if the signal is analogue (varying between 0V and 12V)? \$\endgroup\$ – Michael May 3 '14 at 22:16
  • \$\begingroup\$ @Michael is it? \$\endgroup\$ – Passerby May 3 '14 at 22:17
  • \$\begingroup\$ Yes, the controllers fade the LEDs, so I would imagine so. \$\endgroup\$ – Michael May 3 '14 at 22:19
  • \$\begingroup\$ @Michael so its PWM? That's important to note in the question. That said, a transistor will work with PWM, but the amount of current through the output is important. \$\endgroup\$ – Passerby May 3 '14 at 22:22
  • \$\begingroup\$ I don't really know exactly what it is or how they work. \$\endgroup\$ – Michael May 3 '14 at 22:28

A simple diode from each box to the LED will work, if by "work" you mean "protect the controllers from each other." If you want some way to let each controller take control of the LED, that's a different story.

If each box puts out a DC voltage, using diodes will cause the LED to light to the greatest brightness commanded by any of the 3 controllers. If the output is a PWM waveform, the output will vary wildly with time, from the greatest commanded brightness to as much as full on.

If you insist on connecting all three controllers to the same LED, you'll have to make sure that the "unused" controllers are set to zero brightness. If you do that, a simple set of diodes will work fine, regardless of how the controllers work.


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