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I have a very simple circuit, and I want to use a tri-color LED to indicate which 1 of 3 outputs is on. I currently have a design that works for a common cathode 4-pin LED. However, I have easier AND cheaper access to this function of LED with a common anode.

It has to be red, yellow, and green. Each output has to drive a separate 10Ω resistor, exactly as shown in the pics. Only 1 output is on at a time.

The picture on the left is common cathode, and works as intended. How can you change the picture on the right (common anode), in order to acheive the same result?

enter image description here

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  • \$\begingroup\$ In the right-hand drawing, you could place R5 and RYG just to the left of the Green, Yellow, and Red power sources... (With R5 to GND, and the cathodes to the - supply terminals.) But maybe I'm misunderstanding something. \$\endgroup\$ – TimH - Codidact Feb 13 '14 at 23:01
  • \$\begingroup\$ I am not able to physically connect anything to the negative side of the power supplies unfortunately. \$\endgroup\$ – brett s Feb 13 '14 at 23:05
  • \$\begingroup\$ If you're talking about connecting the cathodes to +, that will not inhibit current flow through the LEDs. \$\endgroup\$ – brett s Feb 13 '14 at 23:06
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You could do this (resistor only shown for one channel). You could use a single ULN2003A in place of all the transistors and all the resistors except for the 500R, and that is precisely what I would suggest doing.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ I am going to avoid adding components to the design if I can. Thanks for this thought though. If I absolutely have to, I'll wait for the common cathode LEDs to hit stock. \$\endgroup\$ – brett s Feb 13 '14 at 23:02
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If you don't want to use additional components then the only alternative I see is reversing the sources, assuming the load nature and what is shown as source allows this.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ With this, what keeps V1 electrons from going to V2 or V3? \$\endgroup\$ – brett s Feb 14 '14 at 22:03
  • \$\begingroup\$ @bretts The resistors. This has the same behaviour as the schematic you have posted. The source is just a voltage potential, it doesn't make a difference which polarity is common to the circuit. \$\endgroup\$ – alexan_e Feb 14 '14 at 22:18

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