Control color of Bi-Color LED using Arduino

Question

I have this push-button switch which has two solder lugs for 12 VDC power and will illuminate red or green depending on the polarity of the wiring. It may not actually use a bi-color LED, but it seems to be a good representation for how it works. I can't find a schematic for the switch, though I expect it may be something like this:

I'm providing the 12V using an LM7812C voltage regulator but I'm struggling to figure out how I could control the polarity of the applied voltage using an Arduino. My idea is to have an Arduino control a MOSFET to switch power to the LED, though I get lost trying to find a way to flip polarity.

Is there a way I could control the LED to be red, green, or off using outputs from the Arduino?

Or alternatively, just have the switch default to one color and switch to the other when triggered by an output from the Arduino?

Thank you!

Answer 1

You could also do it with minimal components with a suitable dual op-amp configured as two comparators driven by two outputs from the micro.

High on one low on the other would turn on a particular color. Both high or both low would turn it off entirely.

^{simulate this circuit – Schematic created using CircuitLab}

NOTE: R1 here is assumed to be built into the switch so your LEDs don't fry at 12V. But you need to verify what that illumination method really is.

ALTERNATIVE APPROACH: You can also hook the amps up as simple amplifiers with enough gain to drive the outputs to the rails like below. It requires four more resistors but gets rid of some issues with driving op-amps as comparators.

^{simulate this circuit}

Answer 2

If the switch has two leds as drawn and does not include a current limiting resistor, you can connect a GPIO pin to each side of the LED. If both GPIO pins are high or low together, no current will flow and the LED is not illuminated. By making one GPIO high and the second low you get one color; by flipping the values of the GPIO pins you get the second color.

The 330ohm resistor value assumes you are operating the arduino at 5V, and that you are not looking for a lot of brightness in the LEDs.

If there are resistors inside the switch for 12V operation, you will need to use external drivers. The two half-bridge solution suggested by jonk is probably the easiest possibility. Most 12V tolerant peripheral drivers are open collector (they only pull the output down to ground and leave it floating for a high output). If you want to use transistors, put a 100ohm pullup resistor on the output of each of the low-side switches. You will be wasting power on the side that is pulled low, but if efficiency is no critical this will work. Ignore the mosfet part numbers, tiny mosfets will work (or something like a ULN2004 or other bipolar transistors).

^{simulate this circuit – Schematic created using CircuitLab}

^{simulate this circuit}

Answer 3

You need three states: COLOR 1, COLOR 2, and OFF. It appears that the layout of your switch is:

If you want to use bubblegum parts (cheap, readily available, not special in any way, and supplied by many sources), I'd probably go with four NPN and two PNP, as in:

^{simulate this circuit – Schematic created using CircuitLab}

The advantage here is that it will supply very close to the full voltage across the LEDs in the switch. The disadvantage is the number of parts involved.

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A simpler approach using passive high-side resistors comes at a far too great a price, in my opinion. The trade off between very significant voltage loss to the LEDs, extremely high excess wasted currents in the resistors, and gross power dissipation in the resistors (like "watts") simply cannot be made acceptable.