Author Options:

Will this RGB LED manual color changer circuit work? Answered

I am working on making a lighted banjo head, and thought to use an RGB LED strip (http://www.sparkfun.com/products/10259) so i can change the colors. I am new to electronics, and have no real programming experience. As such I designed this curcuit to work with basic parts i can find easily at hobby stores.

I reasoned that if i increased the resistance of the circuit with a potentiometer, I could dim the LED's. The led strip takes 0.02 amps at 12 volts and is common anode (it has built in resistors for each LED). I cant find low resistance potentiometers so i thought to put 5k ohm pot in parallel with 680 ohm resistor to get the required  resistance of 600 ohms to shut off the LED.  

I have looked around the internet and haven't seen anyone use this method, so im wondering if it will work. Any help is appreciated as i hope to make an instructable of this project!


Here is a circuit diagram:



Best Answer 6 years ago

Wire the 5K pot's like this:



Using this circuit, wouldn't the 680 ohm resistors just up the resistance instead of lowering it? the original reason for having them was to decrease the resistance of the potentiometer. Also, it looks to me as if this is wired as common cathode, however, the LED strip is common anode.

Sorry if i am misinterpreting this, and thanks so much for the help,

OK, use lower than 680 Ohm then.
And if it's common-anode swap the 12V +/-


Why do you want to do it like that?
It's the same as before, what interests you in doing it that way*?


*I have looked around the internet and haven't seen anyone use this method

My knowledge of electronics is limited so far to P=IV and V=IR, and my programming skills are limited to basic drag and drop commands for things like Vex and NXT. i want to use this project to learn electronics starting form simple circuits, then working my way up in complexity and function until it is eventually microcontroled. This seems to be the simplest way possible to control the led colors therefore a good starting point.

While i see the general function of the circuit you drew the complete reasoning escapes me. my goal is to learn "why" so i can use the same principals in the future.

The change in this drawing is that I drew it with the potentiometers wired properly as rheostats instead of as standard resistors.

Thanks again for the help and sorry for taking so much of your time,

i'm starting to understand it. one question though. using this setup does it mater what resistance the potentiometer is?

You'd be OK with 5K, but too low and you waste energy through it.


so with this the resistance doesn't directly affect the voltage like a rheostat, but just acts to divide it?

Yes, divides it, and you've got a very-certain on-off there.


cool! Thanks for all the help! You'll be seeing an instructable of this project up some time in the next month or so with special thanks to you!


It won't work. There are pot positions that would have very low resistance, and that would blow your LED.

It won't really work. You will only be able to move the pot a little bit to change the brightness. The best way to do it would use a circuit like this: Make the resistor I marked as 0R5 into 68 Ohms.

You'll need 3 copies.


trimmable current source.JPG