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Using Common Cathode Instead of Common Anode rgb led with IC? Answered

So I'm trying to do this: https://www.instructables.com/id/RGB-LED-Color-Sequencer-without-a-Microprocessor/ and I realized that I have the wrong kind of led..the author says you have to "adjust the connections" but I don't really know how. Right now my led will glow but not change colors. Does anyone have experience with this?



3 years ago

Instead of the LED being on the HIGH side, let it be on the LOW side. Instead of the common anode being connected the positive rail, connect the common cathode on your LED to the negative rail instead.http://i.stack.imgur.com/egSqx.png

Thanks for your response! I actually guessed that and tried to do it but it still is only doing one color. Unless you have some sort of idea what kind of mistake causes this I will rebuild the circuit from scratch (it is quite possible that I made a mistake in trying to fix it quickly.)

I have fallen for the simple trap of not paying attention if the device powering the LED has a "totem pole" output, (push and pull, sink and source) or just common emitter or common collector output, in which case it the LED must be on the HIGH side or LOW side for it to work.

Also, it should be noted, although probably not important for your application, that when you change the LED from the HIGH side to the LOW side, or vice versa, that the LED will light up inverted.

So say you are driving it from an arduino, and the light needs to be on. If the LED is on the LOW side, and arduino sources power to it, then setting the arduino pin HIGH will turn it on. HOWEVER, if the LED is on the HIGH side, so the arduino sinks the current from the +Vcc through the LED, then the arduino pin has to be set LOW. I hope that makes sense, it will make more sense when you mess around with circuits like that and learn it yourself. I have fallen for that trap a few times.