Hi,
i have 3 groups of 3 rgb leds which i want to multiplex to safe some pins and ofc safe soldering work.
Just have some basic knowledge on this and need advice
This is the intended shematic.
The current limiting resistors have not yet correct value (they are calculated for one led not for three in parallel). I will turn on one color at a time on 3 leds the same time (so three groups of leds)
i know that the 'color picking' transistors will work and i tryed this but i am not sure about the one connected to the anode as the transistors are not connected to ground but to the leds i guess i will draw current from the arduino and this will not work?! Can u give me some advise on that shematic?
Here's a possible solution as an attachment. I had to mask some of your schematic and was not able to get rid of the black outlines.
Q1, 3 and 5 should be PNP not NPN Transistors, especially if V+ is greater than the V+ of your Arduino.
You will now have to pull the Base on each of those LOW to turn on those Transistors.
Q2, 4 and 6 where wired completely wrong because the emitters should be going to ground.
Here's a modified schematic and an alternative, better way to do this if these are individual and not RGB Led's. R10 - 18 are 10 - 22 Ohm Resisters that help compensate for the slight forward voltage difference that Led's have. These will help to make sure each LED is much closer to the same brightness when in parallel.
The section starting with Q10 is a modified version that places the resisters on the anodes of the LED's and puts these LED's in series. This will eliminate the problems with brightness differences in each of the banks. Another big benefit is that the LED's will draw much less current because they are in series instead if parallel. V+ for the LED Circuit must now be higher than the 3.3 or 5V Arduino Voltage.
Edit: You could also use N and P Channel MOSFET's which would reduce voltage loses and allow you to drive higher currents with much less heat dissipation than BJT Transistors will have.
Ah, okay. Somehow i always connect npn transistors to my arduino didn't even thought about pnp ... makes perfect sense.
I can't put the leds in seriel as i have common anode rgb leds. So i have to stick to that parallel approach.
I wanted to use a usb charger as power device for the arduino and use the 5V pin on the arduino to power the leds.
Three leds in parallel will draw 60mA this should be fine for that pin shouldn't it?
Will do: 5V --> pnp --> three leds --> npn on each cathode --> gnd
Do i need a pull down transistor on the leds to 'switch' em off more quickly when i turn of off one the transistors?
