LED Driver

Hi all,

Just finishing up a project and I need to make a RGB backlight for a touchscreen. At the moment I have a single BlinkM MinM that is handling the colours fine, What I want to do is remove the LED off the MinM and use the data signals to light about 6 similar LEDs. I understand I will need some driver circuitry, Has anyone got any ideas that are compact in size? I have limited space. All LEDs will display the same color at the same time.

According to the BlinkM MinM Schematic, you want pick off the High signals coming from the ATTiny, and then buffer those to drive other sets of RGB LEDs, that I assume you want to have spread out?
Can you get Common Anode LEDs instead? Will make things easier.

You could use something like a ULN2003, ULN2803.
Put out 5V and three lines from the driver chip to pull the individual cathodes low.
I don’t think you’d even need to disassemble the source BlinkM, just carefully attach leads to the ATTiny pins going to the ULN2003, and distribute the leads out from there. Put the bank of 18 current limit resistor with the ULN2003, just fan out the +5 & RGB connections to the LEDs.
I’ll draw up a quick schematic.



So you are saying that there will be enough current left to drive the IC without removing the first LED? Also, I forgot to check whether the LED's were common anode, http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=96680916&uq=634337435716300644 these were what i am proposing, they are common anode. Will this invert the intensity of the LEDs?

Yes, the driver chip needs ~ 1mA to turn on the output. AT Tiny45 IO pins are rated for 40mA, if the LEDs used have Vf of 2v then its outputing ~ 30mA now, 31 would be okay. If vf is higher, then it is outputting even less current and provides more margin for your added 1mA.