RGB LED driver, Any comments?

[paxmowa]

Hi there,

I'm thinking of building a RGB LED driver for 8 LEDs. I want to use a PIC16F777 for this task as it has a very convenient 3 hardware PWM channels.

I've made a little schematic, but have some questions.

larger image:
http://users.telenet.be/maxpowa/schema_RGBleds.jpg

I wonder, are the transistors I've choosen OK for the task at hand? Should I use MOSFETs instead, if so, which mosfets are my best choice?

I would power the circuit with an MC33269T-3.3 LOW drop 3.3Volt regulator. Is this OK or are there better options to power the circuit?

Anything missing or other comments?

Thanks in advance,
Steve.

[keithdoxey]

Shouldnt the placement of the transistors be transposed.

PORT D
PNP collector to +5V feeding emitter to common Anode
Take the base to 0V via resistor to turn on an LED

PWM Outputs
NPN emitter to ground, Collector to R,G,B common cathodes
Take the base to +5V via resistor to turn on a colour.

You might also like to take a look at the RGB LED Controller Project http://www.rgbled.org

I am just waiting for the bits for my kit to arrive

Just my thoughts.

Regards

[paxmowa]

Dude! You're so right.

Lack of sleep makes pax go crazy.

Thanks a bunch, i'll rework it after some sleep... or coffee...

Steve

[keithdoxey]

Dude! You're so right.
Steve

Apart from when I am wrong!!!

PORT D
PNP collector to +5V feeding emitter to common Anode
Take the base to 0V via resistor to turn on an LED

Should read

PORT D
PNP *EMITTER* to +5V feeding *COLLECTOR* to common Anode
Take the base to 0V via resistor to turn on an LED


That'll teach me to reply in a hurry before dashing out to collect the kids from school
DOH !!!!

[paxmowa]

Thanks for the efford Keith.

After a bit of sleep and with a cleared head I made up this:

Updated:


Swapped PNP and NPN transistors around, replaced BC547 by BD139.

It aint finished yet i guess.

I checked the specs for the RGB leds and green and blue chips are rated 3.5 to 3.8 volts, red chip is GaAlAs rated at 1.85 to 2.2 volts. The 3.3 volts from the voltage regulator is to high for the red chip and to low for the blue and green. Will prolly go to 5V supply and put resistors in series with each chip.

Anyways, thanks again for the help so far,
Steve

UPDATE:
This is the third version, added resistors in series with each led chip, this should be almost complete i think. Now only add the PIC16F777 and power supply.

[paxmowa]

hmmm, I figure there's also the voltage drop over the transistors. about 0.6 volt each.

So the resistors for blue and green should be 10 ohm and for the red should be something like 82 ohm.

Getting there... slowly, but getting there none the less.

Steven

[ScaleRobotics]

This is a pretty old thread. But thought I would add a link to a constant current driver circuit for 1 watt LED's. There are a couple other products around, but this is pretty low part count, and allows from 1 to multiple LED's, so its pretty versatile. It also accepts 7 to 30 volts. They also happen to use a PIC.

Project site: http://picprojects.org.uk/projects/hbld/index.htm

Calculator tool for ZXLD1350 chip: http://www.diodes.com/_files/calculators/zxld1350calcv2.xls

ZXLD1350.pdf