Practical PIC Projects

 

 

RGB LED PWM Driver for High Power 350mA LEDs

 


Description

This project combines a PIC and three constant current 'buck' converters to produce an RGB LED controller that will operate with the the high power 350mA LEDs using PWM to control the LED brightness.  By driving the red, green and blue LEDs with varying pulse widths the controller can generate up to 16 million colours using fades, strobe and static effects.

The use of surface mount components and the low power dissipation in the three current sources allows for a very compact design.

The circuit can drive one or two LEDs in each of the three channels and will work with devices from Luxeon, Prolight, Laminar, Lumileds and others.

The project uses a slightly modified version of the code used for the Standalone RGB controller described elsewhere on this site allowing the sequence data file to be used from the other controllers.

As the board uses surface mount components this project is not really suitable unless you have the soldering skills and experience to work with this technology.

Mood Light

Here we have a mood light made using the circuit described on this page and a light fitting I bought from Homebase DIY store.  It's a 26cm metal rim flush light fitting with brushed chrome finish.  The frosted glass dome diffuses the light from the LEDs while the silver metal base helps distribute the light evenly.

Assembly was kept very simple.  A hole was drilled in the base for the 'mode' switch to fit through and the PCB was fixed down with double sided tape.  Another hole was drilled and then filed square for the DC power connector which was secured with two M2.5 screws.  The RGB LED came from www.led-bulbs.com  It wasn't fitted to a heatsink so one was made from a piece of copper plumbing pipe cut open and flattened; the heat spreader in the base of the LED was then soldered to the copper.  When the whole assembly is screwed to the metal body of the lamp fitting it provides excellent heat dissipation for the LED. 


 

Once assembled it can be fixed to the ceiling, hung from a wall or simply placed on top of a table or other furniture.  If it's placed on a flat surface at eye level when you're sitting down, the low profile of the fitting illuminates upwards without being too obtrusive and the overall effect is very pleasing.

The second photo strip above is a Mylonit 45cm white lamp from Ikea fitted with the Rev3 PCB


Schematic

 

 

 

download

You may also be interested in the Revision 3
 PCB and schematic for this project here

  .

  • The Zetex ZXLD1350 and ZLLS1000 parts are available from Farnell and Digi-Key

  • Inductor Panasonic Part No: ELL6RH680M, Farnell Order Code: 1198602.

  • The inductors listed below appear to be suitable on specification only, I have not tried them. 

    Panasonic
    ELL-6PM680M, Digi-Key part No PCD1714CT-ND

    Sumida
    68H Digi-Key part No 308-1536-1-ND CR54NP-680KC
    47H Digi-Key part No 308-1316-1-ND CDPH4D19FNP-470MC
    82H Digi-Key part No 308-1501-1-ND CDRH6D28NP-820NC
    100H Digi-Key part No 308-1488-1-ND CDRH6D28NP-101NC

PCB Artwork rev 2



Component overlay

PCB Artwork (PDF)

PCB Artwork mirrored (PDF)

 

 


Construction photos

Bare PCB

During assembly.

Completed board

Testing with 3.3V / 1.3W zener diodes as dummy loads (they will get hot)

Operating with a
 Laminar 4000 RGB LED

attached to programmer
 using ICSP header

Construction notes

The LED driver circuit is based on the Zetex ZXLD1350 device.  I've basically used their reference design here so if you have any questions about it the best place to go is the Zetex website.

Once you've etched the PCB inspect it closely, especially around U1, 2 and 3 where the tracks are very closely spaced, to ensure the board has etched correctly and there are no copper whiskers between the tracks.

All resistors and capacitors are 0805 sized parts except for C1, 2 & 3 which should be 1206 (you can just about stretch an 0805 here at a push)

C1, 2 & 3 should be at least 1F, larger values up to 10F will work.  However they should be multilayer ceramic types using X7R or X5R dielectric. Don't be tempted to substitute electrolytic or Tantalum parts.  For further information refer to the Zetex datasheet for the ZXLD1350

The Zetex ZXLD1350 driver IC and ZLLS1000 schottky diode are available from Farnell Electronics and DigiKey

The PCB has been laid out to allow a choice of inductor to be used.  The ones used on the board shown above were a 68H Panasonic Part No: ELL6RH680M  but you can use any suitable inductor in the range 47H to 220H.  I've built several boards using different inductor types on each one. Make sure that the inductor is rated for switching at over 350mA and choose ones with low DC coil resistance, ideally less than 0.7ohm.  If you're stuck you can try the following parts

Farnell:
Panasonic Part No: ELL6RH680M, Farnell Order Code: 1198602

Digi-Key:
Panasonic ELL-6PM680M, Digi-Key part No PCD1714CT-ND

I used a BAT54 part for D4, you could substitute a ZLLS1000 part used in the regulator section here without any issues. However do not use a BAT54 for D1,2 & 3

Before attaching the LED's I recommend you test the board with dummy loads. I used 3.3v / 1.3watt zener diodes while I checked the switching circuit was functioning correctly.  This is much cheaper than killing your RGB LEDs due to a fault or construction error.  The base of Q1, 2 & 3 need to be held low for the ZXLD1350 to operate; you can do this by holding the PIC in a reset state by pulling the MCLR pin to ground.  This will work before the PIC has even been programmed.

The RGB LED(s) need to be mounted on suitable heatsinks as they will be dissipating up to 1 Watt each at maximum brightness.  For the single package RGB LEDs where three dies are mounted in a single package this will require a substantial heatsink.

I've provided two push button switches and a serial interface.  I plan on using these with future code releases however, with the code provided on this page only SW1 is used or required.  You may wish to omit  SW2, Q4, R8 & R9 parts.

The green SMD LED indicates when the PIC is powered (either from the ICSP header or the onboard regulator).  It is not essential and you may wish to omit this and R7.

I originally used a single row right angle header for the ICSP connector and mounted it through-hole on the reverse side of the board.  However, as you can see in the photo below, I've swapped it for a R/A header tacked vertically onto the copper side of the board.  This keeps the top side of the board clean except for the switches.


High Power LEDs

Warning: Do not look directly into high brightness LEDs;  it can damage your eyes.

The constant current driver circuit requires that each LED has isolated connections.  RGB LEDs that share a common anode or cathode terminal are not suitable for use with this circuit.

Only use LEDs designed to operate at 350mA or greater.  Standard 3mm and 5mm LEDs will be destroyed if connected to this controller.


Programming

The board features an ICSP header so the PIC can be programmed in-situ.

Connections to this header are shown in the schematic and on the component overlay.

The ICSP connector provides 5 volt power to the PIC during programming. Do not connect power to the board when the ICSP header is connected.

Very important:
When the board is not attached to the programmer via the ICSP header a shorting jumper must be connected between the Vpp and 5V pins of the header to ensure the PIC MCLR pin 4 is pulled high and is not left floating.  The 12F683 part has an internal pull up on the MCLR pin so this only applies when using the 12F629/675 parts (See photo right -->)


Software

Download:

  • Download Source code (zip file)
  • HEX file for 12F629 (right click and save-as)

NEW  (12/04/2008)
Version 3 firmware for this project is available for download here

This code will assemble (using Microchip's free MPLAB IDE  ) for use with a 12F629 / 675 or 683.  The .HEX file above was assembled for use with a 12F629.

The colour sequences included are a minimal set for testing. 
They are:

  1. Constant red 100%
  2. Constant green 100%
  3. Constant blue 100%
  4. All off
  5. White 5%
  6. White 50%
  7. White 90%
  8. Red fade up/down; Green fade up/down; Blue fade up/down
  9. Red fade up / blue fade down; Green fade up / red fade down; Blue fade up / green fade down;
  10. Blue 5% blinking

Sequence data information

Since the supplied SequenceData350.inc file contains only basic sequences for testing you will want to modify the SequenceData350.inc file with your own sequences and reassemble the code. 

The format for this file is exactly the same as that used with the other RGB Controllers on this site.  For an explanation of the sequence data format see Standalone RGB controller SequenceData.inc file.

The link below contains the sequence data for a mood lamp I have in my room at home.  To use this one, download it and replace the one in the ZIP file above, then reassemble the code.

Download: