Giant RGB Digital Clock - Page 1
Forum section: electronics / projects

author: Things
author_rank: Regular Contributor
timestamp: August 14, 2012, 02:24:19 pm
content: Hey guys,So I'd take it most of you have seen Sparkfun's giant wall clock:http://www.sparkfun.com/tutorials/47Well, I've decided to make my own spin off of this, except bigger, and RGBOriginally I was going to have a central uC board for driving/multiplexing, However, I've ditched my original design and have decided to do individual control of the LED strips via I2C.This means that:I only have to run 4 wires to the digits, power and data.Each digit can just be daisychained - so only 4 wires back to my uCNo multiplexing, so I gain a heap more brightness and can also drop the 2nd MCU off my design.I was forced to learn AltiumI decided to use the common WS2801 LED driver chip, with 3 MOSFET's for driving my LED strips. I decided to design them so that they'd solder right onto the ends of the LED strip.Because of this, I had to be able to put SMD components on both sides, which Fritzing doesn't yet do. A friend recommended Altium, and after going in circles trying to work out how to use it, I have a board design and off for fabrication in 24 hoursThe schematic for each driver (1 per LED strip, so around 46 of these). Main power is connected to Byp on the vreg - this was just a simple pinout fix.Comparison with an AAA and how it'll go onto the strip:Have sent in all the PCB's and ordered all the parts, so should have something working within 3 weeksShould be a fun and hugely impractical projectDan

author: T4P
author_rank: Super Contributor
timestamp: August 14, 2012, 02:52:17 pm
content: I like the photoshopping!

author: Things
author_rank: Regular Contributor
timestamp: August 14, 2012, 02:58:25 pm
content: Hehe yeah, it's a nice way to sort of get a relative feel on how it's gonna go together and overall size

author: David_AVD
author_rank: Super Contributor
timestamp: August 14, 2012, 09:36:33 pm
content: I think you have an error with your circuit. Have you tested it?The WS2801 uses open drain outputs. You need to use pull-up resistors to drive the MOSFET gates.

author: Things
author_rank: Regular Contributor
timestamp: August 15, 2012, 12:34:02 am
content: Hah! Indeed, crap!Ah well, the PCB's are cheapThanks for pointing that out! Just a natural thing to put pulldowns on N-ChansEDIT: Thankfully the PCB manu hadn't processed my file yet, so I was able to send a new revision in to themI also switched the G and R pads around, as I'd assumed the letters on the strip itself were a typo, but apparently not. Not a huge issue to change in software if I had to anyway.Don't like it as much as the old design, but had to run the 5V power trace which make it a bit more difficult:Dan

author: David_AVD
author_rank: Super Contributor
timestamp: August 15, 2012, 03:37:59 am
content: Probably too late, but you also need to ground the POL pin of the WS2801. (easily fixed with a bit of wire)This is to invert the PWM as you are using the open drain outputs to turn the MOSFETs off, not on.In case you're wondering, I have used the WS2801 in several projects / products.

author: Things
author_rank: Regular Contributor
timestamp: August 15, 2012, 03:51:43 am
content: Yes, that is what the LPH pads on the bottom are for. The datasheet was a bit indecisive, so decided to break the POL pin out to pads and put GND and +5 pads next to it for a simple solder bridge.Thanks for your inputDan