Friday, October 9, 2009

A well deserved plug

I did not populate my Lightuino LED board with excessive LEDs. In retrospect, that is kind of ironic. But the board is made for you to put the light where YOU want it to go :-).

In particular I left off the TX and RX LEDs so you can't tell if it's downloading the sketch. But it does have power and the pin 13 LED. So... I decided to hack the bootloader to blink the pin 13 LED as the sketch comes down. Well guess what! Its already been done! And it worked flawlessly, without me even having to do a compile.

Thank you LadyAda and Brian! These are people who are building the community, not just selling knockoffs. So I say, if she stocks it here or he stocks it there then buy it from them!

[Disclamer: My only affiliation is as a satisfied customer of adafruit -- I use her USBtinyISP and the Boarduino]

Tuesday, October 6, 2009

Lightuino Design Thoughts

From references

In designing the Lightuino I wanted to improve on my existing CCShield in several ways and provide an alternative to the Rainbowduino (which is also an Arduino compatible LED display board).

New Features!

NOT (necessarily) a shield!

The biggest change is the addition of the ATMEGA CPU, clock, etc logic, making the Lightuino a standalone board!

Of course, you can still use it as a shield, either WITH the ATMEGA populated or WITHOUT (if the ATMEGA is not populated, it MUST be used as a shield of course). And you can stack them. If it is stacked, it ought to let you do some really interesting distributed computing stuff.

By integrating the ATMEGA on-board, the total cost is a lot less, since you don't need the Arduino board. And its a lot smaller too!

Separate LED power Regulator

An LM317T was added to the board, with a 10k adjustable POT, so you can deliver consistent power to the LEDs (or to whatever) even if your wall-wart is unregulated. As long as your wall-wart provides the voltage, you can go up to 30+ volts, allowing each CCShield sink to control a string of LEDs. However the M5451 chips can only handle a 13 volt differential, so you must make sure that the voltage has dropped down to this before it enters the chip.

IDE cable headers

If you are stacking Lightuinos, the IDE cable header does not quite fit in a stack, so right angle headers are available.

The IDE cable pinout was also changed to put all of the power lines on one end, and to NOT provide a ground line unless a jumper is inserted on the board. Additionally, the pinout is the same on both sides (instead of upside down). This makes it a lot easier to wire up the LEDs AND also makes it very difficult to accidently burn out a LED by wiring it up to ground (for example) instead of to a current sink.

Finally, one of the lines is unassigned, and accessible via a pin on the board. So you can push whatever signal out that you want!

Pin Selection

Just like the CCShield, you can select whatever Arduino (digital) pins you want to control the LED portion of the board. But the CCShield's pin selection was difficult as it used extremely small SMT pads. The Lightuino simply has a section of the board with some thru-holes and you use jumper wires to make the selection.

Brightness Adjustment

It turns out that the M5451 chips can drive the LEDs at a barely discernible difference in brightness, so trim pots were added to the brightness adjustment to allow you to make the chips output the exact same light intensity (or actually, you could make one chip output bright, and one dim). Of course, this can be simply populated with resistors to save $ if exact brightness levels are not needed. It is unlikely that these will be needed for casual use.


The ATMEGA 328 QFN package has 2 additional analog inputs, so I brought those out by simply extending the Arduino standard pinout.

Also, I had extra space so I threw in the footprint of a voltage divider per analog input pin since that is a very common circuit. Just add your own resistors, and you are all set!

Comparison with Rainbowduino


Let me start by saying that I don't have a Rainbowduino, so this is all conjecture from reading the specs. It seems to me that it was mostly focused on driving an RGB 8x8 LED matrix display. In fact you can even plug a matrix directly into it. I was more interested in driving individually placed LEDs for art projects. While LED matrices are cool and have their uses in signage, frankly, if I wanted a big screen I'd just buy a flat screen TV! But whatever floats your boat. :-)

Of course, you can always use a LED matrix electrical connection, but not have the LEDs in a grid, but it is trickier to do that wiring... additionally you can use the Lightuino to "sink" the columns of an matrix (70 columns instead of just 24!)... but you would need to power the rows with some other circuitry. So again, its trickier :-).

Shield Compatiblitity

Additionally, the Rainbowduino does not seem to be Arduino shield compatible. This seemed to me to be a major drawback as it does not take advantage of the vast community of shields out there that can add so many cool features to a project!

CPU Power

The ATMEGA 168 is pretty limited in RAM, as I found when working with the normal Arduino, so I moved up to the 328 with double the RAM.

LED Capability

I opted for 2 chips that can drive 70 constant currents outputs at 20mA, the Rainbowduino allows you to drive fewer outputs (24) at higher capacity (120mA).

PWM Dimming

I wanted to experiment with POV art (i.e. a light strip that forms an image floating in space when your eyes flick by it, or spinning a strip to form in image in a circle) and to do that you need precise control of when the LEDs are ON vs OFF. In other words, a separate PWM chip won't work. However, this means that the Lightuino has to drive PWM in software, just like what the Arduino's PWM pins do. At very low PWM levels (say 10 on out of 256 counts) you can perceive the LED blinking (and of course more Arduino processor time is used). Therefore, you end up with another tradeoff, precise control over the blinking of each LED verses faster blink rates (note, I'm just assuming the Rainbowduino has an external PWM, but I haven't verified that).

Saturday, October 3, 2009

Lightuino V2.0 -- An Arduino compatible optimized for driving LEDs

The CCShield board was pretty successful and I had a lot of fun with it, so I decided to do another run. I wanted to attempt some surface mount work but at the same time not produce a lemon if it did not work, so I created the hybrid board you see above. The board has a complete Arduino-compatible subsection on it implemented using surface mount parts and the CCShield (plus some extra goodies) implemented using thru-hole parts. The idea is that the board can be used as a shield if the surface mount parts are not populated, or used standalone if the parts are.

Another cool feature is that you can stack them -- so I can experiment with running a bunch of processors simultaneously.

I'll be adding a couple of posts about this board. This first is about building up the surface mount parts. The image above is the board with the surface mount parts baked on and the minimum thru hole parts added to bring power and the ICSP programmer on.

I used the smallest part I could find for the CPU, QFN-32 and 0603 parts for the resistors and caps. The tricky part of the QFN32 package is that the leads are underneath the part, making it hard to repair solder bridges.

To make my surface mount station, I "rescued" an old toaster oven from the dump and "splurged" on some solder paste, rosin, and wick from dealsextreme dealextreme. Then I bought some .5mm mechanical pencils from Staples, some tweezers from Walgreens. Total cost about $20.

This supplemented my current thru-hole solder station that consists of a plastic clamp (Home Depot) to hold a circuit board, a radio-shack 40 watt soldering iron, and an old a magnifying glass/light on arm combo from the basement (really its the bright light that's important).

I then used the mechanical pencil to "paint" the solder paste on the leads. For the QFN part, I just laid the paste across the leads and then used an Exacto-knife to "cut" the spaces between them. It seems like the biggest pitfall is simply using TOO MUCH PASTE. You want just the thinnest layer, and do not even cover the entire pad!

Then I just placed the components in their positions using the tweezers, and did fine adjustment of the QFN32 part with the edge of the tweezers.

Baking was easy. I followed the temperature profile here. But not really... I used the thermostat on the oven, not a real temperature gauge. Basically, just set your oven for 170 C put the boards in and wait 3 minutes. Then raise the temperature to 220 C and shine a bright light in so you can see the boards clearly. After a minute or two the solder will go from grey to silvery. Wait another 30 sec to a minute to be sure that ALL the solder has gone silvery. Then turn off the oven (but leave the boards in there) and crack open the door. You want to cool the boards evenly. After a few minutes, open the door fully, and when they are cool to the touch, pop them out!

I did 4 boards and had 2 solder bridges on the QFN parts. Every other part worked without any issues.

If you have a bridge (you can see it), paint the entire side of the part with a gob of rosin. Then place your solder wick over all of the leads and heat it all up with a clean iron on top of the wick. The rosin will help the solder flow and it will either flow onto the pins, or onto your wick. Easy!

So if you are a DIYer who has been leery of using SMT parts, I'd say "go for it"!

[edit oct 15, 2009: I just did 12 successful boards without any solder bridges. Mix 50/50 paste and flux so the result is viscous like maple syrup]