Just after getting back from 27c3 in early January Jeffrey and myself got talking about a project for TOG’s Paddy’s Day hackaton.
Our general idea was to create a little badge with LEDs in the shape of a shamrock. We were hoping for something small, light and bright with lots of LEDs. I said that I would do some more research to see if it would work and if it did to come up with a suitable circuit design.
I knew the general idea was to use a step-up (boost converter) to increase the voltage level so to that I could drive a string of LEDs. I then began to search manufactures sites for suitable components and relevant information.
After searching all the main contenders I found a very interesting range of chips in a small five pin surface mount package from ON Semiconductor. The most interesting options were the CAT4137 LED Driver, Boost, 5 LED and CAT4238 LED Driver, Boost, 10 LED. Another option from Linear Technology was the LT1932 – Constant-Current DC/DC LED Driver in ThinSOT. As I wasn’t sure what would work I ordered a couple of each component and the suggested supporting components. Along with lots of green surface mount LEDs.
I then designed and etched a simple proof of concept board. After soldering it up and connecting up a CR2032 coin cell battery it worked but I was disappointed with the brightness of the LEDs.
I then put the project on hold. Roll on a couple more weeks and I looked at this project again.
Investigating the proof of concept board I realised I had used a wrong resistor value and was trying to supply 30 milliamperes instead of 10 milliamperes into the string of LEDS. Changing this didn’t really help increase the brightness. But it did point me in the direction that I was just trying to draw too much current from the battery.
I found the following blog post on using cr2032 coin cells from Marcus from Interactive Matter (who I meet at 27c3) very informative. Connecting up the circuit to two AAA batteries solved the brightness problem.
I have now reworked the board into a near final layout. And as a token gesture to the 14th of February (Valentines Day) I’ve created a design in the shape of a heart.
Tomorrow I’ll finish the prototype of the shamrock badge and post a picture. If I created a kit would you be interested in buying it?
Update: Here is the photo of the prototype for the shamrock badge. When we looked at it after I assembled the badge it was hard to make out the leafs also I might change the resistor value to increase the brightness.
The photo shows a prototype of a small 16×16 RGB LED matrix display that I worked on during 27c3 in the Hardware Hacking Area.
The display is made up of four 8×8 RGB LED modules which are only 32mm by 32mm which doesn’t give much room to work with. Each module has it’s own TLC5947 a 24-Channel, 12-Bit PWM LED Driver from Texas Instruments sinking the columns and a A2982 8-Channel Source Driver from Allegro MicroSystems source the rows. The display is controlled by an mbed NXP LPC1768 micro controller. With a 74HC238 3-to-8 Decoder and a custom switch-mode power supply module rounding out the board.
Coming up with a suitable schematic and layout only took a couple of days as I’ve used all the parts before in other boards. Etching the board and drilling the holes took about two hours on xmas eve. Reviewing the board and schematic on Day 1 I discovered a problem with the way some part were connected. With a craft knife and some future solder bridges I was able to work around the problem. I’ve already corrected the schematic and layout if I decide to make the board again.
On Day 2 after Mitch Altman’s workshop on Arduino For Newbies I began to solder the board. The process was relatively straight forward with mostly surface mount packages in SOIC and TSSOP package types. The most time consuming part was creating vias by soldering a thin wire between the two sides of the board and inserting the last LED module as the drill holes were very narrow. Though in all the soldering took about six hours.
The smoke test was successful in that it found no faults in the board. So I could now move on to programming the mbed by adapting some code from a previous project. The programming results late on Day 2 weren’t great with significant flashing of the display. Though I did discover a soldering fault which didn’t show up during the smoke test.
Day 4 was much more successful after a good nights sleep. So with a fresh head, the basic operation is. Each TLC5947 contains 24 12-bit shift registers which are connected together in series and driven by one of the SPI ports on the mbed, these registers are the source of gray scale data which is controlled by additional pins connected to the mbed. The high side current coming from the A2982’s is controlled by a 3-to-8 decoder also connected to the mbed. The corrected sequence for displaying a line on the display is that the 1153 (24 * 12 * 4) bits of data is loaded into the shift registers through the SPI port, the output is blanked, then the correct row is selected on the decoder, the data latched into the gray scale registers and the output re enabled. Then repeat for the next 7 lines, to draw a full frame.
Now at the end of Day 3 the display and code can easily operate with no flashing, I’m not sure of the refresh rate. I even spent some time working on random graphics and drawing display modes, though I’m sure better results would be achieved by someone else. I’m very happy with the results as I was expecting an out right failure.
Now what? I’ve got some small changes I could make to the board including reducing the brightness of the LEDs by changing some current controlling registers and the software can always be worked on. This was a proof of concept project for a 16×16 display I’ve been working on similar to the cool [Projekt:Bunt] a large 10×10 RGB matrix. So this prototype and any software developed for it should help the testing or running of a larger display.
I’m attending the 27th Chaos Communication Congress – We Come In Peace conference in Berlin for the next couple of days.
I plan to hang out in the Hardware Hacking Area for most of the conference. I brought lots of Mini Mood Light v1 kit’s, which I’ll be selling for €15 at the conference, along with some other stuff to sell.
Also on day one I’m giving an Introduction to gEDA workshop. Where I’ll be showing participants how to use some of the open source electronics design tools from the gEDA project.