I figured this would be a good easy project I could finish in a day, so I drilled out a piece of pine board and set to soldering up some 3mm leds. I wasn’t very careful, so the little matrices look kind of ugly, but it all works and you can’t see the wires in the dark!

It didn’t take very long to toss three of these nine led matrices together. Assembling them into a twenty-seven led cube was a bit trickier. I used some gator clips to hold parts of the cube while I soldered it. Eventually I finished all the connections and had a passable cube with fairly even spacing.

Assembling the matrix is a pretty straight forward task. All you really do is tie all the cathodes together. Each matrix will become one row in the finished cube. Electrically, the cube is built as a 3×9 array, three rows and nine columns. You could probably build it the other way around, anode rows and cathode columns, but it is easier to sink a large current than source it. I think the MAKE: software only lights one led at a time, since they’re relying on the microcontroller to both source and sink current. My design is a bit different. The mcu sources current to each anode column, and N channel fets sink current for the entire row. The N channel is easily able to sink a few amps, so the cube can light an entire row at once without having to multiplex the individual leds.

In order to keep the PCB layout simple, the connections are spread all over the place in terms of the registers inside the pic. It would have been cleaner to organize eight of the nine columns as a single 8bit register on the pic, leaving only one bit left over to deal with. Instead, I’ve created symbols for each column, and set them individually from 9bit numbers.

Each anode column is current limited by a 75 ohm resistor. The value chosen was rather arbitrary, since the leds have such a low duty cycle, a lower value would have afforded me more brightness when the cube is battery powered. I can tweak the brightness a bit in the software, changing the scan rate the rows are multiplexed at.

That’s pretty much it. I’ve found there’s not a lot you can do with only 3x3x3 and 1 color, but it’s still kind of fun. Trying to think in three dimensions while drawing the animation frames is kind of tricky. I started with excel, but that wasn’t very useful – I spent more time copy and pasting formulas than I did ‘drawing’. Luckily my buddy Dan helped me out with that. He whipped up an awesome little php script that lets you draw animations 27 leds at a time, and it formats the resulting numbers so I can copy and paste them right into the code.

There’s a few videos of the cube doing various things on my Youtube Channel. Here is perhaps the most interesting one so far.

Thanks for reading, and Happy New Year!

The basic menu consists of options for setting the time and date; minute, hour, day, date, month, year.   The basic options are pretty self explanatory.  I have also planned some more advanced options:

1. Adjustable display brightness – Varying the duty cycle and refresh rate of the matrix can effect its brightness.  I need to make a simple scale for these variances to allow for two or three brightness levels.

2. Adjustable scrolling speed – The program draws the same information several hundred times per ‘scroll’, changing this counter affects how fast the display appears to scroll.

3. Selectable time format – User can choose between 12 hour AM/PM time and 24 hours ‘Military’ formats

4. Message Mode – User can enable / disable scrolling messages as well as display a single message, a random message or messages in sequence.

Other than that, the code is still pretty basic.  I’ve completed a bunch of internal fixes, like the scrolling code now handles messages of variable lengths. up to sixteen characters.  I’ve also created i2c eeprom reading routines to extract menu prompts and other text strings I’ve stored in the eeprom to save flash space on the chip.

To facilitate the role as an clock, I had to redesign the circuit quite a bit. Two new ICs were added, allowing the LED Sign to keep time, and providing some much needed storage.

The first of the new ICs is a Real Time Clock. Because I’m cheap, I chose the M41T80 from ST Micro. It’s an inexpensive clock, similar to the Dallas DS1307 but lacking a few minor features. First, the clock has no power-on reset detection. It just starts up as soon as power is supplied. The Dall 1307 has a stop bit which gets set if the clock experiences a POR, so the firmware can test if the clock needs to be initialized or not. The T80 datasheet mentions some registers may get set to default values on power up, so I’ll have to read it a few more times to see if there is a way I can check for a POR. Second, the T80 has no support for a separate backup battery. Instead, ST recommends you place a diode in series with the clock, and use a large capacitor to provide backup power. Last, there is no automatic leap year / leap second correction, oh well!

The second IC is a 16 kilobit serial eeprom, similar to the Microchip 24C16, I chose one from Catalyst semiconductor due to lower costs. The eeprom is arranged as eight banks of 256 bytes each. The chip contains a 16 byte write-buffer, I’m not sure if it can cross a bank boundary or not, I’ll program my firmware assuming it can not. The eeprom will be storing character strings related to operation of the LED Sign as a clock, as well as user programmed messages and possibly simple graphics.

I’ve also added some micro switches for adjusting the clock and changing settings, also a 32.768kHz crystal was added to providing the timing source for the RTC.

At this point, the layout of the printed circuit board has become pretty complex. I tried making one of these at home, but didn’t have the patience to exactly align the top and bottom layers of my press and peel sandwich. So, I decided to try a pcb prototyping house. There are a lot of board houses to choose from, many of which cost an arm and a leg. All of the domestic board houses are ruled out, I’m sure they do a fine job, but they cost too darn much. I settled on Spark Fun’s BatchPCB service. They’re not the cheapest board house out there, but their cost is fair. They include a lot of features most other board houses charge extra for, like double sided silkscreen and solder mask, 8 mil pitch and spacing, 20 mil holes, etc. I placed my order on the 6th, and had the PCBs by the 22nd. All the time in between, by mind set to wandering, and I made some POV toys. Once the pcbs showed up, I incurred another delay. Turns out I hadn’t ordered my RTC chips yet! So, another few days wait brought goodies from Mouser (man they are quick, and inexpensive!)

The boards from batchpcb look awesome. Nice bright green solder mask, tinned pads and holes, smooth clean edges. This is the ‘top’ side of my led sign. There are a few passives on this side, along with the two new ICs. This side is covered by the LED matrix once the board is fully assembled. Don’t mind the flux smeared everywhere – I did clean it off before soldering the matrix down.

The bottom of the board contains the PIC processor, mosfet column drivers, a crystal for the clock and some microswitches. Pin headers for power and programming the microcontroller have also been installed. The module is a bit thick at this point, thanks to the socketed IC and the pin headers. On the finished version, I’ll solder the IC straight to the circuit board, and probably use wires instead of a header to supply power.

The firmware is in the early stages right now, so my next post on this subject will try to cover whatever features I’ve decided on. Right now I know I want a few things:

1. Scroll the time
2. Occasionally scroll the date
3. Occasionally scroll short messages, either randomly or programmaticlly
4. Support some sort of software brightness control

I’ve made some revisions to the design since I had these boards fabricated. One big oops I made was forgetting the pull-up resistors for the i2c bus! Luckily there’s plenty of room on the board, and both i2c lines ran near the Vcc rail. So a little quick scraping action to peel back the solder mask and presto, new lands for 0603 sides resistors. I’ve also added a diode and big capacitor for the RTC’s backup power.

I hope to work on the firmware more this week, so I should have more details about how the clock works next time!

EDIT: Added a quick video of the time scrolling

Persistence of Vision is some sort of effect, either psychological or biological in nature, that allows our eyes and brain to ‘see’ motion and patterns in a sequence of rapidly stills (hence a movie projection). This effect can be exploited using basic digital electronics to create a virtual LED sign, which the viewer will ‘see’ when the pov toy is put in motion.

Commercial POV toys are usually some sort of led matrix attached to a spinning contraption, and display either a fixed or scrolling message. Those are fine, but the technical aspects of motorizing the display make it beyond the scope of a simple project. Therefore, my project relies entirely on elbow grease – swing the wand and watch the messages appear.

Revision one of my POV toy is compact. It measures approximately 2.25″ length wise by 1.1″ width wise. The circuit consists of an inexpensive Microchip PIC 12F683 microcontroller, and a 74HC595 serial shift register. The display consists of eight 3mm leds, I chose blue for the first unit, but I’ll probably make a few more with different colors, I especially want to see green and amber – those leds don’t see a lot of action in the world of consumer electronics.

The circuit is very simple. A tiny microcontroller provides the brains, a simple and inexpensive shift register provides the brawn, and a switch and hall effect sensor provide some control. My first unit has no hall sensor, so power is controlled with the push button switch. When “off”, the pic is in a ultra-low power sleep state. Come to think of it, I could have put the shift register into a tri-state mode when “off”, but for now, it holds its gates at Vdd, which provides no bias across the leds, so there shouldn’t be much current going anywhere. The hall effect sensor will eventually control the power to the display, as well as provide some synchronization. Right now when swinging the wand, the message is legible in one direction, and backward in the next. I plan to try putting a small round magnet in a tube, and gluing it behind the hall sensor. When swung in one direction, the magnet will be forced over the sensor by g-force. When the wand is swung back, the magnet will be forced away from the sensor. This should let the microcontroller know not only to blank the leds during the ‘backward’ swing, but also how much time it has to render the entire message, speeding up the display for a quick swing, or slowing it for a long swing.

The circuit board is a single layer layout with two jumpers. I went with as many SMT components as I could afford, since I hate drilling holes in my home-made circuit boards. Power is currently provided by a 3.7 volt 1600mAh lithium ion cell.

I tried making some video of the unit in operation, but my camera wouldn’t play ball. Next weekend I may have more time to fiddle with photography and try to get some ‘action shots’.

Although version one has been a great success, and was easy, I was unsatisfied. The eye candy just isn’t sweet enough. Version two takes the flash factor up a notch, with RGB leds. I’m working on a ‘full color’ POV wand toy. Version 2.0 has been built, but it has some flaws – and is the subject of my next blog entry. Here is a video of “2.0″ running a test pattern on its 24 led array.