Big Mess o' Wires

It seems the Big Mess o’ Wires blog has reached the threshold where companies will send their products to be reviewed. I’m flattered. Newark (or Farnell for those of you in Europe, Element 14 in Asia) has kindly offered to send me a product of my choice from their catalog, anything 25 GBP (about $39) or less.

What kind of electronics parts are people interested in seeing reviewed? Tools? Development boards? Programmers? Kits? Specific ICs? My first thought was a demo board for one of the less common microcontrollers (not AVR or PIC), like the MSP430 or an ARM variant.

Check out www.newark.com, and post a comment if there’s something specific you’d like to see reviewed here.

This Halloween, my daughter and I are working on a large LED display board for the yard. We started with a 3 x 6 foot pegboard with a grid of 24 x 48 holes pre-drilled, and 100 each red, orange, yellow, and green LEDs. 5mm size T 1 3/4 LEDs fit pretty nicely into the pegboard holes. Her job is to design some interesting animated shapes for the board, and my job is to figure out how to wire the whole thing up and power it.

My initial thought was to fill only the holes needed for a few specific Halloween shapes, and connect the LEDs using some ad-hoc point to point wiring. But the more I thought about measuring, cutting, stripping, and soldering hundreds of different ad-hoc wires, the more I hated the idea.

The Grid

Wiring up the board as a regular grid should be much easier, even if not all the grid points are used. The simplest method would be to lay down lengths of bare wire horizontally and vertically between the pegboard holes, using a plastic or rubber spacer at the crossing points to ensure the row and column wires don’t make electrical contact. To add a new LED to the grid, you’d just bend one leg down and solder it to the nearest row wire, and bend the other leg to the right and solder it to the nearest column wire. If the column wire was supplied a positive voltage while the row wire was grounded, then the LED at the intersection would illuminate. The great thing about this arrangement is that it wouldn’t require any wiring changes if you later need to change the shape of a jack-o-latern’s eye or a bat’s wing: you could just add a new LED to the grid, and alter the software that controls it.

Driving this grid would require 24 row lines and 48 column lines. The columns would be scanned rapidly one at a time, with all the row lines driven simultaneously to control the state of the LEDs in that column. If the columns were scanned rapidly enough, the human eye would perceive the whole display at once. The 24 row lines would most likely come from a shift register connected to the MCU, while the column lines would come from some type of decoder that enables exactly one of 2^N columns using N input bits from the MCU.

Splitting the Grid

Scanning through 48 columns one at a time would work, but with a 1/48 duty cycle, the LEDs would look very dim. A 1/8 or 1/4 duty cycle is more realistic in order to create an acceptably bright display. To achieve a 1/8 duty cycle, the grid would have to be divided into 6 independent sub-grids, each with 24 rows and 8 columns, with 192 LEDs per fully-populated sub-grid . Each of these sub-grids would require its own 24-bit shift register for the rows, and a 3:8 decoder for the columns.

Power requirements would likely force the sub-grids to be even smaller. Assuming 10 mA per LED, a fully-populated 24 x 8 sub-grid with all LEDs on would see 1.92 amps of current at the decoder, which is way too much for any common IC. After looking at some typical purpose-made LED driver chips, it looks like 300 mA is a more realistic upper limit.

I have a sample of the MAX7219 LED driver, which combines 8 row drivers and 8 column drivers with built-in column-scanning circuitry into a single chip. It can control 64 LEDs, with a total current limit of 320 mA. Each LED is driven with 40 mA when its column is enabled, and the 1/8 duty cycle results in an average drive current of 5 mA per LED. With 1152 total grid positions, it would require subdivision into 18 sub-grids to control the full grid with 18 MAX7219′s. For a brighter display, the MAX7219 could be configured with only 4 columns and a 1/4 duty cycle, for an average drive current of 10 mA per LED. That would require twice as many sub-grids and twice as many MAX7219′s.

Cost Estimate

At roughly $8 per MAX7219, the cost of driver chips would be $144 for the normal display, or $288 for the brighter display. That’s a bit out of the budget range for a weekend Halloween project! I suspect I could save a significant amount of money by using a simpler driver chip, but so far a specific solution has eluded me. The row drivers need constant current outputs, with a relatively high current limit in the 40 mA range. The column drivers (assuming they’re a separate chip and not combined like the MAX7219) need a very high current limit in the 300+ mA range.

The best option I’ve found for the row drivers is the STP08DP05B1, a $1.82 chip with eight constant-current outputs able to supply up to 100 mA per row and 800 mA total. I’m having a little trouble understanding the datasheet, though, and I’m not certain those are truly the per-pin current numbers, and whether they’re for sourcing or sinking current.

I’ve not yet found any good options for the column drivers, probably because I don’t know what such things would be called. High-power decoders? Analog demux switches? I need something with three digital inputs, that enables one of eight analog outputs, and can supply as much current as possible to that output, but at least 300 mA.

I could also use standard low-power shift registers and decoders, connected to separate high-power transistors. That would greatly increase the total number of components needed, though, since I’d need a separate transistor for every row and column in every sub-grid.

No matter what the eventual solution, it appears the cost for driver electronics will be at least $50 or so, and possibly much more.

Power

The power requirements of the full grid would be non-trivial. Assuming I used 18 of the MAX7219 drivers, the maximum current draw would be about 6 amps! That’s well outside the capability of any wall-wart 5V supply in my collection, and would probably require some type of bench supply or switching power supply scavenged from an old PC. However, the display would only draw 6A if it were fully populated with all LEDs simultaneously turned on. With a less than fully populated grid, and some upper limit on the fraction of LEDs that could be on at once, the power requirements might be reduced to a more reasonable level.

Solutions?

Since I’m not about to give up on this display entirely, nor wire up a few hunded LED’s with ad-hoc point to point wiring, some variation of the grid approach looks like the only viable solution. That means I’ll be spending a lot of money on driver electronics and a high-current power supply. To help keep the cost and assembly time down, I’ll probably only build a subsection of the entire 24 x 48 grid, but do it in a way such that the rest of the grid can be populated over the course of future holidays.

Edit: Thinking about this further, I’m starting to doubt whether this project is a very good idea. 5mm LEDs with a 1 inch grid spacing will do a very poor job of space filling, so this display likely won’t look very good.  It will appear more like a bunch of isolated point lights than adjacent pixels in an image– OK for abstract displays, but not so good for making a recognizable image of a jack-o-lantern.

My bigger concern is safety. Combining a 6A power supply with a grid of bare wires on the back of the board sounds pretty dangerous. Curious fingers could get a significant shock.

While I’ve been working on the Plus Too Macintosh replica,  I’ve also collected a few real vintage Macs in the past couple of weeks. Too many, in fact, so it’s getting hard to even walk through my office due to the Mac paraphenalia everywhere. The trouble is that once your friends hear you’re collecting vintage Macs, they’ll start giving you perfectly good systems for free! Here’s the current inventory:

Mac Plus – aka “Lonely” – I bought this Plus from a Craigslist ad with the photo shown above. It’s in nice working condition, and has only moderate yellowing of the plastic. I’ve paired it with an older Apple M0110 keyboard, which actually predates the Plus. After an upgrade to 4MB of RAM and the addition of a 100 MB external Zip drive, Lonely is in business! The Plus is running System 6.0.7 and has a 68000 CPU running at 8 MHz.

Mac 512Ke – aka “Smelly” – This was another Craigslist purchase, acquired from a guy who was once a Mac repair tech in Japan. He threw a whole box of spare Apple ICs into the deal for free. This machine earned its nickname from the horrible burning smell it gave off when I first turned it on. In an attempt to pinpoint the problem, I later ran it outside with the rear cover off, but the smell never returned and Smelly has worked fine ever since. It’s paired with an Apple M0110A keyboard (the one with the numeric keypad) with a couple of broken keys. The 512Ke is basically a Plus with no SCSI and only 512K RAM, but the CPU specs are the same.

Macintosh Classic – I was given this machine for free, which is good because it has lots of problems. It does mostly work, but there’s no sound, and it sometimes fails to boot up and shows a checkboard pattern on the screen. I believe both of those problems are due to aging capacitors on the motherboard, which could be replaced if I were motivated enough. The floppy drive doesn’t work, and seemingly half the keyboard keys are broken. It’s got a 40 MB internal hard drive and 4 MB RAM, and the same CPU as the Plus and 512Ke. I’ll probably cannibalize the HD and RAM, and recycle the rest.

Macintosh LC II - I bought this pizza box machine cheaply from a vintage Mac web site. The specs are good: 68030 at 16 MHz, 12 MB RAM, 1 GB hard drive, and ethernet card. Unfortunately it’s a real dog running System 7.5. I used to own an LC myself, and I don’t remember it being this slow. It can take nearly a second for the computer to respond to mouse clicks on the tool strip! I haven’t been able to get the ethernet card working, although I haven’t tried very hard. I plan to install System 7.1, and see if that helps improve performance at all. If not, I’ll probably swap out the 1 GB HD for the Classic’s 40 MB one, and re-sell or donate this machine.

Powerbook Duo 230 – This was another freebie. The Duo 230 sports System 7.1, 12 MB RAM, a 120 MB hard drive, and 16 glorious grays. The 33 MHz 68030 CPU feels relatively snappy. Unfortunately without a Duo dock or mini-dock, the only way to get data on and off the Duo is with a LocalTalk network connection. Even if I had a dock, though, the flakey keyboard makes it challenging to use. All of the keys work, but many of them have to be pressed very hard to register a keypress. This is a common problem with Duo keyboards, and I followed the instructions to disassemble the keyboard and clean the contacts on the mylar membranes, but it didn’t seem to help much. I’ll probably donate this machine.

Power Mac 8500 – This Mac is awesome, and I got it for free! There’s something about the vintage tower Macs that’s just irresistable. With a 120 MHz Power PC 604 CPU, 160 MB RAM, and System 9, it’s actually capable of running semi-recent software. It’s only problem is that it’s as loud as an airplane. I think the hard drive mount may be vibrating, which should be simple to fix. If one of the fans is failing, it may be more difficult to find a replacement.

I was able to get online and browse the web using the 8500, although no recent web browser will run on this machine, and the browsers that do run on it (Internet Explorer 5… ahem) have trouble rendering many web pages from 2011. Even when the pages do render correctly, it is S…L…O…W… These days, even a simple web page is likely to involve downloading dozens of individual image and style sheet files and running a bunch of JavaScript. I plan to keep this machine for general Mac nostalgia use, and as a file server for the Plus and 512Ke.

Piles of Peripherals

I’ve got external Zip drives, assorted RAM, system saver fans, SCSI cables and terminators, serial cables, LocalTalk cables, and a bunch of other stuff that I can’t even identify. I need to learn to say no to donations, even when they’re free!

Hot diggety damn! I believe I’ve successfully soldered the Max II in its cruel 100-pin package with 0.5 mm pin spacing. At least, I’ve assembled enough of the board to connect it to a JTAG programmer, program an LED blinky routine to the Max II, and confirm that it works. There could still be all kinds of pins shorted or broken, of course, but at least I know I didn’t fry it completely.

Soldering that chip wasn’t fun. It took me about two hours just for that single TQFP 100. I used the drag soldering technique, where you goob tons of solder onto the pins and create pin-to-pin solder jumpers like crazy, then go back afterwards with solder wick to clean up the jumpers. At least that’s the idea.

I had the absolute worst time trying to wick away the jumpers. No matter what I did, the extra solder on the pins wouldn’t soak into the solder wick. I set the wick on top of the pins that had jumpers, then set my iron on the wick, and pressed down onto the sandwich. The solder underneath the wick would melt, but it wouldn’t go anywhere. When I removed the iron and wick, the jumpers remained right where they’d been, with nothing at all soaked into the wick. I applied flux everywhere, over and over, but it didn’t help.

This pattern went on for ages, and I got more and more upset. I started swearing at the board, using every profanity I could think of. I had to close the windows so the neighbors wouldn’t hear. After a while, I started singing random profanities to the tune of opera while I worked on the jumpers. I burned the soldermask off several of the traces, burned the board, and re-heated the same pins so many times that I was certain nothing was left inside the chip but melted slag. All in all, it was not a good time.

Eventually I stumbled onto a few techniques that helped a little, raising my wicking success rate from 0.1% to maybe 20%.

• Some jumpers can be cleared simply by touching the iron briefly to the pins, without any wick.
• Don’t stretch out the wick braid. Keep the strands pressed together, like stranded wire.
• If a stubborn bit of solder refuses to be wicked away, add more solder. A huge glob is actually easier to remove than a tiny fleck.

Assuming the other as-yet-untested pins on the Max II are OK, then assembling the rest of the board should be cake.

Sometimes it seems like there are a million different LCDs you might use with your microcontroller project, and deciding on one can be hard. Once you’re ready to move beyond a basic text display, you’ll find graphic displays have a dizzying number of options for technology, color depth, interface type, driver, and power. Recently I’ve been collecting info on display options for my own projects, and here I’m presenting three options that look promising.

The Nokia 5110 is a low-resolution monochrome LCD that’s cheap and extremely low-power. A family of small monochrome OLEDs provide crisp, bright displays with a bit more resolution, in a teeny-tiny package. A 1.8 inch color TFT provides even greater resolution and 18-bit color, while still limiting power to about 90 milliwatts.

Nokia 5110

Colors: monochrome
Resolution: 84 x 48
Active display area: 2.8 x 2 cm
Interface: SPI
Power: 3.3v
Current: 0.4 mA for logic, 1-10 mA for backlight
Cost: $10
Vendors: Sparkfun, Adafruit

This is the display I’m currently using for my Backcountry Logger project. It’s cheap, easy to use, and consumes very little power. You don’t really need the backlight for daytime visibility, and with backlight off, it consumes just 400 uA! Awesome for battery-powered projects. Just 1-2 mA worth of backlight current is plenty for night-time visibility too, although more would be nicer. This puts the display well within the capabilities of a single CR2032 coin cell.

Working with the display is simple, as long as you remeber it’s a 3.3v device and not 5v tolerant. There are lots of great tutorials and examples available for this display, including this nice one from Adafruit. Communication with the display controller uses SPI. Writing a single byte sets 8 pixels at a time, so pixels are not individually addressable. If you’re mostly displaying text or bitmaps this isn’t a problem, but if you need a more complex image consisting of many overlapping elements, you’ll need to composite them in software before sending the result to the display.

The weak points of the Nokia 5110 display are resolution and looks. The contrast and sharpness are pretty good for a display of this type, but there’s still no getting around the fact that this is a low-res, dark-gray on light-gray display. The 84 x 48 resolution allows for just 14 x 6 characters using a typical 5 x 7 font (allowing for space between letters). If you seek utility, low cost, and low power, it’s a great solution. If you want something with a bit more bling, look elsewhere.

SSD1306/SSD1308 OLED

Colors: monochrome
Resolution: 128 x 64
Active display area: 2.4 x 1.2 cm
Interface: SPI, I2C, or parallel
Power: 3.3v, and maybe 7-12v
Current: roughly 5-13 mA, depending on how many pixels are lit
Cost: $15-$20
Vendors: Sparkfun (bare LCD), Adafruit, eBay

This little OLED display comes in a few slightly different flavors, but all of them are tiny. It’s one thing to read the dimensions (0.96 inches horizontally), but another to see it in person. All three of the displays discussed here are small, but this particular OLED is smaller than a postage stamp. It’s about the size of the last joint on my thumb.

Despite its small size, the OLED display is very readable. It’s sharp and bright, and a pleasure to look at. Since it’s an OLED, there’s no backlight, and current draw varies between about 5 to 13 mA in my testing, depending on how many pixels are illuminated. The 128 x 64 resolution is a nice bump from the Nokia 5110 display, allowing for 21 x 8 characters. Communication choices are SPI, I2C, or parallel, selectable via configuration pins. Like the Nokia, each byte sets 8 pixels at a time. Adafruit has a tutorial and library.

The display controller chip runs at 3.3v, and the display itself needs 7-12v, although you might be able to get away with a single supply depending on which display variant you have.

Adafruit sells this display on a break-out board, with white pixels, with a SSD1306 controller. The SSD1306 has a built-in charge pump, and can optionally generate 7.5v for the display from a 3.3v supply, which is a very convenient option. Sparkfun sells the naked LCD, with white or blue pixels, and a SSD1308 controller that lacks the charge pump. You’ll need to mount the 0.5 mm pitch connector somehow, and provide a separate power supply for the logic and display. The most common eBay variant comes on a break-out board, with the SSD1306 controller, and yellow pixels in the top quarter and blue pixels in the bottom three quarters. Mine also came hard-wired to use the parallel interface, and switching to SPI required wicking away some solder jumpers and adding new ones.

This display meets a narrower range of needs, but is awesome for its target niche. I’m strongly considering doing a version 2.0 of the Backcountry Logger using this display, to gain the benefit of smaller size, higher resolution, and better looks. Unfortunately with the amount of current it needs, it’s probably outside of what a CR2032 can provide, and will require 2 x AAA or 1 x AAA with a DC boost converter.

Adafruit 1.8 inch TFT

Colors: 18-bit (262144 colors)
Resolution: 160 x 120
Active display area: 3.6 x 2.8 cm
Interface: SPI
Power: 5v or 3.3v
Current: 1 mA for logic, about 26 mA for backlight (can be dimmed)
Cost: $25
Vendors: Adafruit

This is the display I plan to use for Tiny CPU. Unlike the others, it’s a full color display with individually addressable pixels. It supports 18-bit color, but can also be configured for 16-bit color or monochrome (I think? Haven’t tried that yet). It’s a bit larger than the other two, but still quite small compared to most displays. It is natively a 3.3v device, but the Adafruit breakout board includes an LDO regulator and level-shifter chip, so it can be used with 5v microcontrollers as well. Current demands are the highest of the three displays, but not excessive at about 1 mA for logic and 26 mA for the backlight. The backlight can be dimmed using PWM to further reduce the current. This is still well within the capabilities of an AAA-based battery-powered project.

The TFT is very attractive, not quite on par with the OLED, but certainly nicer than the CSTN used in some other cheap color LCDs. The 160 x 120 resolution feels giant in comparison with the previous displays, allowing for 26 x 15 characters. The display controller allows for a few different protocols, but the break-out board hard-wires it for SPI.

With the larger resolution and greater bit depth, a screen’s worth of data requires many more bytes than the previous two displays. Depending on the speed of your microcontroller and SPI interface, this may result in noticeably slower refresh times. I tested it using hardware SPI on a 16 MHz Arduino, and found the refresh time to be acceptable.

Adafruit has a nice tutorial and library for working with this display (I’m sensing a theme here). Unexpectedly, the breakout board also has a micro-SD card reader on it. Ignore it, or use it as a bonus peripheral in your next project.

I just applied 4 years worth of WordPress updates in one go, without making any backups first. What could go wrong?