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Two Birds

Monday, April 27th, 2009

I want to replace the abraded power cord on my brother’s sump pump in exchange for his letting me borrow it. Cort needs four panel-mount BNC connectors for an amateur radio handheld direction finder project. Convergence.

Don’t panic. If for some weird reason you need one of these, I have more where this came from.

It’s a little hard to see from these shots, but the case profile is a weird trapezoidal shape.

Physically large linear power supply; two main PCBs.

Lithium 1/2 AA cell from April of 1989. Rated for 3.6V and still holding 3.69V after twenty years.

Not sure why they used two PCBs (don’t tell me they really needed the extra 20 square inches), but it’s cute the way this one slides out.

Every one of these had its shield pins broken free of both solder joints. Looked like cold solder, but I assume it was just mechanical stress.

Voila! Four connectors for Cort. And a power cord for my brother, that I’m out of time to swap onto his pump tonight. Tomorrow, then.

Posted in Inside, Salvage, Slim | 1 Comment »

Repairing a Soundcraft Spirit E6 Mixer Switching Power Supply

Sunday, April 19th, 2009

A few weeks ago, John mentioned to me that one of the audio mixers in the art and technology / Internet radio station lab had stopped powering on. I said I’d have a look at the power supply and see if I could fix it, and he sent it with me. Turns out we both got a little more than we bargained for — I in terms of effort required and he in terms of time without the mixer.

I didn’t know whether it’d be a linear or switching power supply, and it turned out to be switching. I figured it’d just have some baked electrolytic capacitors I could replace, and it turned out that was just the beginning.

I’ve got it mostly fixed now, and it’s been a long and interesting road.

(more…)

Posted in Circuits, Inside, Repairs | 154 Comments »

DIY Power Injector for Axis 2100 Network Camera, Part I: Investigation

Tuesday, March 24th, 2009

A little while back I bought a couple of used Axis 2100 network cameras, intending to use them at home (driveway cam? front yard cam?) to replace my lousy Linksys WVC54GC. The Linksys’s video stream is viewable only in Internet Explorer or with VLC; the Axis (although older) streams motion JPEGs that can be viewed with just about anything, provides still JPEGs, and has FTP server upload capabilities to update a statically-served image on a web server.

Power Over Ethernet and Power Splitters

Having got them, I decided I’d like to play with one at work, installing it in our 3rd-floor bay window overlooking campus, so those of us with windowless offices can get a little sunshine in our lives. That would be easiest if we didn’t have to run both ethernet and power to the camera separately, so I started reading up on the camera’s power capabilities.

Power over Ethernet (PoE) is a standard for delivering 48VDC over an ethernet cable — depending on the implementation, possibly on the same pairs used for data, possibly on the spare pairs. The data pairs use differential signaling (the difference between TX+ and TX- is the signal, like balanced audio, as opposed to a single signal referenced to ground), so the DC power can be provided on the two wires of one pair and grounded on the two wires of the other pair (similar to the way phantom power is provided on balanced audio cable).

The product web page almost intimates that the camera supports PoE:

Access to a power outlet not needed with use of Power over LAN Midspan and Active Splitter from Axis
No need for power outlets and electrical cabling using Power over Ethernet products

But in fact the camera doesn’t operate when hooked to a PoE connection. Additionally, another Axis page lists a “PoE splitter” for the camera, which strongly suggests that power may be delivered over the ethernet cable by standards-based or non-standards-based means, but must be split externally before entering the camera.

Still, pp 59 and 62 of the users guide show intriguing connections among the I/O port, the power connector, a bridge rectifier, and a switching power supply; and p 65 lists the power supply as accepting 9-15VAC or 9-15VDC, which is kind of interesting. Even though the unit clearly doesn’t function on PoE, I thought it was worth opening up to see whether it would accept non-standards-based power on the ethernet port.

By the way, recognize that green thing?

Going In

Here’s the camera with the bottom of the case lifted off. Note that the top of the circuit board faces the bottom of the enclosure.

The CCD is on a separate PCB stuck onto the end of the main board.

Another view of the top side of the board with the CCD board separated.

And the bottom of the board (facing the top of the enclosure).

Continuity

I started by trying to trace connectivity from the power jack to the unit’s ground, not remembering that the camera accepts AC input. I got no continuity there, as my tester doesn’t indicate continuity through diodes. After noticing the bridge rectifier (last photo above, just to the left of the upper right corner), I realized my error and was able to trace ground from the rectifier output throughout the unit.

Next I traced from the power jack to the I/O connector and confirmed the connections shown in the user guide — you can provide or draw rectified and filtered but unregulated DC power on the I/O connector.

Feeling bold from my discoveries, I checked continuity to the ethernet jack. I got DC continuity between ground and the center pins 4-5, but no other connectivity. Bah! No non-standards power on ethernet for me.

Hm, but I also hadn’t successfully traced V+ from the rectifier throughout the unit. I figured I needed to pick it up from the other side of the LM2596 voltage regulator — which turned out to be a switching step-down regulator.

LM2596

Pin 4 is the feedback that connects to the filtered, regulated output — and indeed, it connects to the V+ power supply pins of all kinds of things throughout the circuit. Cool; progress! Maybe the regulated DC voltage connects to the ethernet jack???

Mm hm, it sure does. I get DC continuity with .4-.5Ω resistance from the V+ rail to ethernet pins 1 and 2 (TX+ and TX-), DC continuity with .2Ω resistance to pin 3 (RX+), and no connection to any other pin.

So?

Although the datasheet’s application circuit above shows the 5V step-down regulator, the camera actually uses the LM2596S-3.3 for 3.3V power. That this appears on both TX pins of the ethernet port suggests that I could provide DC power on pins 1-2 and ground on 4-5.

But it would have to be regulated 3.3VDC, which (1) isn’t standards-based PoE and (2) isn’t going to travel well over 100+’ of cable. This suggests that a non-standards-based switch-end power injector directly in through the camera’s ethernet isn’t going to work well.

More plausible: Build a PoE-attached power supply with a 48V-3.3V step-down converter and hang it off the back of the camera. At first I thought that doing PoE would require a dedicated PoE chip to do the negotiation, but all that’s needed is a 25kΩ resistor across the powered pairs. So it really would be feasible to do, without a special chip just for the PoE.

However, if the solution is going to require external PoE down-conversion anyway, why bother going all the way down to the regulated 3.3V? Why not go 48V-12V and come in the regular power jack, which also eliminates any risk of making the LM2596 unhappy by feeding power into its output? (Its block diagram looks like that shouldn’t be a problem, but I don’t know enough to be certain.)

If using an external down-converter anyway, the only drawback I can see of splitting the power out to feed the the power jack separately rather than the ethernet is the requirement for an extra connector.

Or the Simple Way

Regardless, I don’t really want to have to build an external step-down regulator just to get the cameras working. So for now, I’m more likely to build simple cable splitters to inject 12V onto unused pairs at the switch end and split it out to the power jack at the camera end.

Posted in Hacks, Inside | 18 Comments »

Fixing My Wife’s Curling Iron

Saturday, March 14th, 2009

When my wife leaves her curling iron on top of a note that says “Non-connected wire somewhere? goes in & out of power,” my first thought is, who taught you English?; and my second thought is, I better stop making fun of her long enough to get this fixed before she gets home from work.

My belated third thought is, I finally get to see how the infinitely-spinning power cord connection works . . . and I bet it’s the problem.

It’s tough always being right.

The power cord has a connector that looks a little like an RCA plug, except the barrel portion’s outer surface contacts the jack’s leaf instead of the inner surface contacting a barrel jack.

You can see the pitting on the leaf where the contacts had arced, probably at least in part due to oxidation of the surfaces. Also, the leaf was twisted so it was making contact only with the leading edge of the barrel instead of with the whole face, which exacerbated the problem by reducing the contact area to a very small point that became completely pitted.

After smoothing the pitted area and polishing the tip and ring leaves and the tip and barrel with 600-grit sandpaper, I reinstalled the power cord and carefully twisted the barrel leaf parallel to the barrel so they mate over a larger surface area.

Once the whole iron was reassembled, I plugged it in and watched its power lamp while spinning the cord around. No further problems that I can see. So it now, ah, has a connected wire somewhere and only goes in power.

Update 16-Mar-2009:

Turns out the power plug fit loosely in the bathroom light fixture receptacle and I just needed to bend the prongs out a bit. Oy vey.

Posted in Inside, Repairs | 5 Comments »

Fixing an LED Sign

Sunday, March 1st, 2009

My wife is working a couple of evenings a week for a tax preparer who used to work for her when she was an H&R Block office manager. Friday night he was taking down his $300 sign out of the window to throw it away because it had a few dark LEDs. She told him no promises, but I might be able to fix it because I do stuff like that every night.

And yeah, she was right.

I’m actually not sure I’ve seen all the failures the sign has to exhibit, but I’ve fixed two overt and two covert. The obvious ones were the dark LEDs in the white “M” and “E.”

The less obvious ones were that the pull-chain switch at the bottom doesn’t shut the sign off, and that the DC power plug pops out of the jack.

Opening It Up

The sign is made of two plastic sheets sandwiching a thicker plastic frame. The front must be glued on, but the back is screwed on for repairability. Actually, given what the inside looks like, I have my doubts that the manufacturer ever considered repair; I suspect screwing the back on was just the first thing that occurred to them.

I’m (mostly) not knocking the schematic — many parallel series chains of LEDs is the only reasonable way to design something like this. And making chains of three each blue and white LEDs but five red LEDs is also reasonable, given the respective voltage drops.

But I do take issue with stacking three LEDs with 3V drops on a 9V (nominal) unregulated power supply. The supply happens to run about 9.7V under the load of the sign, which leaves .7V across the 150Ω current-limiting resistors, hence just under 5mA LED current.

Leaving that low a voltage drop across the resistors makes the LED current (and brightness) incredibly susceptible to variation in the actual voltage of the unregulated power supply: a power supply change from 9.7V to 10.4V seems insignificant but would double the (white and blue) LED current. Worse, a drop from 9.7V to 9.35V would halve it.

Mainly, though, it strikes me odd that the sign is wired with leftover four-pair UTP (network cable). Was this thing built in some guy’s garage with stuff he pulled out of the dumpster at work? (Wait, did I build this thing??? )

Fixing the Power Connection

First things first — I started with the power connection so I wasn’t fighting it the whole time I was testing and repairing the rest of the sign.

The jack is recessed into the frame. The manufacturer made some effort to get it close to the outside and minimize the amount of recess; but the correct recess for this jack is zero. The plug is designed to make good mechanical contact when it’s sunk completely into the jack; before that point, the springiness of the jack’s outer contact pushes the plug back out of the jack, which is exactly what was happening.

The solution, or I should say hack, was to remove a corresponding amount of rubber from the plug’s molded barrel insulation, so it could once again fit the depth of the jack properly. No further problems.

Dead LEDs

Next I tackled the dead LEDs. A series string of three was dark in the M, leading me to supect one LED burned out and open. Measuring the voltage drop across each, I found the entire ~9V drop across the uppermost LED, so it appeared to be open.

After jumpering across the suspect LED, the other two in the string lit (of course, too brightly relative to their peers), so it was indeed open and the problem.

The right fix would be to replace the broken LED, but I don’t have any white 10mm LEDs on hand, it would take a while to order, frosted 10mm LEDs seem to be more difficult to find (or to find clearly specified as such), and I’d have to wade through long lists of nearly-identical products searching for the one that was actually right.

Immediately I thought of a way I could fix it using materials I had on hand — drill a hole into the back of the 10mm LED and sink a white 5mm LED into it. This idea made me cackle with glee, so you can imagine my disappointment upon realizing I don’t in fact have any white 5mm LEDs here. I need to get me some so I can go back and try that yet.

Really, this hack is just as good, though. (Ah, I realize I’m using the word “good” in a perhaps somewhat nontraditional sense.) The SMT LED soldered across the dead 10mm LED’s pins diffused nicely through the 10mm LED lens.

There was also one lonely dark LED in the “E.” Turned out when it failed, it didn’t open, so it was still passing current through for its friends.

I used the same hack on this one, although I did clip one lead off so that the dead LED wouldn’t pull the voltage drop too low for my SMT LED.

I was surprised at how well the hacked LED in the “M” matched the brightness and color of the other white LEDs, given how different its physical construction is. The one in the “E” is a little more noticeable (picture down below), but probably not objectionable if you’re not specifically looking for it. And I’d say a replacement 10mm LED has fairly good odds of being a little off in color or brightness, too.

At 5mA I should have nothing to worry about; but I watched the first SMT LED with my infrared thermometer for a minute with the power on, and it didn’t get above ambient temperature. Should last a good long time.

“Fixing” the Power Switch

I got the pull-chain switch out of its housing and couldn’t see why it didn’t work. Since my wife said the owner leaves the sign on all the time anyway, I don’t reckon it’s worth replacing the switch, so I removed it and soldered in a bypass wire.

Because the switch was broken in the “on” setting, I could have left it installed and the sign would be on. But the switch is already broken, and who knows when it’ll further break “off” instead of “on.” Under the circumstances, I’d rather bypass it now than have to go back and reopen the case to replace the switch later.

There’s a sizeable cutout in the bottom edge of the back cover for the switch, so it wasn’t really an option to leave the switch cover off. It does look a little odd having the cover on there with no switch protruding, but it’s not awful.

Putting It All Together

And thar she be, in all her working glory. For the moment, anyway.

Shortly after taking that picture, I moved the sign and a couple of blue LEDs on the border went out; then one of them came back on. Half an hour later, both (all) were back on. I’m not sure I’ve seen the end of this yet, and I may have a sizeable job cleaning leads and resoldering cold joints sometime in my future.

Physical Construction

One last thing: the frame and skin construction of the sign forms a rudimentary torsion box. The plastic face and back are quite flimsy by themselves, and even the face with frame glued on was flexing as I was moving it. But the moment I got the back skin screwed on, the entire assembly was quite rigid and immune to flexing and racking. Pretty impressive for such a simple technique.

Posted in Hacks, Inside, Repairs | 3 Comments »

Fixing a Buzzing Clock Radio

Wednesday, December 31st, 2008

I’ve been using this clock radio for at least twenty-three years, and I love being able to read its huge 2″ digits when I wake up during the night and my eyes are blurry and unfocused. It’s having a little trouble here competing with the sunlight streaming in the side window, but it’s nice and bright at night when I need it.

Lately it’s picked up an annoying habit of buzzing at, you guessed it, 60Hz. The intensity and timbre of the buzz vary, sometimes coming and going at the same time I can hear the furnace fan starting up and shutting down, sometimes apparently at random.

Listening carefully has led me to believe that the buzz isn’t coming from the speaker end of the clock, but rather from the power supply end. Speaker buzz would suggest bad filter capacitors (and one could certainly forgive twenty-five-year-old electrolytics for needing to be replaced); but power supply buzz makes me think of transformer windings coming loose and needing to be re-epoxied, metal brackets near the transformer working loose and needing to be tightened, and that sort of thing.

Today I had time to open it up and — I think — fix the problem.

Most of that is radio circuitry, and I don’t even use the radio. The clock part appears to be two ICs underneath the raised pushbutton circuit board. The transformer is in the “basement level” between the two posts to the left of the display’s ribbon cable.

Right away I could see something I suspected was at least part of the problem. The transformer’s forward mounting tab was bent at other than the proper 90° angle from the body, only one edge of the mounting screw’s head was in contact with the tab, and there was a lot of slack between the tab and the mounting boss.

After removing the main PCB to make room to get a screwdriver in there and straightening the mounting tab with pliers, I got the screw tightened down properly. I could tell that the screw hadn’t worked loose over time but had been assembled this way at the factory: I could feel that I was tapping new threads into the mounting boss as I turned. Expecting the plastic to be fairly brittle after twenty-plus years, I worked gingerly, and successfully tightened the screw without breaking the plastic.

While turning the screw, the entire end of the transformer’s cover was rocking from side to side, and a different angle revealed the reason and a second likely suspect for the buzzing sound (of which I carefully took this out-of-focus picture). The mounting cover had an edge bent out away from the laminated core and a loose tab.

After a little work tightening things with pliers and pressure, the cover seemed to be pretty well fastened. I checked the power supply electrolytics with my Capacitor Wizard since I had the clock open anyway, and they all tested good. I reassembled the clock, retesting for buzz several times along the way, and so far the buzz appears to have been banished.

The transformer problem was clearly a manufacturing defect; and it’s interesting to think that it took over twenty years to manifest itself. Here’s to the next twenty!

Posted in Inside, Repairs | 4 Comments »

Cleaning (and Cleaning . . . and Cleaning) Akai Headrush E2

Wednesday, December 31st, 2008

I just bought an Akai Headrush E2 delay and looping effects pedal. It’s the box that the amazing KT Tunstall uses in her solo performances of “Black Horse and the Cherry Tree” to lay down her own rhythm and backing vocals before playing guitar and singing, all by herself. It’s a really slick setup.

I got a great deal on eBay — they retail for ~$200 and typically run about $150 on eBay, but I picked mine up for $76 missing the 9V power supply. It turned out to be an even better deal than I imagined, because the seller forgot to mention that the pedal came with a generous helping of organic matter, lovingly applied all over the pedal. This picture is after a preliminary 3x cleaning with Goo-Gone.

Not only was the case unsightly, but the foot switch action was dodgy as well, and that’s not easily cleaned from the outside. To really get the case clean, I wanted to get the panel loose of the controls so I could give it a good soak; to fix the buttons, I wanted to spray them with contact cleaner. I figured I may as well show off the inside while I had it open anyway.

There’s the upper PC board. Note the filth that crept down onto the mode select pushbutton, and even the potentiometers:

I had already got the outside of the lower case fairly clean, but wanted to clean the inside as well, so I went ahead and took both boards out. Here’s the lower PCB, where all the magic happens:

I actually had a plan for dealing with the filth. Goo-Gone is great at softening and removing sticky adhesives like the remains of the world’s largest velcro strip the former owner had attached to the bottom of the pedal; but there’s no cleaner like Fantastik for removing the evils that men have done from arcade game control panels and (apparently) guitar/keyboard effects boxes.

I gave the top case about a ten-minute soak in Fantastik — I had already soaked and scrubbed in Goo-Gone three times, mind — and after the soak it literally rinsed completely clean under the faucet, with the tiny exception of a little bit of gunk still in one of the potentiometer mounting recesses. The foot switches left behind a pool of brown liquid (and I had only sprayed the mounting threads), the potentiometer knobs had white inset lines again, and the washers that I’d had to pry loose of the case with my knife were clean and shiny.

I also used an “acid brush” (dry, no cleaner) to knock the accumulated dust and grit loose of the PC boards around the base of every component that penetrates the top cover and get the boards nice and clean again.

The pushbutton foot switches aren’t quite sealed, and I was pretty sure I could see in to the inside along the solder lugs. I sprayed with wiper cleaner/lubricant, worked the switches a bunch of times, and repeated the spray/work process twice more. Shiny!

Same pedal reassembled about an hour later, with towel lint added for your viewing pleasure.

And remember, folks, you can’t go around calling yourself anal-retentive if you don’t bother to align your fasteners.

The pedal now works perfectly — no more double-taps from switch bounce and missed taps from stickiness, etc.

It makes such a difference that before cleaning, I couldn’t understand the operating instructions (because the pedal didn’t seem to be doing what they seemed to be saying), and afterward they’re much more clear (although incomplete in failing to mention that it does not record while you’re tapping the tempo in delay mode, pity; and confusing, incomplete, and inaccurate in their description of looping mode operation; but who, bitter, me?).

Seriously, after an order of magnitude shorter time poking around than I had already done before cleaning, I feel pretty comfortable that I understand how to get it to do the things I want to do. Using the delay mode, I can now play a pretty passable rendition of the “Fly Like an Eagle” synthesizer intro. Wonder what to tackle next . . .

Posted in Equipment, Inside, Repairs | 41 Comments »

Trying to Repair Roomba Scheduler

Monday, December 29th, 2008

About two and a half years ago, I did something foolish in trusting the wiring instructions from a Roomba battery rebuild supplier and blew up my brand new Roomba Scheduler. Hasn’t worked since, because I wanted to see if I could find the problem and fix it myself before sending it back for factory service. Turns out I can’t (or at least haven’t), but the inside is interesting nevertheless.

Opening

Getting into the Scheduler is easier than getting into the first generation, mainly because it (almost) all comes apart with screws, instead of the (few) latching tabs in the original Roomba. These instructions on fixing the Roomba Discovery “Circle Dance” do a good job of showing the screw locations, although the site then goes on to describe processes specific to cleaning the wheels’ optical sensors that weren’t relevant to my problem.

The most important part that wasn’t obvious to me from the instructions is that the front bumper holds down the front edge of the top, so you must remove the bumper, even if you don’t need to work on the bumper area.

Guts and Wiring

Once the cover is off, the inside looks pretty tidy. As on the original Roomba, there’s one main board sandwiched between the battery compartment and the brush deck, and all the sensors and motors cable up to it.

I had to pull all the cables before I could get the board out, and most of the cables had only one place they’d logically plug back in, but I still took pictures to make sure I’d know how to put it back together again, shown here for the convenience of all the king’s horses and all the king’s men who might be trying this themselves at home.

Having removed all the cables, there’s still an optointerruptor at each end with a bumper lever latched into it. It took some prying to get those loose — port (left when in motion, right when facing it to work on it) side first, then pull the board itself loose of the starboard side.

Main Board

And here’s my main board, with nothing that I can see wrong. No scorched components,

no scorched traces. Foo. (The battery connector is J7 on the solder side, in case you want to trace out from there and try to debug this for me.)

With the component spacing so tight on the board, and no obviously damaged components to investigate first, I didn’t feel like bothering to plug all the connections back in and trace battery voltage while the board was out of its little home. So I gave up (for now, anyway), contacted iRobot to ask about repair, and reassembled the Scheduler.

Reassembly

The most noteworthy thing about reassembly is getting the bumper’s port-end (I think) mounting bosses back into their mating holes. Do those first, then the starboard (I think) end of the bumper, then ease the rest of the port end the rest of the way on. Whichever end it is, do the posts first.

Dirt Sensors

The last thing to mention while we’re in here anyway is the dirt detectors. The second or third generation of Roomba introduced dirt detectors that are supposed to be able to tell when Roomba is actually picking up dirt, so it can spend more time vacuuming that area. I think my dirt typically has a fairly uniform distribution on my floor; but maybe some folks like to send Roomba out to clean up knocked-over flowerpots and whatnot.

Anyway, I’ve always wondered how it could tell when there was dirt — some fancy-schmancy optical sensor pointing at the floor??? — and here’s the answer.

Piezo sensors. Dead simple. Dirt hits them, ting-ting pting tang, and they translate the sound / force into an electrical signal that the Roomba interprets as the influx of dirt. Brilliant!

iRobot promises to respond to a customer inquiry within one business day of receipt, so . . . they’re late. But we’ll see what they say about repair service and cost. I understand they also sometimes have returns and refurbs available for purchase; and at the right price, that could get me a new Scheduler and leave me a spare for parts.

Hm, looks like entire used Schedulers are running ~$100 on eBay, and I just found someone selling the circuit boards for $20 plus shipping. I can’t imagine iRobot touching that price for a factory repair, so it looks like I may be able to do this myself after all. Maybe get a spare Scheduler just for the fun of it, too.

Posted in Inside, Repairs, Roomba | 6 Comments »

A6276 LED Controllers for “Organic Energy Cloud”

Monday, December 29th, 2008

My earlier edge-lit plexiglass demo was a study for an art/technology collaboration with Lisa Rundstrom that became known as “Organic Energy Cloud,” installed at Diver Studio for the November 28 Final Friday.

Lisa and I ended up agreeing on 200 LEDs (and ultimately installing 160). I had decided early on that I wanted to use two A6276 16-LED drivers to a board, for distributed LED control, all run by an Arduino. Once I got all my parts, over the November 22 weekend, I designed and began assembling the driver boards.

Schematic and Boards

The schematic is dead simple — daisy-chained A6276es, common clock and latch lines, output enable tied active (low). For speed and cost, I planned card-edge connectors to solder all the LED and “umbilical” connections to. Although the A6276 doesn’t have separate digital and analog grounds, I used separate wires in the umbilical for digital and LED V+, and I doubled the GND and LED V+ lines for current-carrying capacity.

I hand-etched a prototype PCB to make sure the design worked, but it was pretty time-consuming and not my most beautiful work (especially being a two-sided board). Tom McGuire milled me a much more beautiful set of boards (although not using that mill), and did a very nice job using the drill holes as registration marks to line up the milling for the back side.

Assembly

Monday the 24th, I started pressing my friends into service for slave labor. Jeremy and Mindy were my first victims, and together we got all the PCBs and about half the LEDs assembled. Jeremy was stripping wire ends and Mindy and I were soldering — she did a fantastic job, especially for someone who had never soldered electronics before.

The key to retaining a shred of sanity while soldering wires onto SMT LEDs is a good soldering jig. Sunday night while assembling my prototype control board, I had tried to assemble LEDs with just the helping hands vise, and the LEDs kept going crooked in the alligator jaws. It took me about an hour to solder sixteen, and I knew that wouldn’t get 200 done in time.

I figured my mom still had some wooden clothespins, and after a little quality time on the disc sander, I had some perfectly elegant, incredibly functional SMT LED soldering jigs. Production speed skyrocketed; frustration plummeted; stabbiness dissipated.

With the LED wires attached, and especially once the umbilicals were on (not shown here), the controllers really reminded me of facehuggers.

Rather than try to size LED wires specifically for LED placement within the piece, we made them all the same length. Lawrence, Gail, and the kids (especially Phill and Jake) stayed up late the next night helping me finish soldering LEDs and assembling the controllers.

Bus

I designed the controllers so that the clock and data lines would run in parallel to every board, but each board would have a separate latch line. It doesn’t matter what’s in the A6276′s internal serial buffer — it only matters when that chip gets its buffer latched to the outputs — and this arrangement made for a minimum of connections to the Arduino.

I knew we were going to be plugging and unplugging the umbilicals from the Arduino multiple times before we got everything assembled; but because of the parallel bus arrangement, there were too many wires to plug the umbilicals directly into the Arduino’s headers. I thought about plugging the bus together on a breadboard, but it would have made for a lot of jumper wires for the paralleled lines.

It ended up feeling easiest to design a small bus board with female headers on it, with the power, data, and clock lines bused, and the latch lines run to a separate 8-pin header at the top. (The bus board could run eight controllers — 256 LEDs — although we only ended up using five controllers.) I would have chosen to use ribbon cable to connect the latch header to the Arduino, except I forgot about that jumper and had to make something quickly out of borrowed wire while we were doing the gallery installation.

All Together

Because I was prepared to run 200 LEDs at ≥ 20mA each, the small power supplies I have weren’t going to be up to the task, and I used a PC power supply, not visible here except for its DC power cables.

The Arduino is plugged into the bus and the PC power supply, and in the foreground is a breadboard for a part of the project that didn’t come to fruition. This is actually how it sat during the show — Lisa is fascinated by technological infrastructure, and the mess of wires was part of the “organicness” of the piece. More on that in a follow-up post on the installation.

Controller Enhancements

Soldering all the LED wires to the card-edge connectors turned out to be a bit of a chore with someone helping with two vises and a needlenose pliers, and incredibly tedious to do alone. All along, I’ve been thinking about how I could design or redesign the controllers so they’d be practical for artists to use without me there to assemble everything, and these boards just aren’t suitable. I have three crucial considerations that weren’t anywhere near met:

Wiring the LEDs to the board needs to be easy. (Soldering individual wires was time-consuming and hard to do alone.)
Replacement of burned-out LEDs needs to be easy. (Desoldering wires from the controller and resoldering new wires isn’t practical, especially once the board is installed in a piece.)
Replacement of burned-out boards needs to be easy. (Desoldering all the wires from a bad board would be a nightmare.)

So ideally I’m looking for a connector that would make it quick and easy to change individual connections on the LED side, and also quick and easy to change a whole batch of connections on the board side — like a daughtercard with an edge connector on the bottom and jacks for LED wires on the top.

Coincidentally, the week after the show, I was at Anixter’s Chicago facility evaluating replacement keycard door access control systems for work, and I found what looks like the perfect connector on the back side of some HID card readers. The connector has screw terminals on the top for the wires that go to the door strike, motion sensor, etc., and plugs on the bottom to snap the whole thing into the socket on the card reader. The guys from the lab told me they think it’s called a Phoenix or Buchanon connector, and kindly gave me one to take home!

Now I just need to spend some time going through about a hundred pages of the Digi-Key catalog to see whether they carry this, and maybe to see what else they have that’s similar and might be even better.

Meanwhile, this past weekend during a scheduled power outage in our computer room for some kill switch and fire suppression work, I found this connector (upper left) inside the fire alarm control panel. (BlackBerry camera + closeup == fuzzzzzzz . . .)

I really like the orange quick-release levers on this — it’d sure be handy for hooking up the wires in the first place (presuming, as with the screw terminals above, that one is using a large enough gauge of wire that one can clamp them and make good contact). I also love that the wires feed out the top of the connector instead of the side. But it doesn’t offer bulk plug action like the above Phoenix-or-Buchanon connector.

Especially if the two had the same pin spacing, maybe one could offer boards with choice of connector. I think I’d pick the quick-release connector for use in a project with only one or two controllers, and the two-level connector for use in a project with lots of controller boards.

More quality time with Digi-Key for me.

Posted in Arduino, Inside, TechArt | 25 Comments »

X-Ray Control Panel???

Sunday, November 16th, 2008

One of the electricians at work gave me a bunch of circuit boards from decommissioned equipment this week. It’s usually elevator stuff, so I hadn’t paid much attention to it other than to note the pretty colored wires.

The I was taking it out of my trunk to put into a “process later” pile when I noticed the front.

“X-Ray.” 60kV tube voltage. “Fine focus.” Whaaaa???

Combined with the way all the front-panel switches and knobs are bent and broken off, I’m guessing this was in one of the laboratories, some unfortunate researcher accidentally stepped in front of the beam and mutated into a creature with superhuman strength, and in the resulting chaos demolished the equipment. Since it was destroyed, now I have it. Cool!

I went ahead and disassembled it tonight. Here’s a different view of the meters:

And there’s a big pile of connectors, resistors, and lovely wires on my workbench.

Cool Rotary Switches

Here are the two rotary switch assemblies. Their knobs were broken off and their shafts bent, so they’re not working terribly well, but I’ve improved them a little.

They have circuit boards that are ganged together, with the inner shaft turning the back set of switches (of course) and the outer shaft turning the front set.

The mechanics of the assemblies are fairly intricate. You can click the picture (as always) for the full-resolution version if you want to follow along.

Clockwise from the bottom:

Every switch position is a separate trace on the PCBs, with 24 positions on the wide assembly, 12 positions on the rear part of the narrow assembly, and two sets of 5 positions on the front part of the narrow assembly. A wiper on the plastic rotor connects the PCB’s inner ring trace to each outer pad in turn.

In the upper left, you can see how the limits of rotation are set by two discs with tabs sticking out, which bolt onto the head end of the (sub-)shaft. The discs’ tabs stop against a rear-pointing tab on the head-end mounting plate, which is on the underside in this picture.

Shown at the top, the rear portion of the narrow assembly is still in good condition. It was pretty gummed up, but a few sprays of silicone lubricant got it turning nicely. The PCBs are mounted on a set of multiple threaded rods, threaded spacers, and unthreaded spacers. The rotor’s detent action is provided by the wavy disc on the back side of the mounting plate, a ball bearing sitting in a hole in the plate, and a leaf spring on the front side of the plate.

In the upper right, the front portion of the narrow assembly is okay, but the outer shaft that used to rotate it was sheared off at the base (shown immediately below it).

I had to saw the front knob off the narrow assembly’s shaft to get the assembly apart and make part of it usable. You can see that the kob appears to have been threaded onto the end of the shaft; but with a vise and a pliers, I was unable to turn it loose. The shaft had been pretty badly bent anyway, so I have no hard feelings about having to saw it off.

Idea for Rotary Switches

So I’ve actually been looking for rotary switches like this, and thinking of trying to make some myself. This unfortunately is not the form factor I need, but it shows the idea is sound.

The high school robotics team has strict rules they have to play by, and one of them is that the joysticks used to control the robots in non-autonomous mode must work like PC joysticks (I think the PIC that runs their control panel is charging and timing an RC circuit to determine the joystick position) and cannot have any supplemental power.

This wouldn’t matter, except that the linearity of the joysticks they have is poor; and with (apparently) only 8-bit sampling, there’s not as much they can do programmatically to correct the linearity as they’d like. So it takes a bit of programming effort to eliminate drift when the stick is physically centered; and then when they start to move the stick, the robot lurches into action with not much fine control over low speeds, and at high speeds is pretty much just maxed out.

Of course I assume if it were my robot, I could correct most of that in programming. Still, that’s a lot to ask of high-school kids who are already making amazing engineering accomplishments on a very tight timeline.

So Ron (of the fundraising concert, and father of the team captain) would like to figure out how to build a new joystick that abides by the letter and spirit of the rules but gives finer control over low speeds and has really significant jumps up to maximum speed only when you floor it. Obviously he wants pots with an S curve response (log taper in both directions from center), and he hasn’t been able to find that commercially, at least not that he could retrofit into a joystick.

His idea was to do it discretely — come up with some sort of switching action, then connect that to a resistor ladder. He could play with the resistor ladder to his heart’s content until he got something that “felt” right for the application — make it pluggable and let the kids swap resistors until they got a response curve they liked. And he wasn’t too worried about the robot lurching as the joystick went from step to step on the ladder — he feels that relatively few values would suffice.

If these 24-position rotary switches could fit into a joystick’s gimbal assembly, they’d be fantastic for that! Reserve the middle 3-4 positions for a broad center band to eliminate home-position drift, then have ten more positions in each direction for different speeds.

Except I’m pretty sure the gimbal assembly has little 3/4″-diameter pots right there on it, and these big PCB things just wouldn’t work. Feh.

I thought about etching my own PCBs to replace the wafers inside a couple of sacrificial pots, but I hadn’t figured out quite how to route all the wires out.

Open to suggestions here.

Posted in Ideas, Inside, Salvage | 3 Comments »

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