Sponsored Material

I got the first of the sponsored material in! 100 ATtiny45's! Thanks, Atmel!

Chibots Digital Medal

The design is a success! All five prototype medals worked perfectly on the first try. They look great! With sponsorships and donations the cost is at $12.10 per a medal and is falling as more donations come in. I'm really excited to be able to deploy this medal to the club! I can't wait to award the first one!

I put the LEDs in and put it in a 1/3 drop resin medal. Red for this prototype. I'll make white and blue medals as well. I ended up going with 180 ohms in the reds after all. That brings them to just at 20mA whereas on the white LEDs its more like 14 or 15mA. The whites actually seem a touch brighter than the red even so.

Overall the effect is wonderful. In the picture the area around the LED is saturated but it looks great in person. Woot!

Success for the Serial Medal

The very first PCB for the serial medal worked flawlessly on the first attempt! I know it looks a little screwed up in the picture, but that is because of my crappy cell phone camera's very slow shutter speed. Anyway, I am terribly excited! This thing is finally coming together. I need to run out to get resistors for the red LEDs I want to put in it. 180 Ohms works great for the white LEDs but the reds have a much lower voltage drop. And I probably need to snag some 47s for the row selectors as I just had 58s on hand. Although, it looks fine with 58s.

To be certain I need to make up the other four boards and sit on them for a few days to be sure they are stable. Assuming that goes OK the next step will be to finish the work up on the Bill of Materials. I'm pretty sure I'm going to need some sponsorship to get the cost under $15 per a medal. However, I have high hopes for getting the PCBs themselves sponsored which may land me right in the $13 range.

Brazing

I gave aluminum brazing a shot this weekend. Its pretty cool really. I got a pound of rods from DuraFix. (Check their sales video. Another company that sells the stuff and also has a video is Aluminum Repair.) I also picked up a mapp gas torch kit from a hardware store. The idea is that you get the aluminum up to a certain temperature such that the silver alloy in the rods will melt, but the aluminum itself won't. The alloy act as a sort of metal glue.

It turns out that the metal needs to be pretty clean and further it has to be cleaned the right way. A stainless steel brush was provided with the rods but I soon discovered you really need two: one for pre-cleaning and one for manipulating the alloy while hot. To to make a joint its a good idea to tin both sides.

It also turns out that mapp gas is plenty hot enough to melt aluminum. I slagged a couple of pieces before I got the hang of it. Probably need to back down to propane or even butane.

After around five or six attempts I got a pretty strong joint. Its the one in the picture. I managed to crack the joint open but it's still pretty strong and it took a hell of a beating just to get to that point. To be able to make that strong of a joint with a couple hours practice is pretty cool. Supposedly, after I master the techniuqe the joints will be stronger than the original metal.

I figure this brazing thing can be very useful for making custom aluminum chassis for my robots. When it comes time to make my RoboMagellan bot water tight I think I'll be using this technique.

Anyway I just wanted to do a quick post with a couple links. Sorry its not longer and doesn't share the experience more in depth.

The Serial Digital Medal PCB Layout

I've also finished the PCB layout for the circuit. I want to fuss over it a bit more, because 5 prototypes boards run me about $95 . Thus any mistake is an expensive one. I'm still kicking myself for the too-small hole sizes from the very first version of the board.

However, the last layout took about a week and a half of work to get it 100% routed to where I was happy. This board auto-routed fully on the second placement attempt. I still needed to do some manual re-routing but nothing too serious. Plus, all the normal lines are 10 mil and the power lines are 15 mil. Each chip has its own 0.1uF anti-noise cap to boot. I'm hopeful the intermittent noise and dust problems of the past will be gone from this design. I'll still probably get it a good solid flux cleaner bath though.

Chances are I'll put PCB layout in today or Monday. I kinda wish I could have them for tomorrows meeting, but that meeting is going to be busy anyway and I absolutely don't wont to rush this. Rushing was a big factor in all the mistakes of the previous version of the medal.

This is starting to get exciting again!

Serial Digital Medal now on ATtiny45

So the medal is now working off of the ATtiny45 instead of the ATmega32. You can see the 8 pin DIP just to the left of the red dip switch. The display is working great with no glitches. I'm not sure what caused it before but if the problem doesn't come back I should be good. It might have been a loose connection or something. There are a lot of wires for this circuit.

In the picture the text looks pretty bad, but its really my cell phone camera. In real life the display is perfect. I still can't find my good 3MP cannon. :(

If you look to the lower left in the picture you'll see two more ATtiny45's. Those are the two MCUs I bricked before realizing that the programmer was still set to ATmega32. I wondered why the display started out slow and then actually got SLOWER when I set the fuses for a faster clock. Doh! I hope someone in the club with an STK500 can get them back for me.

Serial Digitial Medal working!

So the problem with the breadboarded serial medal was that the data and the strobe line was backwards and I had shifted the pins connections by one on one of the displays. Nothing that required an o-scope/logic analyzer at all. If I had bother to trace the lines I would have found it. Once I fix the problem the code started working pretty well. Oh well, the device I bought looks fairly neat. At the very least the software is super simple to use.

The breadboard is still using the ATmega32 at the moment but there is not much logical difference in this case. Still, to be safe I will through the ATtiny45 in there before I put the board order in.

Also the display isn't perfect. It does seem to glitch some times on the capital letters. It doesn't look too terrible, but I'd like to figure it out. It seems to be associated with the character rather than a particular location on the display. So it could be the algorithm. Overall it still looks good even with the glitch.

The Serial Digital Medal Design

So I mentioned before that, to reduce cost and pin count, we were changing the digital medal design over to some serial driver chips. I'd like to talk about the new design a little.

The big cost saver is the ability to use the ATtiny45 (2.13USD Qty: 1) instead of the ATmega32 (8.28USD Qty: 1). Previously, the driver chips that we were using were really just gangs of transistors. Each transistor had an input and output pin. Hence the MCU pin count requirement to operate the chips was high and forced us to the ATmega32.

After some searching our members came up with transistor arrays that, instead of individual transistor inputs feature a clocked serial data input line that feeds into a shift register and a strobe line that latches the shift register bits up to the (now internal) transistor inputs. Additionally the chips feature a serial data out line that leads away from the far side of the shift registers. The source driver, U1 in the schematic and used to select rows in the 5x7 LED matrix displays is the MIC5891. The sink drivers, U2 and U3 in the schematic, used to select columns, are two A6821's.

So in the new design the three driver chips and the MCU are daisy chained: Serial Data Out connects to Serial Data in, one chip after another with common clock and strobe lines. From the point of view of the MCU this forms a single shift register that is 24 bits wide. The first two bytes shifted out become the column selectors and the final byte becomes the row selectors. The idea is for the MCU to clock out three bytes, hit the strobe and then repeat. This basic scheme uses a mere three pins to reset the state of all row and all column selectors and is why the ATtiny45 will work.

Of course, this is just the design. I have the circuit fully breadboarded up and can see the clock, data and strobe lines popping away at the first chip in the daisy chain, but nothing is coming out of the other side of the shift register on its Serial Data Out line. Most likely this mean that something is wrong with the clocked data that the MCU is emitting and thus the first chip isn't properly receiving the data in the first place. I only own voltmeters so it is hard to tell what is wrong with the signal. I've ordered a cheap O-scope/Logic Analyzer from Hobby Lab that should help me out. It should present me with timing diagrams of the signals on clock, data and strobe lines that I can then compare to the example timings in the data sheets for the chips. Hopefully there will be something I can do to correct the signal. My concern is that since I'm using the Bascom Shiftout command, that the only solution will be to shift to C for greater control of the timing. Maybe it won't suck too bad if I really do have to port the code though.

Chibotica Fall 2006

Chibotica went pretty well. The turn-out was lower than I had hoped for. To my knowledge we only got one person from iHobby Expo. Still, I'm convinced the the Outreach program is the right thing to do. If I can convince the member to have our next fall contest at iHobby Expo, I think we will be quite well attended.

But again the show itself was very fun. The Mini-Sumo especially was dramatic. Its great that Mel's grand-daughter won it. I wish I could have given her a medal as reward. But oh well.

Digitial Medal Super Funness!

Well, I'm still working on the damn digital Medal. I have one fully working. It looks great actually. (I'd post a pic but I can't find my camera) But of course, we are going to change the design. The board is just not stable. I had one that worked great after construction but then it failed a couple of days later. It improved a lot with a alcohol bath. So at least part of my problem is just dust and debris. I need to buy flux cleaner so I can clean these boards too.

However, my ability to experiment further is limited because I have no good boards left. Basically i can try to repair one malfunctioning board. I will do that, but it won't bring me the confidence that I truly know what is going on with the boards. I am NOT going to order more boards. At $95/five they are just too expensive.

Anyway, another problem with the board is that it is expensive. The massive number of pins required on the MCU means that a ATmega32 is required. However, the MCU can't drive the LED matrix directly so driver chips are required. Some of the members rounded up some driver chips that take a serial input and can be cascaded. This means that the MCU now only needs three pins instead of 22. So we can go from an ATmega32 to an ATtiny45. Its a pretty big savings.

Still More Digital Medal

Weirdness.

Having missed all my deadlines for the medal I can now relax and go back to the drawing board to get the stupid thing right. I have breadboarded the entire circuit to see where I went wrong.

I got the same sort of weird errors on the breadboard. That's good from a troubleshooting perspective. When I traced it back I came all the way to the MCU pins. Some of the MCU pins can't hold an Active High under any condition except open. The inputs to the ULN2803A are decently high impedance I think. But some of the MCU pins can do it. Others can.

I may have been mis-handling the MCUs the whole time and destroying outputs at random. That could explain what I've been seeing in all my prototypes. I'm going to swap MCUs tonight and see if the symptoms change.

iHobby Expo

iHobby Expo was pretty cool. Its the largest outreach I've done so far. I learned a lot. The Hobby RC crowd was actually pretty interested in what we were doing. That was a little bit of a surprise. I think we'll get at least a couple new members out of this. Woot!

Disappointment

Pretty bummed tonight. I got the new corrected PCB boards today as well as the components to populate them. The boards went together easy. No more hole size silliness. I knew already to solder the UDN2981 on the bottom of the board.

However, the board doesn't work. A few columns light up. The best thing I can say is that soft power works. I have no idea of what's wrong at this point and the chances of correcting it by meeting time are slim.

Digital Medal sort of working Part 2

So, some heat applied to the row seven current limiting resistor fixed the dead row number seven. As for the doubled up column, I've determined that the column is shorted to Vcc on the MCU to transistor array link. That has to be an assembly issue. An unpopulated board doesn't have the problem.

So now the only worry is the soft power. Can it handle the load? Why is it dropping out to two volts in the present circuit? The short doesn't draw that much current, but since I don't know exactly where it is, I can't be sure of its full effect on the circuit.

I'd like to just cut the column signal line to see if I can get rid of the soft power problem, but its really hard physically. Plus there is no guarantee the as to the extent of the short. Maybe I should just test the load carrying capability with a false load? But what if the load is not the real issue? Hmmm. Hmmm. I will have to ponder it for a while.

Arrrr

My pirate name is:

Black Roger Rackham

Like anyone confronted with the harshness of robbery on the high seas, you can be pessimistic at times. You have the good fortune of having a good name, since Rackham (pronounced RACKem, not rack-ham) is one of the coolest sounding surnames for a pirate. Arr!

Li-ion Batteries

So today, bored at work, I began to think of the Li-ion batteries I got from Eddy some time back and how I might connect them. A lot of folks just solder the pads, but I kinda wanted to do a removable pressure contact type thing similar to what cell phones do. (The battery is actually intended for a cell phone.) I remember my old cell phone had theses sort of spring loaded plunger style contactors the rear battery door pushed the battery pads onto.

Anyway I went looking for the contactors, thinking they were custom and I'd never find them. I found these .1" Mill-Max spring loaded headers but, having only my faulty memory, felt sure that the battery pads were well over a .1" pitch. To my delight, when I returned home and bothered to check I discovered that my cell phone plunger things looked exactly like the Mill-Max headers and that the battery pads were in fact .1" pitch.

This looks like it will be a cool way to connect my batteries if I can come up with a good way to put pressure on the battery.

Digital Medal sort of working

Well I made some progress tonight. I have to short out the soft power circuit, but I can get the digital medal to mostly run. One column appears to be doubled up and the bottom row is completely dead. But the text is scrolling! Woot! I'll review the PCB design for the affected row and column, but it could easily be a drilled out hole issue. Its possible that heating the affected pad for a bit will get me a connection.

Anyway I'm happy to have progress.

Digital Medal Struggle

Sigh. So, the digital medal has problems. Not the least of which is the time and energy I have available to work on it. The worst problem is the damn hole sizes. I trusted the print out as a sort of eye-ball measurement that all the hole sizes were OK. They weren't. I have to drill out a lot of holes. The worst of that is for the LED displays that can't be soldered on both side. I have to drill a second hold very close the pad and run wire-wrap wire through it and the main hole to get a good mechanical and electrical connection to both sides of the pad.

The next problem is the third transistor array that was thrown in at the last minute. I guess I miscommunicated with the fellow who did the schematic for me. I though I had understood him to say that he'd prototyped the circuit. He hadn't done it for this last minute add. First I discovered that power was backwards. I should have looked more carefully once I discovered that. But, no. I started on a second drilled out board only to discover that the inputs and outputs were also backwards.

So now I'm onto to the third drilled out board. Ironically this board is a touch easier because I don't have to cut traces to fix the power. Since the inputs and outputs on the UDN2981 were also reversed, if I solder the chip to the other side it will be connected perfectly on the current board. If this works I don't think I'm going to edit the board except for hole sizes.

Anyway, I should be able to get a bit of work on it tonight. Maybe I'll be able to light it up, who knows. *I WILL* get it working for the November games dammit.

Digital Medal

Well the Digital Medal Project has been consuming almost all my free time for what seems like a month now. It's almost done! Today I got the prototype boards and tried placing the components. It was designed to be tight. I was astounded when the components snapped into the center hole with the precision that I had measured them for. Its a perfect fit.

All was not perfect, however. The PCB program I used to make the board shows the .02 in holes a lot larger on print out than .02 really is. Either that or the board shop used the wrong size bit. $90 worth of prototype board an I have to drill out half the damn holes and solder on both sides. That going to be a bit of a trick for the LEDs but I think I know how to do it.

Oh, well, the finals should be damn near perfect.

Now to solder up a board and see if the circuit actually works! Boy that it would be amazing to have everything fall into place right at the last minute here. The Table top competition is this Sunday!