Grr! Apple has updated the firmware and USB handling on all their new iPods (and upgrades) so that they do not play with Mintyboost so nice. There’s a simple fix on the Mintyboost site, (just moving a resistor) and this will be updated in MintyBoost v1.2 which is in the works as we speak!

Now that i’ve finished porting icky C++ code I can return back to thoughts of sweet sweet hardware design. Next up is a second MIDIsense board.

The next board is pretty simple: An analog and digital IO board. This is what most MIDI interface boards are used for. The atmega8 chip has 6 10-bit analog inputs so there will definately be 6 analog inputs. Then I decided to put in 5 digital I/O too. Not only will you be able to get digital inputs (switches) but also control the pins as outputs including PWM outputs. OK not very interesting, I admit.

So then I thought about improving that a little bit:

For the 5 digital I/Os, instead of just having the pin go to the microcontroller and thats it, I put in spot for installing a capacitor to +5V and a series resistor. (To install the resistor a trace must be cut).

Of course, the RC pair can be used as an output filter: so that PWM output can be smoothed out nicely (the output will be PWM’d at around 16Khz)

The resistor can also be used to choke the output, for driving an LED for example.

But of course the real trick is that now the board has 5 extra analog inputs for measuring resistive sensors: by setting the output of the pin to +5V (discharging the capacitor, its tied to 5V) and then measuring how long it takes for the capacitor to discharge through the resistor! Its a poor method of measurement and takes a while to get a stable result (as compared to the onboard A/D). However, it certainly works and for many situations, and its good enough for most MIDI sensor applications where the 10-bit A/D may be overkill.
(You can read more about the RC timing A/D trick from this old-school app note from phillips although its rather outdated, you can get an idea of how to implement it!)

So now the board has: 11 Analog inputs, or 11 digital/PWM outputs (5 with RC filters).

I make kits so I’m always interested in seeing what other people have up their sleeves. For my bike stereo last year I used 2 class-d amps based on the TPA3001D (1 x 20W). This year, I bought 2 AMP3 kits from 41Hz. Each kit drives 2 x 25W which is perfect since I have one speaker on each handlebar. They are designed to run on a 12V SLA battery (up to 14.5V input).

Stuff I liked about the kit:

• Silkscreened, soldermasked PCB for easy assembly
• Lots of thruhole vias, heavyweight copper
• Winding the toroids was totally painless
• Chip works great!
• Has a configurable pre-amp that is good enough to take line-level input

Stuff that was a little annoying

• PCB came with copper shorts, apparently a fluke but still frustrated me
• 0805 parts: Why? 1206 would be much easier! (Jan says the particular resistors were thin film and only available in 0805 although I’m still a little suspicious…)
• Output connection and power connection is adjacent for no particular reason, and holes are much too small for speakergauge wire. I think having large speaker-style terminal blocks for the speaker outputs would be a reasonable design upgrade.
• If you’re going to use 2oz copper, have thermals! Soldering this requires a very powerful iron which also needs to have a fine tip. That’s a bit of an oxymoron.

Overall: I give this kit a B+ Its a great design, with only a few technical issues. At $25 for 2 channels of 25W classD action, its a mega-bargain!

I <3 weller irons and xcelite tools. If you've looking to get a wes51 (my strong suggestion for anyone who's doing serious electronics) or 170M shear cutters (get 2, they get broken by misuse) this is a good time. Check out the flyer: its a stupid mail-in rebate but I’ve never had a problem with these. The irons are $20 off ($80 total) and the shears are $4 off ($0.60 total). You can get the tools at All Spec or Jameco.

Sometimes its really important to have a stereo system for your bike. SCUL has a ’standard’ method by which they use a 12V SLA hooked up to a car stereo which is then wired to some car speakers. It’s cheap and easy but I’m never one to leave well enough alone. Thus my ultra-light/ultra-efficient pimped-out bike stereo system:

• 2 x 7.4V 4Ah Lithium Ion Camcorder batteries = 15V at 4A = 60W of power! ($20 each)
• Class D amp. I use the TPA3001D1 you can get kits for this, I made my own PCBs cause I wanted to make at least 4 amps. Either is A-OK. up to 20W output! Is TSSOP so a pain to solder.
• Cheap MP3 player (this one was ~$10, uses MMC cards and a AAA battery) You dont want to get your iPod all dusty. Requires an MMC card, a 256M is like $25 or something.
• Speaker(s)! I recommend old car speakers. Making an enclosure for them will make them sound better but they’re sort of engineered for poor enclosure design. Cheap/thin cookpots work great and come with an attachment point even!

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OK so boards came in, and there’s 80-100 to be assembled. They’re pretty simple boards: 1 8-pin micro, 3 resistors, 1 capacitor and 2 4-pin header plugs. Since it takes a long time to solder, I designed the board for mass-manufacture: surface mount parts!

Eventually everyone needs to make a lot of PCBs (at home), the best way to do this is to use solder paste and a reflow oven. Since EYEBEAM has a laser cutter I can use it to make a screening stencil and a registration frame. (You can also buy stencils from your 4pcb.com and probably a bunch of other PCB manufacturers) Then it’s super fast to make tons of PCBs. Just silkscreen on solder paste (available at digikey), place the components, and bake!

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Last weekend I found an hour or two to relayout the PCBs to fit into the square tubing better. The boards are thinner and theres a tab below the microcontroller so you can grab it with a pair of pliers without damage.

There’s also a white silkscreen square to write an address to identify each PCB (since they’re addressable)

Then I checked the gerbers with GC Prevue and sent them off to 4PCB.com tuesday afternoon. With 2 day turn and overnight shipping that means they arrive Friday. Since it’s Kate’s first order with them, she gets $500 off the order… Basically we pay for shipping ($50).
Tomorrow I have to solder up 80 Duel Nature PCBs (maybe try out making silkscreens on the lasercutter)

At HOPE I got to look at a cell-phone jammer owned by a friend. This one is interesting because 1) its extremely well designed 2) it’s very small and 3) it actually has a PLL instead of just being open-loop controlled. The entire package fits into a pack of cigarettes.

(For more disassembly photos & comments…)

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Yay I’m done with another small kit. This one is a nice battery-powered USB charger. You can plug in anything that charges over USB like iPods, cameras, cell phones, etc. to get a lot more run-time. It runs off of 2 AA batteries, alkaline or rechargeables and has 2.5x more juice than a 9V-powered design.

Some numbers…
iPod video (tested, using alkaline batteries): 3hrs more video (1 full recharge)
iPod shuffle (unverified): 60 hours more (5 full recharges)
iPod mini (unverified): 26 hours more (1.5 full recharges)

This project is suitable for beginners, some soldering tools are necessary but even if you’ve never soldered before it should be pretty easy. You can etch a circuitboard and/or breadboard this up, or simply buy a kit.

I also spent a bunch of time documenting the process by which kits are born, so that people can learn about how to design stuff like this.

You can read all about it on the MintyBoost page or at Instructables and buy kits from the adafruit webshop.

Thanks to EYEBEAM Openlab for supporting this project as part of my fellowship!