1. Misadventures of the skeletonDAC
Because of
a̶l̶i̶e̶n̶s̶ bad engineering, my Dell E6410 laptop makes copious amounts of high-pitched hissing, beeps, and other funny digital noises out of its headphone jack. While occasionally amusing, it's also incredibly annoying when trying to listen to music, so I decided to obtain an external USB DAC. One of the cheapest decent options is the DIY skeletonDAC, which can be built for about $20. It has a completely imperceptible noise floor and sounds great.  |
| Not mine... |
The skeletonDAC has some substantial SMD soldering involved, including soldering a PCM2704 DAC chip with 0.65mm pin pitch. Yikes. All I had was a cheap $5 iron and some 22 gauge 63/37 solder.
SMD soldering actually isn't too bad. On my first attempt, I used the flood and wick method, which is exactly what it sounds like - you flood the pins with solder, and then remove the solder bridges with wick. The results were acceptable:
As documented previously, I accidentally soldered the chip in backwards during attempt 1 and destroyed the chip attempting to desolder it.
In attempt 2, I tried flood and wick again, but I accidentally went too aggressively with removing the solder bridges and bent and destroyed several pins of the PCM2704 while wicking.
I switched to flux-based SMD soldering in attempt 3, which works absolutely fantastic. With flux, you can do cool things like drag soldering chips:
With flux-based SMD soldering, you apply solder to the tip of your iron and transfer it to the pins. This is a huge no-no with through-hole soldering, but works exceptionally well for SMD if you flux the pins of the part you're trying to solder.
Unfortunately, DAC attempt #3 started exhibiting strange issues. It first refused to turn on, then started sporadically spamming volume down and mute (shorting pins 22 and 24 on PCM2704 to pin 27). After several soaks in isopropyl alcohol (I suspected uncleaned flux residue was shorting the pins), the thing worked, but had serious high-frequency attenuation.
Thinking I might have damaged the chip with heat, I made a 4th build, which also attenuated the highs.
It turned out the flux paste I was using was acid flux, the kind of stuff people use to solder pipes and really bad for soldering electronics.
Attempt 4.5 (using attempt 4's board, a new PCM2704, and proper flux) finally worked. Unfortunately, 3 days later, it fell victim into the same issues as #3. At this point I had spend $100 on building a $20 DAC.
Currently, it looks like the left and right channels are being mixed seeing as the output on the two channels looks identical in Audacity:
 | |||
| Intro to Jaco Pastorius' Continuum |
 |
I probed the outputs of my DAC, both before and after the output DC-blocking caps, and there seem to be no shorts. Still investigating...
2. RGB swing arm Lamp
 |
| I didn't just use the Colorize function in GIMP, I swear. |
This was my final project for my EK 156 Design & Manufacture class over at BU, which I worked on with Vinny Celeste and Nick Maresco.
It's a swing arm desk lamp that uses RGB LEDs, specifically 3 of these $5 ones. The lamp is mechanically made pretty much entirely out of 6061 aluminum, save for a sandblasted polycarbonate "lamp shade".
For the electronics, I decided to etch a board at MITERS and installed it in an inexpensive aluminum die cast enclosure. It uses an LM2576 buck converter to take 24V from a wall wart down to the LED forward voltage (2.5V for red and 3.5V for green/blue), and feeds it into a PWM circuit.
 |
| Complete with lazily drawn inductors. |
Admittedly, I voltage drove the LEDs (LEDs are current driven devices and are much better done using constant current) and furthermore used some obscene 555-based PWM circuit instead of say, just using a TL494, both of which are major design issues. On the other hand, it works (well, at least until the LEDs blow up because lol voltage driven LEDs).
Also, I learned TO-220 MOSFET tabs aren't ground and will lead to issues if you ground them.
I ended up getting a B in the class, but whatever.
3. Imaginary Over Due Scooter
Inspired by Noel Hwang's scooter , which he built last summer, and following Charles Guan's excellent guide, I decided to build own electric kick scooter.
Design goals:
- 15mph top speed
- 2-5miles of range
- Regen braking (this is more out of curiosity than actual necessity)
- <$450
- Be sort of lightweight
Following Noel's advice, I decided to use 5" wide aluminum 6061 U-channel from McMaster as a chassis. I would have preferred to fabricate a T-nutted aluminum chassis or a welded steel one, but I don't have easy waterjet access, and steel would require painting.
Other tentative specs:
Fork: Crufted off a Razor A3
Motor: Hobbyking SK3 6364-190KV brushless DC motor
Controller: Kelly KBS36051
Wheels: 5" x 1-5/16" Colson Performa 65A solid wheels (donut tread because turning)
Gear ratio: Probably 9:25, #25 chain sprocket
Battery: 10S 2P A123 26650 LiFePO4 cells, 33V bus
Chargers: 2x Turnigy Accucel 6S chargers in series (because screw paying $120 for a 10S charger)
If I wanted to, the U-channel could easily fit 8S 5P A123s, which would net some obscene amounts of range. However, my use case for a scooter is mostly for short 2-4 mile hops rather than 10+ mile trips (which I would either take the subway or use my bike for), and fitting in that much battery would make the scooter really heavy. My expected use case for the scooter involves bringing it on the subway and using it for the short hop between the subway station and my final destination, which favors a more lightweight scooter. I decided to shave down the U channel and run 10S 2P instead, for lighter weight and a slightly lower ride height.
In reality, I would like to build something very lightweight with hub motors, like Charles Guan's RazEr or RazEr rEVolution. People warned me against using hub motors for my first ever EV, however. Also no waterjet access.
Anyways, random incomplete CAD drawing:
 |
| This photo probably shows nothing other than laziness at CADing |
