bitquill - Spiros Papadimitriou

W-Ear enclosure and LiPo controller

Under construction

Some time ago I backed the  W-Ear kit on Kickstarter. I was originally planning to draw a simple enclosure (rather than an Altoids tin) but it was summer and I was itching to design some non-trivial circuit, so I decided to make it rechargeable ( related blog post).

The LiPo charger is a derivative of  Adafruit's MCP73833 board, with a few minor tweaks (basically some additional input protection). The switching boost converter is based on the MIC2288 datasheet (with some extra components on the output, so I could experiment slightly -- most of those capacitors and inductors are not needed in the final version). The irregular board outline was a bit of a pain to do in Eagle; I ended up doing it in a proper CAD program (by tracing the outline of a scan of the W-Ear PCB), then importing the DXF into the outline layer in Eagle.

After I had the PCBs made (OSHPark FTW!), my fears that the output ripple would be too much were confirmed. Instead of redoing the entire boards and/or using a properly designed passive filter (which would have made the PCB much larger), I decided to just swap out the feedback resistor to increase the output voltage, and then add a second regulation stage using a low-noise, high-PSRR linear regulator. Searching DigiKey, I found the ADP7102. Not exactly a cheap component, but for a one-off build I didn't care; just wanted to minimize re-build hassle and cost.

After adding the second regulations stage, everything worked as expected! I'm using a cheap LiPo off of E-Bay (I think it's a 1000mAh one). Here is what the PCB "sandwich" looks like, in the custom-designed enclosure):

Assembled

I added a piece of paracord as a loop strap (that's what the two holes are for), and it works nicely. The only casualty was a blown LM386 (about a dollar from DigiKey, no big deal); not sure how, probably shorted something when it was lying around my desk. The W-Ear itself is decent, but the LM386 perhaps wasn't the greatest choice (9V is a bit of a pain to work with, and as used it is *quite* picky about what headphones it likes). However, the phased array works reasonably well. It picks up some ambient noise (possibly transmitted through the enclosure, not sure), but others wearing it can hear my whispering when I can barely hear it myself. Impressive what you can do with a couple of capacitors and resistors! Overall, quite happy with it. Plus, it was a fun hack overall, and a useful learning experience (although it will likely be a while before I attempt another non-digital circuit! :) ). Only problem is that battery is drained fairly quickly (within a couple of days?) even when the mic is off.

All the design files are attached to this page; released under a CC-BY-SA license. If you do anything with them, please drop me a line!

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