Design notes

This is a decoration; the form is the function!

First Law of Christmas Decorations: A decoration must not harm humans (such as by catching fire or causing anguish by burning out their computer's USB port).

Second Law of Christmas Decorations: A decoration must look nice and blink its LEDs when the human wants it to, as long as this does not conflict with the First Law.

Third Law of Christmas Decorations: A decoration must not harm itself (such as by blowing out its power filter when plugged into a cheap USB charger), as long as this does not conflict with the First or Second Law.

Regarding the electrolytic capacitors: I'm using aluminum polymer for caps >1μF because they look more like decorations on a tree, rather than boring-looking MLCCs. I'm avoiding wet electrolytics, since the worst thing would be for them to leak on grandma's Christmas decorations during storage. Also avoiding tantalums to prevent burning down a nearby dead fir tree in the event of a failure.

Using electrolytics means that the LDO voltage regulator selection is harder; most LDOs these days want a super-low ESR, or a narrow range, and electrolytics' ESRs are pretty variable.

ESD is a concern with dry winter weather and sweaters, and capacitive touch sensors. The leakage current and capacitance of TVS diodes is a problem on the cap sense lines, so this design uses series resistors instead (along with the AVR pin's parasitic capacitance) to slow dV/dt to the point that the pin's protection diodes are fast enough to keep ESD away from the pin's gate. Not sure how well it'll work, but here's hoping.

Keep the cap sense lines short and thin, and try to keep them away from other lines to prevent capacitive coupling. Especially keep away from the LED cathode lines (which will be modulating at about 15mA @ 1kHz), and also the +5V line feeding them (which will be a bit smoother).

Program with either the TagConnect connector or the .1" ISP header (with pogo pins http://sfe.io/p11591 or solder in a header). Either one uses the same pinout as the Arduino's ICSP connector. I use USBtinyISP http://adafru.it/46 at 100kHz SPI clock (the default). Be sure your programmer is 3.3V safe, and don't have it feed the Vcc line (remove the jumper from the USBtinyISP). The Vcc line here is connected to 3.3V; it can be disconnected altogether with JP3 and/or JP4.

Be prepared to work with a cheap crappy USB charger that puts out 4.3V and is half ripple; use protection. Don't blow out your uncle's laptop with ESD; use protection. Supply suitable chargers with the devices. And use protection.

Keep protection close to the USB input; there's lots of places for liquid infil etc. to ruin your day.

The USB spec says max 100mA before enumeration, and 500mA if it successfully enumerates for higher power. We draw 500mA and never enumerate. This seems common among cheap USB-powered toys.

The USB spec says inrush current is limited to 10μF ∥ 44Ω. We ignore this and have about 25μF inrush current. This seems common; even the Arduino has 100μF inrush current.

Need to test against a few different motherboards to see if they trigger overcurrent warnings.

This won't work with most USB batteries; it doesn't draw enough current to keep them awake. (Move this design note to user notes.)

Keep space for a shroud between the micro USB connector and the board edge. So far, 0.6mm seems sufficient.

See schematic for a few other notes.