Put a ring on it...

Me and Anna got engaged yesterday!  For a few years I've been trying to come up with an idea for a special ring that I could present to her... I wanted it to be something geeky, so a regular ring wasn't going to do... it had to have lights and stuff... otherwise who'd know what'd happen...

As these things often do, it started out quite simple.  I'd read about Charlieplexing before on Hackaday, and it seemed like a great way of doing a little array.  I figured I could just about get away with a 5x6 array on a board measuring 18x23mm... small enough to make into a ring.  It wouldn't need anything fancy... an 8-bit micro would be plenty enough to drive and display a few scrolling messages.

The schematic in progress ended up like this...

Using a QFN16 package let everything fit on neatly, with IO to spare to connect to other doodads when I actually figured out how to do it.  Processor was to be a PIC16F684.  There's quite a few pics of similar sizes, so I liked the idea of doing one with touch sensing.

The LTC3525 DC/DC converter went on the underside.  Power source was to be a supercapacitor, which was going to allow for very fast charging... a few amps for a few seconds, and you're done - excellent!  There are many companies who make these things, but after doing some research, the best I could find in small form factors were from an australian company called Cap XX.  They do them in prismatic packaging which allows for a very thin form factor... ideal for a design like this!

I bought a few and did some experiments...

This here is a series-stacked pair of HA130 cells, which are charged to around 5V with a very low power levelling opamp circuit, using the MAX4470... this works out to be much more efficient in terms of energy consumption than a lower resistance balancing ladder.  That small gap between the two bits of copper on the right hand side is a crude (but effective) resistive touch switch.  This controls a PMOS FET (with some gate protection circuitry) which allows current to pass to the LED.

While it worked pretty well on the bench, I noticed that the supercapacitors were discharging much faster than my simulations had anticipated.  After some discussion with CAP-XX, it turned out that self-discharge rate and leakage current are treated as two very different things... only the latter is specified.  The rate was so high that it wouldn't stay at a useful level for more than a few hours.... gah.  And paralleling cells instead doesn't help the standby current one jot as the discharge rate will be high on both cells.  A boost converter would allow a longer standby but the on time would be very short near the end.

Ah well.  It didn't look like this was going to be practical.  The thing was, if I started looking again at a conventional battery, pretty much anything would be overkill for just a few lights.  Hmm.  Maybe I could look at doing something a bit more complicated...