january update

Let’s see… last time, I was struggling with what processor to use, now I’m struggling with how the hell to keep this thing charged. The idea is to use a couple solar cells to make it autonomous. I’ll have to figure out how to flash firmware over the air too, but that’s not super hard… the hard part, as usual, is power. I ordered one batch of cells to try, but they were too delicate and only had one solder point on each cell. I should have known not to order from Edmund Scientific. After destroying those, I started working on a boarduino that’s more or less equivalent to what I’m planing on for v4.

So, let’s talk about v4. It’s based arount the atmega 1284p, and the nordic NRF80001. Same accelerometer and RTC as version 3, and for now the power supply is mostly the same. They may change as soon as the second round of solar cells come in, but the charge IC I’m using now might work, at least if I’m reading the spec sheet right. I have been wrong before. For example: I thought I was going to run this version at a much lower clock speed, to save power. As it turns out, I wouldn’t ever be able to put the processor to sleep if I ran it that slow, so I’m stuck using it at 8mhz. I may try it anyway; 1/2 the power means 1/2 the surface area of solar cells to keep it charged. I’m also hoping that the AVR is more effecient than the nordic. We’ll see.

I do have most of a schematic for it down in eeschema, kicad’s circuit design program. I don’t have the bluetooth part or the buttons. The buttons are trivial, and the radio is based on the reference design in the data sheet. So, mostly copying that over and tailoring it to fit my needs. FCC certification is a problem for when it goes on sale, if ever. My luck, it’ll be leaky in all the bands, and require massive sheilding. TBH, I think that’s how a lot of designs go, those little folded metal covers over parts of the board.

I have some preliminary thoughts for the shape and dimensions, too; hopefully thinner, but roughly the same area as the current iteration. This board version should have a cutout for the battery, and thus be a bit thinner overall. I’ll also probably farm out a 3d-printed version of the case, and use thinner plastic for the face of it. 8mm is the target, not counting the strap lugs. The solar cells may be links on the band, too, but that’s just a concept for now. I need to see them to know if I can use them that way or not.

Next post: how to use an arduino micro as ISP for a ATMEGA 1284p. If I feel up to it.

Posted by Matt on 2015-01-19 05:04:19 +0000