Overview

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After jumping on the bathroom scales following a recent holiday, I decided it was probably time to pay a little more attention to my weight! Something that's worked well for keeping on top of our home energy consumption is just having better visibility of the numbers, which I think in itself leads to better behaviour patterns. So, the idea was born that if I could get a graph of my weight onto (for instance) my iGoogle home page, then I'd be more inclined to skip the danish pastry with my morning coffee (or even grab an espresso instead of a cappuccino).

I've already got a number of XBee enabled devices around the house, but thought this project may have wider appeal, so decided on a direct WiFi connection. Googling for WiFi scales, I found what looks like a very slick consumer offering already out there http://www.withings.com/en/bodyscale, but (a) it was relatively expensive, and (b) where's the fun in that when you can make something yourself..

Mechanical platform

First step was to look for some donor scales to use as a base platform. Trawling eBay, I found some basic scales that were widely available from multiple vendors, not too slimline (so hopefully plenty of space for electronics), and remarkably cheap (less than £8 delivered). On the downside they only offered pounds and kilos as units, whereas in the UK we're still quite used to weighing ourselves in stones. (As it happens this turned out not to be a problem).

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Once received, and taken apart, they revealed a 3-wire load cell in each corner, a simple PCB containing an ASIC and a handful of passives, and a bare glass LCD.

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Initial prototyping

For anyone interested, I have documented my initial prototyping steps here.

Initial design decisions

Schematic capture and board layout

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toi import KiCad schematic, plot to svg, and then convert to png (need to manipulate alpha channel else background is black)

convert schematic.svg -flatten -resize 1024x768 schematic.png

Board manufacture

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Soldering

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