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Weigh-Fi wireless scales

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

Following the prototyping stage, I decided on the following components :-

Atmel ATmega32U4
TI ADS1311
NXP PCF8566
LiPo battery
Maxim MAX1555 charger IC
Microchip xxx LDO
Roving Networks RN-171 wireless module
Chip antenna / U.FL connector
Mini-USB horizontal connector

Although I dearly wanted to get everything inside the original enclosure, the layout was just too tight & I just wasn't getting anywhere. So reluctantly, I split the design into two boards. One board inside the original case to handle the LCD and ADC interfaces, and a main board inside a new enclosure to handle everything else (USB, LiPo charging, MCU, Wireless etc). The plan is to stick this new enclosure to the underside of the glass platform, with a short ribbon cable connection to the display daughter-board.

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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BOM Costs

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Firmware

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Server software

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Links

All source code, schematics, and pcb design files can be found in the git repository https://github.com/dresco/

The source code is released under an MIT license, schematic and pcb files are released under a Creative Commons Attribution ShareAlike license.