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oliexdev
2014-12-16 09:39:31 -08:00
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@@ -23,6 +23,99 @@ openScale is an Open Source App for Android to keep a log of your body weight, f
## Reverse Engineering of the scale
First of all I had to find a suitable bathroom scale that I wanted to reverse engineer. I was searching for a cheap bathroom scale that can analyse not only my weight but also my body fat, water and muscle. The scale design should be clear and the display of the scale should have some kind of a simple seven segment display (I hoped that a simple display would be easier to reverse engineer). The [Sanitas SBF12 scale](http://www.sanitas-online.de/web/en/products/weight/SBF12.php) that I found in a department store seemed to be right for my purpose. So I bought one for only 20€ (around 25$), see figure 1.1.
<p align="center">
<a href="https://github.com/oliexdev/openScale/raw/master/doc/sanitas_sbf12/package.JPG" target="_blank">
<img src='https://github.com/oliexdev/openScale/raw/master/doc/sanitas_sbf12/package.JPG' width='300px' alt='image missing' /> </a> <br>
<sub><b>Figure 1.1: Sanitas SBF 12 scale</b></sub>
</p>
To gain access to the inside of the scale I had to unscrew only a few screws on the backside of the scale, see figure 1.2. Surprisingly the circuit board is well labelled and structured. As you can see on figure 1.3 the pins for GND, VDD, UP, DOWN, SET and the pressure sensors pins are easy to find. On the left side on figure 1.4 you can see the vibration sensor and the two outgoing wires going to a 3.3V coin cell battery. On the top you can see the connector pins for the scale's display. But unfortunately the microprocessor of the scale was hidden under a big black blob of some kind of plastic on the middle of the circuit board.
<table border="0">
<tr>
<th>
<a href="https://github.com/oliexdev/openScale/raw/master/doc/sanitas_sbf12/scale_overview_raw.jpg" target="_blank">
<img src='https://github.com/oliexdev/openScale/raw/master/doc/sanitas_sbf12/scale_overview_raw.jpg' width='250px' alt='image missing' /> </a> <br>
<sub>Figure 1.2: Scale overview</sub>
</th>
<th>
<a href="https://github.com/oliexdev/openScale/raw/master/doc/sanitas_sbf12/circuit_board_back_raw.JPG" target="_blank">
<img src='https://github.com/oliexdev/openScale/raw/master/doc/sanitas_sbf12/circuit_board_back_raw.JPG' width='250px' alt='image missing' /> </a> <br>
<sub>Figure 1.3: Back side of the circuit board</sub>
</th>
<th>
<a href="https://github.com/oliexdev/openScale/raw/master/doc/sanitas_sbf12/circuit_board_front_raw.JPG" target="_blank">
<img src='https://github.com/oliexdev/openScale/raw/master/doc/sanitas_sbf12/circuit_board_front_raw.JPG' width='250px' alt='image missing' /> </a> <br>
<sub>Figure 1.4: Front side of the circuit board</sub>
</th>
</tr>
</table>
Because I couldn't identify the name of the used microprocessor and I also didn't have access to the microprocessor’s pins, I had only two options left to read the measurements from the scale. One option was to read the raw values from the sensors and do the weight calculation and the impedance calculation on my own. The other option was to decode the scale's display. The idea of the second option was that all calculations of the body analyses are done by the scale as usually and you only had to decode the scale's display with an attached microprocessor. The first option was too complicated and the change that I am doing something wrong with the calculation was too high, so I tried the second option.
The first step was to connect all pins with wires that are routed to the display connector. Luckily on the circuit board are testing points for the display, which you can perfectly use for soldering a wire on. I recommend using enamelled coper wire because it's very thin and you can solder it very easily. You can see my result in the figures 1.5-1.9.
<table border="0">
<tr>
<th>
<a href="https://github.com/oliexdev/openScale/raw/master/doc/sanitas_sbf12/circuit_board_back_wired_unorderd.JPG" target="_blank">
<img src='https://github.com/oliexdev/openScale/raw/master/doc/sanitas_sbf12/circuit_board_back_wired_unorderd.JPG' width='250px' alt='image missing' /> </a> <br>
<sub>Figure 1.5: Connected wires unordered</sub>
</th>
<th>
<a href="https://github.com/oliexdev/openScale/raw/master/doc/sanitas_sbf12/circuit_board_back_wired_ordered.JPG" target="_blank">
<img src='https://github.com/oliexdev/openScale/raw/master/doc/sanitas_sbf12/circuit_board_back_wired_ordered.JPG' width='250px' alt='image missing' /> </a> <br>
<sub>Figure 1.6: Connected wires ordered</sub>
</th>
<th>
<a href="https://github.com/oliexdev/openScale/raw/master/doc/sanitas_sbf12/scale_overview_back_wired.JPG" target="_blank">
<img src='https://github.com/oliexdev/openScale/raw/master/doc/sanitas_sbf12/scale_overview_back_wired.JPG' width='250px' alt='image missing' /> </a> <br>
<sub>Figure 1.7: Back side of the scale wired</sub>
</th>
</tr>
<tr>
<th>
<a href="https://github.com/oliexdev/openScale/raw/master/doc/sanitas_sbf12/scale_overview_front_wired.JPG" target="_blank">
<img src='https://github.com/oliexdev/openScale/raw/master/doc/sanitas_sbf12/scale_overview_front_wired.JPG' width='250px' alt='image missing' /> </a> <br>
<sub>Figure 1.8: Front side of the scale wired</sub>
</th>
<th>
<a href="https://github.com/oliexdev/openScale/raw/master/doc/sanitas_sbf12/wired_with_notes.JPG" target="_blank">
<img src='https://github.com/oliexdev/openScale/raw/master/doc/sanitas_sbf12/wired_with_notes.JPG' width='250px' alt='image missing' /> </a> <br>
<sub>Figure 1.9: Attached notes to the wires</sub>
</th>
<th>
<a href="https://github.com/oliexdev/openScale/raw/master/doc/sanitas_sbf12/display_connector_pin_layout.png" target="_blank">
<img src='https://github.com/oliexdev/openScale/raw/master/doc/sanitas_sbf12/display_connector_pin_layout.png' width='250px' alt='image missing' /> </a> <br>
<sub>Figure 1.10: Pin layout of the display connector</sub>
</th>
</tr>
</table>
Next step is to analyse the signals that are controlling the display. I had the opportunity to use a 16 digital channels oscilloscope ([Agilent Technologies MSO7014B](http://www.keysight.com/en/pd-1788165-pn-MSO7014B/mixed-signal-oscilloscope-100-mhz-4-analog-plus-16-digital-channels)) for this step, see figure 1.11. Alternative you can use a microcontroller like [Arduino](http://www.arduino.cc/) to read the signals.
<p align="center">
<a href="https://github.com/oliexdev/openScale/raw/master/doc/sanitas_sbf12/oscilloscope.JPG" target="_blank">
<img src='https://github.com/oliexdev/openScale/raw/master/doc/sanitas_sbf12/oscilloscope.JPG' width='300px' alt='image missing' /> </a> <br>
<sub><b>Figure 1.11: The 16 digital channels oscilloscope for analysing the signals</b></sub>
</p>
## How its started ...
I gained a little weight a while ago, not overweighted, but I decided to do something about it. So I started my first diet. I bought a diet book in a book shop and followed it strictly about 2 weeks. At the beginning it was really tough, but in the end my diet worked out. I lost around 10kg (around 22lb). In that time I checked my weight every two or three days with my normal digital bathroom scale. That's helped me to stay motivated because I was happy to see that I gradually lost some weight. The disappointment was that my diet progress was not tracked in any way. Of course I could create a spreadsheet and wrote down my weight values but that would be troublesome and not seasonable in a world of smartphones and tablets.