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Hardware components:
Bosch Scales (PPW7170)
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Wunderbar relayr 02 1024x1024
WunderBar
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Code

Project logs

Reading resistor values
Project log #2
On Mar 1, 2015 at 9:45 AM - 0 comments

Since we gave up on hacking the Bosh digital scale the "digital" way, we opted to lobotomise the thing and just add our own ADC. However, this comes with an entirely new set of problems: reading the tiny resistor changes of load cells requires some analog circuitry. Not or strongpoint at all.

The first issue: all the load cells in the scales seem to have 3 wires. Somewhere in the back of our head we remembered something called a Wheatstone Bridge. However, all the load cells in the fancy drawings just have 2 wires. Where is that 3rd wire coming from?

Luckily, the internet always provides the answer. The 3rd wire is used to compensate for wire resistance. Using this knowledge we where able to draw up a simple schematic of how we should connect the load cells.

Next comes the amplification. We used a simple differential amplifier circuit based on the common LM741 opamp. TI has some nice application notes on how to do this.

The original plan was to use a grove ADC and the bridge Wunderbar module. However, due to problems with the module's firmware, we had to improvise an other way of digitising the analog signals.

We opted to use the Spark Core we brought with us, and transfer the data to the Wunderbar using UART.

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Gilles De Mey
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Wd attp012
Tom Collins
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Brit, runner, strummer, tech anarchist, cocktail, etc
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Seppe Stas
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Michiel De Mey
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Full-time geek, Full Stack Engineer and Full Metal Hero. NodeJs, AngularJs, API design, WebSockets, WebSec & IoT enthusiast. Former San Francisco resident.
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