Hacking a digital bathroom scale

[Micah] was inspired by projects he had seen of people using the Wii balance board as an input. He realized the balance board was overkill, and pricey for many applications. Since it is basically just 4 weight sensors, he thought, why not just use a scale? Often, only one sensor is needed and they’re really cheap from big box stores. He picked up a digital scale and cracked it open. As he moved forward, he wanted to keep this pretty simple. There are other ways of getting the information from a scale, but they have been generally more complicated than what he had in mind. He ended up bypassing the internal unknown microcontroller and just connecting the analog sensors to his parallax setup. You can read all about the process an download some source code on his site.

[via Flickr]

22 thoughts on “Hacking a digital bathroom scale

  1. Ah, this is useful – I had been thinking of using a digital scale to monitor how much salt remained in a water softener tank.

  2. Funnily enough I had been tinkering around with exactly the same thing (but on a really old set of scales). I was trying to work out the mass/resistance relationship but from 0KG to my 91KG only gave an 8ohm difference. I was trying to avoid reading in from the LED display but I’m not sure how to continue.

  3. @Alex: That’s why scales need a pretty sensitive amplifier- the resistance change on a strain gauge will be really small. (I’m actually surprised it’s as high as 8 ohms in your case)

    If you can’t reuse the original analog amplifier, you’ll probably want to build your own op-amp circuit to amplify the strain gauge’s resistance into a voltage or frequency that can be measured more easily. I’ve heard wheatstone bridge circuits are good for this sort of thing. A google search for “strain gauge amplifier schematic” turns up a few things.. first hit is a wheatstone bridge + instrumentation amplifier.

  4. @Micah
    Brilliant. I now have it hooked up like this: http://www.sensorland.com/Images/SG-007.gif
    I am trying to balance out the bridge to get a 0.0mV for V by using a trimpot in place of R1.

    Is it a case of now calibrating the voltage to resistance to generate a curve / graph by placing known weights (I have a load of metal gym weights)

    I hope to hook this up to an arduino to do the maths and give me an actual mass.

    Thanks for the help :)

  5. @Alex: Cool! What kind of amplifier are you using? With the wheatstone bridge, you’ll get a very small differential voltage, and I think the Arduino/AVR only has single-ended inputs. You might need to use an instrumentation amplifier or a simple op-amp circuit to amplify the signal enough that you can detect the weight changes with the Arduino’s analog inputs.

    Good luck!

  6. I have a bet and I am trying to make a digital bathroom scale read whatever I choose…how can I do that? I want it to read 15 lbs lighter lol

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