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Using a full bridge weight sensor (Load cell) with 430F2013

Colin Ives
Prodigy 100 points

Other Parts Discussed in Thread: MSP430F2013

I need to use a single full-bridge weight sensor (5Kg) and was wondering if I can just measure the load using the differential 16bit signature-delta converter in the 430F2013 directly without external amp? I /don't/ need great accuracy or resolution (128 levels would be plenty). Looks like the output impedance is just over 1K.  The application includes to weigh a bottle (& its contents) to determine if 1/4, 1/2, 3/4 and full - the installation will be calibrated with an empty/full bottle so absolute weight is not needed - just relative to max/min. I've picked up on the F2013 as its available in simple packages that I can solder and has a differential AD - if there is a more suited product please advise.

Thanks

  • 0
    Guru 74080 points
    Hi Colin!

    It was me on Facebook. From your points it looks like this isn't your first contact with E2E. The differential 16 bit sigma delta ADC of the MSP430F2013 is very well suited for this application. You can source the bridge by the 1.2V reference voltage of the MSP. The buffered reference can deliver 1mA, so the bridge resistance has to be 1k2+, measured from IN+ to IN-, not to any output. Otherwise you would need to add an external buffer to stay within spec. The MSP430F2013 also includes a programmable gain amplifier. You should measure the differential voltage of the bridge under no and under full load when sourced from 1.2V to have an idea of it's behaviour. How do you want to make your data visible to a user?

    Dennis
  • 0 in reply to Dennis Eichmann
    Prodigy 100 points

    Hello again Dennis and thanks for your reply.  To be honest I only dipped my toe in here earlier in the year and forgot about it!

    The application now is, as then, a controller for our 'fancy' compost toilet. One of the elements is a level monitor for a urine container and I was playing around with a capacitive sensor approach but it proved unreliable.  I've since realised that weight measurement can be made to work (I need to mount the sensor in such a way that the bottle is only raise a few mm.  in this approach I'm only planning on putting the sensor under one edge as it's the relataive volume I need to, initially, display o LEDs with a 'full' warning buzzer. Later i may add one of those neat low-power sharo displays.

  • 0 in reply to Colin Ives
    Prodigy 100 points
    The compost loo is a product we manufacture and I thought it was time to add a little technology to what is a basic process (inc intelligent fan control)
  • 0 in reply to Colin Ives
    Prodigy 100 points
    This is an example of the type of low-cost sensor I'm considering:

    www.ebay.co.uk/.../252572469572
  • 0 in reply to Colin Ives
    Guru 74080 points

    As you said, this type has a minimum of 1k input resistance which is not within spec for the MSP. You might need to add an external buffer or feed in an external reference (max. 1.5V) that also drives the bridge. There is about 1mV/V output span for full load, so you would have 1.2mV when sourcing it from 1.2V - not much, but possible. The PGA of the MSP430F2013 has a maximum of 32, so this would mean 38.4mV/5kg which is 7.68mV/kg. The 16 bit converter has a resolution of ~18µV at 1.2V reference, so at least in theory you would have a resolution of 2.4g, not bad, but noise will be your contrary with that low voltage levels.

  • 0 in reply to Dennis Eichmann
    Prodigy 100 points

    Thanks,

    In the light of the above, I'm going to give the HX711 chip a go - using its internal amplifier and gain.  

    Thanks again for your help/insight.

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