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Thread: Power load cell high and Teensy for high precission

  1. #1
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    Power load cell high and Teensy for high precission

    Hi.

    Currently I have two 20kg load cell with raspberry and ads1115. They work good, but differents are 5-10 grs for each meassure. Now i'm using a good 5V to MicroUSB adapter for power the raspberry and also the load cell at 5V.

    I just buy a new 10Kg sensor and i like to use the ADC of the Teensy. The new load cell need 6 to 14V (old are 5V same as raspberry).

    Because the supply is very important for the noisy, i'm thinking in the following options:
    A) 24V switch power supply as main
    LM7805 with 2 capacitor for Teensy
    LM7812 with 2 capacitor for LoadCell

    B) 24V encapsulated transformer.
    With same LM7805 and LM7812.
    Because the encapsulated transformer is small is good for put into the board with all components.
    I'm thinking in any like this:
    http://es.farnell.com/myrra/44306/tr...24v/dp/1689098

    C) Get the 12V directly using A or B method with 12V and remove the LM7812.

    What will be the best option for get more stable supply for items?.


    For make the measurement really i don't know what option do.
    A) ADC from teensy using voltage divisor for down to 3.3V
    B) ADS1115 i2c using voltage divisor for down to 5V (i'm familiar with this)
    C) INA125, 128 ...

    The objetive is the better meassurements because i use for largue items (500 grs) but with small items like 20 grs too.

    Any help will be apreciated.

  2. #2
    Senior Member Constantin's Avatar
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    You are likely better off with a differential ADC that features a higher level of precision than the 13 bit SAR that you can find in a Teensy. While you can use the PGA, differential mode, and a lot of decimation to help you come to more consistent readings, you'll likely do better with a external ADC. There are some very good 24bit breakout boards for sale on ebay, I'd consider them first.

    I agree that choosing a good power supply will be a significant contributor to keeping the readings consistent. I would advocate for a 5VDC wall wart for the Teensy, followed by a external voltage regulator like the ADP150 series from Analog that features a very high PSSR to keep noise out. I'd dedicate that voltage regulator to the ADC and power the Teensy off the 5V.

    Not sure about the power needs of the load cell. Isn't it a simple 4-way resistor circuit that is shaped like a bridge with the ADC in the middle?

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    Thanks for your reply.

    The load cell are a chinesse load cell from Aliexpress rated from 6 to 15VDC. Really now i have one using at 5V without problem with ADS1115 and Raspberry, also I have another that is rated from 5V and the error are similar. This have been working for some months.
    http://www.aliexpress.com/item/High-...431463335.html

    I think in power the load cell with 12V because i think that is the recommended are 6 to 15, must work better using the rated voltage instead 5V.

    I found a ADS1220 24bit from TI, that works similar to the ADS1115 i currently use. But need to check if make shipping to spain, because i didn't found any ebay supplier selling breakout boards with this chipset.
    https://www.olimex.com/Products/Brea...ource-hardware

    I'm checking the datasheet for ADP150 and only supply 150mA. This will be sufficient for use with teensy and the load cell?

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    I have been using :
    https://www.sparkfun.com/products/13330
    https://www.sparkfun.com/products/13230

    Getting 1 gram repeated resolution powering everything off 3v3 from the teensy, running the example out of the box from the sparkfun site.

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    I bought one HX711 break board time ago for use with raspberry but the library are not good at this moment and finally i change to ADS1115.

    Have you power on the load cell with 3V too?, rated are from 6V.

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    Yes, everything running on 3v3. Seems to want to drift around a bit but well within what I need for coffee pour overs Brand new nickle will weigh in at 5 grams +- a few tenths after a tare.
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    Last edited by cartere; 10-14-2015 at 07:50 PM.

  7. #7
    Senior Member Constantin's Avatar
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    Quote Originally Posted by potasto View Post
    The load cell are a chinesse load cell from Aliexpress rated from 6 to 15VDC. Really now i have one using at 5V without problem with ADS1115 and Raspberry, also I have another that is rated from 5V and the error are similar. This have been working for some months.
    http://www.aliexpress.com/item/High-...431463335.html
    Hard to tell what's on the inside but if the strain gauge is a regular gauge, I don't see why using a lower voltage power supply should be a problem. It's basically a four-resistor setup with small changes. A better ADC may be more valuable here than a higher voltage. The only thing I can think of is that lower voltage ADCs may have more issues with spurious noise getting introduced into the system. Decimation will be your friend!

    Quote Originally Posted by potasto View Post
    I'm checking the datasheet for ADP150 and only supply 150mA. This will be sufficient for use with teensy and the load cell?
    Si!

    The teensy can only supply about 100mA from its internal regulator. I thought you'd make use of the internal regulator just for the Teensy while using the ADP150 just for the external ADC. Since the Teensy likely communicates with the ADC using I2C or SPI, it doesn't need a 'quiet' power supply (though it would not hurt). Either way, you need to cut the VUSB-VIN trace on the underside of the Teensy board if you plan on using a external power supply and may be using the USB connection at the same time.

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    Hi Constantin.

    I see that your suggestion are, use 5V normal (not chinesse) adapter for power on the teensy, and even a ADP150 from the 5V to a ADS1115 i2c or HX711 (all are 5V capable), because as you tell Teensy don't use his own reference.

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    Quote Originally Posted by cartere View Post
    Yes, everything running on 3v3. Seems to want to drift around a bit but well within what I need for coffee pour overs Brand new nickle will weigh in at 5 grams +- a few tenths after a tare.
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    Sorry i don't understand the last. Do you tell that when the scale are time power on, the meassurement change +-5 ?.

    The 27.6 grs are with the 10 Kg sparkfun load cell?

  10. #10
    Senior Member Constantin's Avatar
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    Quote Originally Posted by potasto View Post
    I see that your suggestion are, use 5V normal (not chinesse) adapter for power on the teensy, and even a ADP150 from the 5V to a ADS1115 i2c or HX711 (all are 5V capable), because as you tell Teensy don't use his own reference.
    Not sure if I understand. So I'll try again:

    Buy external power supply to go from 120/240VAC to 5VDC

    5VDC external power supply goes to Teensy VIN (VIN-VUSB trace must be cut)
    5VDC external power supply also goes to ADP150AUJZ-3.3-R7 which produces smooth 3.3V voltage

    3.3 Voltage from ADP150AUJZ-3.3-R7 goes to ADS1115

    By running the ADS1115 at 3.3V, you avoid the need for external voltage translation chips (to allow I2C communication between the ADS1115 and the Teensy).

    Besides the two I2C lines, you also have to share a ground between the ADP150, the ADS1115, and the Teensy. By using the ADP150, you remove most, if not all noise from the output of the 5VDC power supply. In this setup, the teensy doesn't care much about power supply noise since it only operates in the digital domain.

    Besides the two I2C connections (and the required pullup resistors which may be already on the breakout board), you may also want to consider using the ALRT/RDY output from the ADS to let the Teensy know when a conversion is ready. Then use a pin interrupt or similar approach to read out results as they become available.
    Last edited by Constantin; 10-14-2015 at 08:46 PM.

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    All understood Constantin.

    I will order some ADP, because i have replacement ADS1115 from Ladyada in the office (https://www.adafruit.com/products/1085).
    My 5VDC is a farnell for raspberry power AC, and is more stable than chinesse AC adapter.

    The only i don't understand, because i'm really a newie is how to handle the ALRT/RDY from the ADS1115 and how to use it. Now with the raspberry, when I access to a webserver i get 1 sec measurements and send the media value.

  12. #12
    Senior Member Constantin's Avatar
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    Simply set up a Teensy pin to be an input to activate the internal pullup. The pin voltage will be 3.3V with a weak pullup (~40k).

    When configured correctly and the ADS 1115 finishes a conversion, the ADS ALRT/RDY pin will go low. Then your teensy knows the conversion is done and uses I2C to read out the data from the ADS. You can monitor the ALRT/RDY output continuously in loop() or you can configure a pin interrupt to do it in the background. Either approach minimizes the number of I2C transactions to only occur as needed.

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    No I only see an error of a few tenths of a gram. Yes this is with that sensor (10Kg), in the calibration I used 1 kilo ( water in a glass weighted with commercial gram scale) to generate an out put of 1000.

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    Senior Member Constantin's Avatar
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    Hi,

    I am new to play with raspberry.

    I am trying use loadcell (10 kg) with raspberry pi 3 and ADS1115. I have installed the ADS1115 python library.

    Could you please help me with next steps to find the weights of things.

    Thanks

  16. #16
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    Hello,
    I have a question please

  17. #17
    By providing power to the load cell from an independent supply you have to factor that supply voltage into the system calibration. This is why I prefer a ratiometric system where the sensor excitation and ADC reference voltage are identical. The absolute supply voltage drops out of the equation. So instead of requiring two stable and accurate supplies you only need one stable supply. Absolute accuracy not required.

    The only advantage to a higher excitation voltage is improved signal to noise.

  18. #18
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    Your main goal here is to be able to get more reliable readings for lower weights. Therefore, you need a good power supply and a load cell with good sensitivity and good division ranges.
    Learn more here: https://tacunasystems.com/knowledge-...inimum-weight/
    For the power supply, you basically need a less noisy power supply, so using options B without a well-designed power circuit will generate more noises than A or C. Option C is totally dangerous because you expose your load cell and ADCs to dangers of voltage spikes and current surges; Option A seems the best. My best advice is to get an ADC chip that can power your load cell from its own power supply, e.g. an HX711. Learn more about how this can be done here:
    https://tacunasystems.com/knowledge-...eighing-scale/

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