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Design kitchen scale based on arduino

3D printable design with all resources - free to download under the Creative commons License CC BY-SA

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This kitchen scale can be printed on an ordinary 3d (FDM) printer. The project consists of 3D printed mechanics, electronics and software. Here you can see the software features of the scale: https://www.youtube.com/watch?v=eGyq23pixPM Wiring diagrams,software,3d print data and instructions can be downloaded for free: Software wirings and build instructions on: http://bechele.de/pages/english/154-0.html 3D construction data and 3D STL files on: https://www.thingiverse.com/thing:4551860 The complete project is published under the following License: Creative commons License CC BY-SA

This project is about a kitchen scale.

Operation:

See the following video to understand how to operate and calibrate the scale:

https://www.youtube.com/watch?v=eGyq23pixPM

The challenges for this project were:

  • To provide a nice looking flat design kitchen scale, where reasonable size vessels can be put on.
  • To be able to print all mechanical parts on a 3D FDM printer without extra support structures.
  • To use standard parts well known in the maker scene.
  • To allow zero power consumption in switched off mode though the operation uses only two buttons and no power switch.
  • To provide a flexible software that may change weight unit and display accuracy.
  • To make it very accurate. This means there must be an easy way of calibration even the non-linearity of the sensor should be calibrated out.

I could find lots of scale projects, but none of them fulfilled neither more than two of the above challenges nor all of them.

So I built my own ... See below how the challenges have been achieved.

Mechanics:

The scale mechanics has been designed with 3D print in mind. This means all parts of the scale can be printed on a standard size 3D FDM (Fused Deposition Modeling) printer without any extra support.

Properties of the mechanics of the scale

All case and chassis parts are 3D printable.

Since Thingiverse is obviously not able to provide all 3D files correctly, here you will find all necessary stl and obj. files in one zip archive:

http://bechele.de/images/attach/155-table_scale.zip

>> apparently not fully true >>  All 3D and CAD files are ready for download on thingiverse: https://www.thingiverse.com/thing:4551860  << apparently not fully true<<

No extra support is required for printing on a FDM printer all download files are already delivered in their proposed printing direction.

Two parts need a raft to stick properly on the print bed. This is the front cover and the button support. All other parts need just a brim.

For the front cover I used PETG, because there the inter layer adhesion is quite good and that is important, because there are very thin parts that have a rather high mechanical load in the inter layer direction. For the weight plate i used ABS, because due to the big number of faces, it is easier to clean off printing smudge marks and chips.  PETG would be a mess there, because chips stick very hard on PETG.

The scale was designed with the idea to be as small as possible, so the electronics compartment has very limited space. Even the electronic parts for the power circuit need to be selected with this in mind. 

Bill of Material:

Required mechanical parts:

6 Screws M3 x 6 DIN 965 2 Screws M4 x 20 DIN 912 2 Screws M4 x 16 DIN 912 10 Mini screws 1 x 4 plastic thread similar to DIN 7981

Electronics:

For the electronics I had in mind to use standard modules that are commonly used in the electronic maker scene, that can be purchased easily and cheap from the internet. Also I focused on low power consumption. Therefore I designed a zero power voltage supply that drops the power consumption to zero during off mode, though power on is realized by a key press. So I realized the Electronics with an Arduino pro mini and a weight cell  and controller based on HX711. These parts can be usually purchased for less than 10€ all together from the internet. If you search for "HX711 module" and/or "arduino pro mini"you will find plenty of offers.

For more about electronics and software visit: http://bechele.de/pages/english/154-0.html

Software:

For the software I had flexibility in mind and the possibility to use different range sensors and different weight units. Therefore weight calibration is not fixed in code, but flexibly adaptable and calibrateable via software. In other words with no changes in the arduino sketch you may calibrate your scale by software. So the accuracy of your scale depends only on the accuracy of your calibration weights and the fluctuation...

Read more »

waage4.ino

This is the sketch for the scale

ino - 14.82 kB - 07/29/2020 at 17:09

Download

scale_wiring.pdf

The wiring for the scale including the zero power circuit to save battery

Adobe Portable Document Format - 109.25 kB - 07/29/2020 at 17:08

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  • All stl and object files in one zip archive

    medeiros108/22/2020 at 08:32 0 comments

    Since Thingiverse is not able to provide all 3D files correctly, here you may download the complete stl and Obj. file set in one zip file:

    http://bechele.de/images/attach/155-table_scale.zip

  • Power consumption of the Arduino and the Display

    medeiros108/21/2020 at 19:50 0 comments

    Because the project is powerd by a 9V battery, every mA of current plays a role. By default the display background illumination is quite bright. that cost about an extra of 15 mA of current. By replacing the jumper of the display illumination by a resistor (in my case 1KOhms) the current dropped a lot. So the display was still bright enough to read the text. Also I removed the Power LED on the Arduino Pro mini. This saved me another 5 mA. Finally the Scale takes in operation about 55mA of current from the 9V battery.

  • HX711 weight cell controller inaccuracies

    medeiros108/21/2020 at 19:39 0 comments

    During the build of the project i noticed that the scale values were drifting quite a lot. After a deep investigation into the problem it showed that the resistance of some of the used SMD parts on the controller are very sensitive. Even by breathing over the electronicss slightly the values drifted dramatically. So I decided to cover the electronics with some transparent varnish. The varnish after it was dry caused the drift of the controller to drop to almost zero. So my recommendation is to put some varnish onto the complete board after soldering everything together.

  • Different mechanic Weight cells with different sizes and threads

    medeiros108/21/2020 at 19:03 0 comments

    Meanwhile I ordered a few new weight cells to build more units and i noticed the weight cells had suddenly a different size. Though the article image was correct the delivery was different.

    Instead of a lenghth of 80 mm the delivered once had only 75 mm and also the distance in between the mounting threads  changed from 15 to 10 mm, so I re- desigend both the scale stand an also the scale dish. Now you will find two different parts of each for download on Thingiverse.

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