How weighing scales work, and how to make your own

You'd think that making a pair of accurate custom weighing scales would be complex. But the availability of mass produced electronic weighing components means that is is very easy to create a set of surprisingly accurate scales.

The key components here are the load cell, which converts weight to a voltage, and the dedicated HX711 integrated circuit that converts that voltage into a very high resolution number and communicates it to a microcontroller via a simple data and timing interface.

Also featured in this video is a very convenient microcontroller based module that receives the number and can compute and display the weight based on a zero-weight reference and an adjustable scaling factor. The unit stores the set values to non-volatile memory so that it does not need to be calibrated for each use.

Note that there are a few different types of the digital display units, and each has its own features and setup requirements. Here are the instructions for the three button version shown in the video.

The load cell is connected to the HX711 module as follows:-
E+ Red
E- Black
A- White
A+ Green

The HX711 module is connected to the digital display module as follows:-
VCC Positive supply voltage (usually 5V)
GND Negative/0V supply
DT Data/DT/RX
SCK Clock/SCK/TX

5V (USB power supply) is supplied to the digital module via its onboard jack, USB connector (if fitted) or two solder pads - ensuring correct polarity. It's a good idea to test a USB cable for the correct polarity, as some cables don't follow standard colour codes.

Once your weighing assembly is set up - ensuring that nothing is on, touching or impeding the movement of the weighing platform, then power the module and allow it to stabilise.

Initially it probably won't show zero. Press the left button to set zero. It will be stored.

Now place an accurate weight on the platform. The display will probably show a random number that does not represent the actual weight. Now you have to use the middle button and right hand button to adjust the prescaler/calibration value to get the correct reading. You can make small adjustments with individual clicks, or if there's a large difference then you can hold the buttons to scroll faster.

IMPORTANT NOTE! The prescaler is just a reference number and NOT the actual weight.

When you stop adjusting the prescaler up or down there is a brief pause and then the display shows the measured weight. If it is still not correct then keep adjusting as needed.

For the final accurate calibration (for example) a 100g weight, find the point where the display is jittering between 99g and 100g and note the calibration number. Then adjust the calibration value until it is jittering between 100g and 101g and note that number. Your perfect calibration is right in the middle of those two values. Once you have adjusted back to that value your scales have been calibrated.

Note that the buttons should not be accessible to general users of the scales as random pressing of buttons will void the calibration. If a fixed size container is used on the scales then it can be nulled out by putting the empty container on the scales and pressing the left button to set the new zero reference.

Note that I've only tested the display module with a 1kg load cell. Also note that I described the load cells as being made of steel. The big ones are actually an aluminium alloy, and the small body scale version is steel.

If you fancy playing with the components used in the video, here are some links. These are affiliate links, but that will not affect the price you pay.
Some listings may not work for some countries and some may just be sold out or disappear.

When paying on AliExpress, let the transaction complete before leaving the purchase page as otherwise the purchase may not go through.

Here's the three-button digital module:-
https://s.click.aliexpress.com/e/_c2JLZTJV

Load cell and HX711 module - I recommend the 1kg set:-
https://s.click.aliexpress.com/e/_c3A9bAuF

The platform kit minus the digital display:-
https://s.click.aliexpress.com/e/_c4S5NfvZ
Only buy it as a kit, as the assembled units may be damaged in shipping.

Stand-on scale style load cell set:-
https://s.click.aliexpress.com/e/_c2utB2Mj

There are also kits with a large round PCB, but note that it involves surface mount components.



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