I have bought these 50-kilogram bathroom scale load cells
and they come with the HX711 amplifier module
In this video, I'm going to show you how to connect
the 50-kilogram load cells to the HX711 amplifier
and how to read the weight values from it with an Arduino
I am going to show you different possible connection diagrams
four cell circuit
two cell circuit
and a single cell circuit
Theoretically, you could use an analog input of your microcontroller
to read the weight directly from the load cells
but the voltage-change inside the cell is so small
that the sensitivity of the analog to digital converter inside Arduino
isn't high enough to do that
That's why you need to use an amplifier
The working principle behind a load cell is
that there are four resistors laid out in a diamond pattern
It's called a Wheatstone bridge
The circuit has two compression and tension resistor pairs
When you put some weight on the load cell then the compression
and tension resistors are going to change their values in the opposite direction
Now we can let current flow from top to bottom
and then measure the output voltage between the two remaining corners
If you apply force to the cell then the resistance values start to change
and you get a voltage difference that you can measure
With proper calibration, you can calculate the weight of the object on the cell
When no pressure is applied
then all the resistor values are equal and the measurement voltage is zero
A full Wheatstone bridge has four wires
but these 50-kilogram cells only have three
These are only half-bridges, but you can combine them to form a full-bridge
You can connect four of them as a loop
you can connect two of them as a pair
or you can use a single one with two external resistors
I will go over all those scenarios one by one
But first, you should verify the wires with a multimeter
You need to find the middle wire
I have white black and red
The resistance between white and black wire is two kilo ohms
The resistance between the white and red wire is one kilo ohm
and the resistance between black and red wire is also one kilo ohm
This means that the red wire is the middle one
Let's start with the four-cell loop diagram
This module doesn't have any screw holes.
So I 3D printed these frames to mount them onto a wooden board
I will link my article about the load cells to the description box below
From there you will also find the Thingiverse link to the mounting frame
Since black and white were the outer wires for me
I need to form a loop by connecting black to black and white to white
Now one diagonal will be the power wires
that must be connected to the E+ and E- outputs of the HX711 amplifier
And the other diagonal is to take measurements
and must be connected to the A+ and A- inputs of the amplifier module
You can validate the wiring with a multimeter
The diagonals should be about two kilo ohms
The polarity here is irrelevant
Changing the power diagonal or switching polarity
will just invert the calibration constant in the Arduino code
On the Arduino side connect the VCC pin to the Arduino 5 volt pin
and GND to the Arduino ground
Connect the amplifier's data pin to Arduino digital input pin 4
and clock pin to pin 5
Now download the HX711 ADC library
The link is in the description box below
Copy it to the Arduino libraries folder
Now you can open File, Examples, HX711 ADC and Calibration
If you used pin 4 and pin 5
then you can load this example to your Arduino without any modifications
Now let's open Arduino terminal
It asks for a known weight
iPhone 5 is 113 grams
It takes some time for it to settle back to zero
Now I will desolder the wires to demonstrate the two cell circuit
The red middle wires will be the measurement wires
and I will connect them to the A+ and A- inputs of the amplifier module
Now form a loop by connecting black to white and white to black
And now we need to power the cells from the connection points
so one white-black wire pair goes to E+ and the other one to E-
Now the resistance between the E+ and E- wire should be about one kilo ohm
And the resistance between A+ and A- wires should also be one kilo ohm
Now we need to recalibrate
This setup is not very accurate
since the other side of the scale is partly supporting my weight
To use only one cell
we need to use two external resistors to complete the Wheatstone bridge
I will use 1k resistors since internally the load cell also has 1k resistors
I will power the cell from the white and black wires
and measure between the red wire and the other end of the connected resistors
In this case also the resistance between the E+ and E-
and A+ and A- should be about one kilo ohm
Let's recalibrate
This is very inaccurate
since a big part of my weight is supported by the other corners of the board
I also added a screen to make it look more like an actual scale
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Thank you for watching!
