In this how to guide we are going indepth on how to build a digital weighing scale,we go over how to set up some of the hardware and much more.

1. Phone Number: 1-800-550-0280
Contact Email: contact@tacunasystems.com
Website: https://tacunasystems.com/
How to Build a Digital Weighing Scale
Preface
Build a digital scale for your next DIY electronics project. Whether you’re looking to
increase your background learning around force measurement or in search of a
school science project, this DIY eletronics project can be completed by anyone with
a basic understanding of electronics and computer programming.
The components used to build a digital scale can be found on Tacuna Systems.
What is a Digital Weighing Scale?
Digital weighing scales are common in our everyday lives, from the produce section
grocery stores to our own bathrooms.

2. Figure 1. ANYLOAD ES310 Postal and Shipping Scale
A digital scale is an electronically operated device that displays the weight/size of an
object placed on it in numbers and units. This display is usually liquid crystal or 7-
segment display.
Tacuna Systems offers a full range of scales below, if you are not inclined to build
your own digital scale.
 AnyLoad Precision Balance Scales
 AnyLoad Bench Scales
 Amcells and AnyLoad Crane Scales
 Amcells and AnyLoad Floor Scales
 AnyLoad Load Bars
 AnyLoad Pallet Jack Scales

Scale Components and Function
Now that we have established what a scale is, this section lists and describes its fun-
damental internal components.
Load Cell Transducer
This device senses an applied weight and produces an observable output electrical
signal proportional to this applied weight. There are different types of load cells and
not all load cell sensors are transducers, but all load cell transducers are sensors. To
clarify, a sensor is a device that detects a physical phenomenon such as a force and
then produces a change in a physical parameter. In the same way, the transducer
senses this physical phenomenon but converts it to an electrical output signal.
Examples of load cells are the pneumatic load cell, hydraulic load cell, strain gauge
load cell, or capacitive load cell (See “An Overview of Load Cells” for more infor-
mation). The most common type used in digital scales is the strain gauge load cell.
The strain gauge load cell is a transducer device that has an underlying mechanism

3. called a strain gauge. The strain gauge has a resistive and elastic metal foil material
that is fixed and attached to the bending parts of the load cell as shown in the figures
below.
Figure 2: Strain Gauge Detail
Figure 3: A Strain Gauge Load Cell
The Signal Conditioner
This is a circuit device that has two functions. It amplifies load cell output voltage and
filters both its input and output signals.
Since the output of the load cell is always very small, measured in mV, this device is
critical to amplify the load cell output to a usable level. Load cell signal processing

4. requires a special type of amplifier called the instrumentation amplifier. The filter cir-
cuit can be embedded together with the amplifier on a single chip. This circuit filters
noise signals and electromagnetic interference (EMI) that might compromise the load
cell’s amplified signal. It decouples AC signals from the DC output signals. Tacuna
Systems offers a signal conditioning device described at this link.
The Microcontroller
This is the central processing unit of the whole digital scale. The analog output of the
signal conditioner flows to the analog input port of the microcontroller, which in turn
converts this analog signal to digital. Note that some signal conditioners have an an-
alog to digital converter circuit together with the amplifier circuit and the filter circuits
on a single chip.
This reduces the workload on the microcontroller, as the output of the signal condi-
tioner instead goes to the digital input port of the microcontroller. The microcontroller
then reads the clocked digital pulses from the conditioner to obtain the data it pro-
cesses to determine the weight displayed. Finally, the microcontroller computes the
weight and then displays it through its connected LCD screen.
The Liquid Crystal Display
As explained above, this is the screen that connects to the microcontroller to display
the weight.
Coupling These Components
To build your digital scale, we will use the following components coupled together:
 A 5kg load cell
 An HX711 amplifier
 An Arduino Uno Microcontroller Unit (MCU)
 Any suitable output screen (in this case an Arduino IDE Serial Monitor)
The physical process to build the digital scale is outlined in the next two steps.
The Hardware Setup
The 5kg load cell for this project has four wires (some load cells have 5). The wires
are as follows, according to the load cell’s datasheet.
 Red wire is the positive power supply
 White wire is the Ground of the power supply
 Black wire is the positive output
 Green wire is the negative output
Any extra wire, which might be any color such as blue or yellow, is the ground wire
for EMI. It should be connected to the power supply ground.
Note the wiring diagram depends on the datasheet of the purchased load cell. There-
fore the colors listed above could match differently (such as green being positive,
etc.). Be sure to always request a datasheet from your supplier with your purchase.
Load Cell to Amplifier Connection
The wires listed above attach to the appropriate pins of the HX711 amplifier device
as shown in the sets of pictures below. The HX711 is a special amplifier that has 2

5. channel inputs A and B, each with different amplifier gains adjustable by a computer
program. The channel A has a gain of 128 and 64, while the channel B has a fixed
gain of 32.
Note this HX711 device matches the description of the type of signal conditioning de-
vice that has an amplifier, filter, and ADC embedded together on a single chip. This
means its data output goes to the digital input pin of our microcontroller instead of its
analog pin.
Therefore, connecting the HX711 and the load cell is as followed:
 Red wire/Positive supply -> E+
 White/Negative Supply -> E-
 Green/Negative Ouput -> A-
 Black/Positive Output -> A+

6. Figure 4: Load Cell to Amplifier Connections
Amplifier to Microcontroller Connection
After connecting the amplifier, the HX711 connects to the Arduino MCU as follows:
 VCC -> 5V supply on Ardunio
 GND ->GND
 CLK -> D2
 DOUT -> D3

7. Figure 5: Amplifier to MCU Connections
The Code
Once the hardware connections are complete, the program code uploads to the
MCU from a PC through the Arduino IDE. The process and the code have two subdi-
visions: the calibration code and the weighing code.
Calibration Code

8. The calibration code is the program used to calibrate the load cell transducer. Cali-
bration configures the load cell output to always give a precise value of weight and at
the particular S.I unit such as Kilograms, Grams, Tons, Pounds etc.
The HX711 library code contains a good guide on how to calibrate the load cell. The
calibration code is shown in Figure 6a-b.
Figure 6a. HX711 Calibration Code

9. Figure 6b. HX711 Calibration Code
The results of the calibration process are shown below in the Serial Monitor of the
Arduino IDE.
Figure 7: Calibration Process Output
Note in Figure 7 the “FINAL Scale Factor” and the “FINAL Offset Value”. These val-
ues are critical to obtain before uploading the weighing code.
Weighing Code
Once calibration is complete, the weighing code uploads to the MCU. This code ap-
pears in Figure 8. Figure 9a-b shows the readouts for a 500g weight (top) and a 1kg
weight (bottom).

10. Figure 8: Weighing Code
Figure 9a. Digital Weighing Scale Output

11. Figure 9b. Digital Weighing Scale Output
Phone Number: 1-800-550-0280
Contact Email: contact@tacunasystems.com
Website: https://tacunasystems.com/