What it shows
Digital weighing scales in shops use strain-gauge load cells, which change resistance by a tiny amount when bent. An HX711 amplifier and Arduino turn that into grams. The project calibrates the scale with known masses and reports its accuracy.
The science
A load cell has four strain gauges in a Wheatstone bridge. Bending changes their resistance by a fraction of a percent, producing millivolts. The HX711 has a 24-bit ADC and a gain stage that reads this and sends numbers to the Arduino. A calibration factor, counts per gram, is found with a known mass. Zero error is removed with a tare.
Materials
Cost: under Rs 1,500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A 5 kg bar load cell and an HX711 module
- An Arduino Uno or Nano
- A 16x2 I2C LCD or the serial monitor
- Two small acrylic or wooden plates and spacers
- M4 or M5 bolts and nuts
- Known masses: coins, a 100 g and 500 g standard weight or filled bottles checked on a shop scale
- A push button for tare
Safety
For this project: Do not overload the cell past its rating. Handle the bolts and plates with care; an adult must help with drilling.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
10 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Fix the load cell between the base plate and the top plate with spacers so the cell can bend when loaded, as shown by the arrow on the cell.
Connect the cell wires to the HX711: red to E+, black to E-, white to A- and green to A+ (check the cell colour code).
Connect HX711 DT and SCK to Arduino pins 3 and 2, with VCC and GND.
Install the HX711 library and upload the calibration example, reading raw counts with an empty plate.
Place a 500 g reference mass and note the counts; compute scale factor = counts divided by mass.
Write the final sketch: tare on startup and on the button, and display grams on the LCD.
Weigh masses of 50, 100, 200, 500 and 1000 g and record the reading against the true value.
Compute error and percentage error for each and the repeatability from five readings of the same mass.
Test the temperature drift by warming the cell with a lamp and noting the zero change after 10 minutes.
Write the report on bridge circuit, calibration, the error table and limits.
What you should see
The scale reads within about 1 percent for masses above 100 g after calibration, and readings repeat within 1 to 2 g. Zero drifts slightly when warmed, which the report discusses.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is a strain gauge?
A thin foil resistor whose resistance changes slightly when stretched or compressed.
2Why is a bridge circuit used?
It turns a very small resistance change into a measurable voltage and cancels temperature effects partly.
3What is tare?
Setting the display to zero with a container on the plate so only the added mass is read.
4Why does the plate need spacers?
So that only the cell bends, and the plates do not touch the base.
Take it further
Add a price-per-kg keypad and compute the cost of the item for a small shop-style scale.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.