What it shows
A load cell is a metal bar whose resistance changes by a tiny amount when it bends under a weight. The HX711 amplifies and digitises this change so the Arduino can show the mass in grams.
The science
The load cell has four strain gauges in a Wheatstone bridge. Bending the bar by a few micrometres changes their resistance, giving a signal of only a few millivolts. The HX711 has a 24-bit ADC and amplifier that turns this into a number. The reading is proportional to weight, so scale factor = (reading with known mass minus zero reading) / known mass, and mass = (reading minus zero) / scale factor.
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.
- An Arduino Uno or Nano
- A 5 kg bar load cell with an HX711 module
- Two plywood or acrylic plates about 12 cm square and four spacers (nuts and bolts)
- A 16 x 2 I2C LCD
- A push button for tare
- A known reference mass, such as a 500 g packet of sugar or 1 kg of packed rice
- Jumper wires
Safety
For this project: USB power only. Do not place more than 5 kg on the scale or the load cell is permanently damaged. Mount the plates so that the force acts at the free end of the bar.
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
8 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 one end of the load cell under the lower plate and the free end under the upper plate with spacers so that the bar can flex.
Connect the load cell wires to the HX711: red E+, black E-, white A-, green A+ (colours can vary; check the sheet).
Connect the HX711 VCC to 5 V, GND to GND, DT to pin 3 and SCK to pin 2.
Install the HX711 library by bogde, and read the raw values with nothing on the plate.
Use the button on pin 4 to tare: store the present reading as zero.
Place the known mass on the plate, read the raw value, and calculate the calibration factor.
Show the mass in grams on the LCD; check with other known masses like 100 g and 250 g.
Average ten readings to steady the display and record the accuracy and the smallest mass it can show.
What you should see
The scale shows the mass of objects in grams within about 1 percent after calibration and returns to zero after tare.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1How does a load cell sense weight?
Strain gauges change resistance as the metal bar bends.
2What is a Wheatstone bridge?
A circuit of four resistors that gives a small voltage when one resistance changes.
3Why is calibration needed?
Each load cell and amplifier has its own scale factor.
4What does taring mean?
Setting the display to zero with a container on the scale.
Take it further
Add a price-per-kilogram keypad entry so the scale shows the cost of loose goods.
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.
Free ebooks for this project
Science projects
Mini and Major Project Guide
For diploma and BTech students: choose, plan, build, document and present