This guide builds a digital weight scale using a load cell and Arduino. The load cell's small analog signal is amplified through two stages of LM358 operational amplifiers—first a gain stage and then a subtractor stage—to produce a clean signal that the Arduino can read and convert to weight measurements.
Following this guide, makers will receive a complete wiring diagram, a parts list with recommended suppliers, Arduino firmware with tare and calibration functions, and step-by-step assembly instructions. The finished scale can measure weight by taking a tare reading, then comparing loaded readings against a known mass for accurate calibration.
Wiring diagram
Gather all the parts
Assemble it in 7 steps
1. Place the two LM358 chips
With USB unplugged, push lm358_1 and lm358_2 across the centre gap of the breadboard, each with its small notch facing the same way. The notch marks the pin-1 end, so you can count the legs correctly.
- Use two bare LM358 DIP-8 chips, not amplifier modules with extra parts already connected.
- Putting a chip in backwards swaps its power connections and can damage it when USB is plugged in.
2. Power the amplifier chips
Connect pin 8 of lm358_1 and lm358_2 to Uno 5V (power), and connect pin 4 of both chips to Uno GND (ground). Connect the positive, longer lead of c_decouple_1 to 5V and its striped negative lead to GND close to lm358_1. Do the same for c_decouple_2 beside lm358_2.
- The stripe on a 10 µF electrolytic capacitor marks its negative lead.
- Do not reverse either capacitor: reversed electrolytic capacitors can heat up and fail. Do not connect Uno 5V to LM358 pin 4; pin 4 is ground.
3. Connect the load-cell power wires
Connect load_cell_1 E+ to Uno 5V (power) and E- to Uno GND (ground). On many load cells these are red and black, but follow the E+ and E- labels or the seller's sheet rather than wire colour alone.
- Use the same 5V and GND rails used by the LM358 chips so every part shares one ground connection.
- Connecting an A+ or A- signal wire to 5V gives incorrect readings and can damage the measurement circuit.
4. Build the first amplifier stage
Connect load_cell_1 A- to lm358_1 pin 3 (signal) and A+ to lm358_1 pin 5 (signal). Fit r_gain_a, a 10 kΩ resistor, from pin 1 to pin 2, and r_gain_b, another 10 kΩ resistor, from pin 7 to pin 6. Fit r_gain_link, the 100 Ω resistor, between pins 2 and 6. Resistors have no direction.
- The two 10 kΩ resistors should be the same tolerance, preferably 1%, for stable measurements.
- Do not join A+ and A- together; they are the two separate measurement wires from the load cell.
5. Build the subtractor stage
Use four 10 kΩ resistors: connect r_diff_in_minus from lm358_1 pin 1 to lm358_2 pin 2; r_diff_fb from lm358_2 pin 1 back to pin 2; r_diff_in_plus from lm358_1 pin 7 to lm358_2 pin 3; and r_diff_ref from lm358_2 pin 3 to GND. This stage keeps the small difference caused by weight.
- Keep all four 10 kΩ resistors the same type and tolerance; matched values give the least drift.
- A loose connection in this stage can make the output stick near 0 V or 5 V.
6. Send the weight signal to the Uno
Connect lm358_2 pin 1 through r_adc_filter, the 100 Ω resistor, to Uno A0 (signal). Tie lm358_2 pins 5 and 6 to GND (ground) and leave pin 7 unconnected.
- A0 is the Arduino pin that reads the changing voltage from the load cell.
- Connect this wire to A0 only, never directly to 5V; 5V would prevent the Uno from measuring the changing signal.
7. Mount the load cell safely
Bolt one end of load_cell_1 firmly to a rigid base and bolt the load platform to the other end using its mounting holes. The metal bar must be free to bend in the direction marked by its maker.
- Keep the thin load-cell cable away from moving metal and USB cables.
- Do not exceed the 5 kg rating or clamp both ends so the bar cannot bend; either mistake can permanently damage the sensor.
Review all connections
1. Connections between "load_cell_1" and "Arduino"
2. Connections between "lm358_1" and "Arduino"
3. Connections between "r_gain_a" and "Arduino"
4. Connections between "r_gain_b" and "Arduino"
5. Connections between "r_gain_link" and "Arduino"
6. Connections between "r_diff_in_minus" and "Arduino"
7. Connections between "r_diff_fb" and "Arduino"
8. Connections between "r_diff_in_plus" and "Arduino"
9. Connections between "r_diff_ref" and "Arduino"
10. Connections between "lm358_2" and "Arduino"
11. Connections between "r_adc_filter" and "Arduino"
12. Connections between "c_decouple_1" and "Arduino"
13. Connections between "c_decouple_2" and "Arduino"
Deploy the firmware
#include <Arduino.h>
const uint8_t LOAD_CELL_PIN = A0;
const uint8_t SAMPLE_COUNT = 64;
long tareCounts = 0;
float countsPerGram = 0.086f; // Rough starting value; calibrate with a known mass after wiring the ~x201 LM358 circuit.
unsigned long lastReportMs = 0;
long readAveragedCounts() {
long total = 0;
for (uint8_t i = 0; i < SAMPLE_COUNT; ++i) {
total += analogRead(LOAD_CELL_PIN);
}
return total / SAMPLE_COUNT;
}
void takeTare() {
tareCounts = readAveragedCounts();
Serial.print(F("Tare stored at ADC count: "));
Serial.println(tareCounts);
}
void calibrateWithKnownMass() {
float knownGrams = Serial.parseFloat();
if (knownGrams <= 0.0f) {
Serial.println(F("Calibration cancelled. Send c followed by a mass in grams, for example: c 1000"));
return;
}
long loadedCounts = readAveragedCounts();
float difference = (float)(loadedCounts - tareCounts);
if (difference == 0.0f) {
Serial.println(F("Calibration cancelled: the reading did not change. Check the load cell and try again."));
return;
}
countsPerGram = difference / knownGrams;
Serial.print(F("Calibration saved for this power session: "));
Serial.print(countsPerGram, 5);
Serial.println(F(" ADC counts per gram."));
}
void handleSerialCommand() {
if (!Serial.available()) {
return;
}
char command = (char)Serial.read();
if (command == 't' || command == 'T') {
takeTare();
} else if (command == 'c' || command == 'C') {
calibrateWithKnownMass();
}
}
void setup() {
Serial.begin(115200);
analogReference(DEFAULT); // Uses the Uno's 5 V supply, the same supply used by the bridge.
delay(500);
takeTare();
Serial.println(F("Load-cell scale ready."));
Serial.println(F("Leave the platform empty for tare (t)."));
Serial.println(F("For calibration, place a known mass on it and send c followed by grams, for example c 1000."));
}
void loop() {
handleSerialCommand();
unsigned long now = millis();
if (now - lastReportMs >= 250) {
lastReportMs = now;
long rawCounts = readAveragedCounts();
float grams = (rawCounts - tareCounts) / countsPerGram;
Serial.print(F("Weight: "));
Serial.print(grams, 1);
Serial.print(F(" g | ADC: "));
Serial.println(rawCounts);
}
delay(20);
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