This project combines an Arduino Uno, a strain-gauge load cell, an HX711 amplifier, and a SIM800L GSM modem to measure weight and send an SMS when a configured limit is crossed. The video below shows the original build. Use it as a project reference, then follow the wiring, calibration, and power notes here before connecting the modules.

Parts

• Arduino Uno or compatible 5 V board
• Load cell and HX711 breakout
• SIM800L modem, suitable antenna, and an active SIM card
• Stable power supplies, jumper wires, and a mounting platform

Connect the load cell and HX711

Connect the load cell to the HX711 terminals marked E+, E−, A+, and A− (or the equivalent labels on your board). Wire colors are not universal, so check the load-cell datasheet or identify the bridge leads before powering it. Connect HX711 VCC and GND to the Arduino supply and ground only if the breakout board supports that voltage. For a simple Uno example, connect DOUT/DT to D3 and CLK/SCK to D2; use the same pin numbers in your sketch.

Power the SIM800L safely

Connect the modem UART to two chosen Arduino serial pins with TX and RX crossed, and join the grounds. SIM800L boards differ in their supply and logic-level requirements. Use a regulated supply sized for the exact module and its transmit-current peaks; do not assume the Arduino 5 V pin can power it or that the modem UART is 5 V tolerant. Check the module’s SIMCom hardware design documents before wiring. Attach the correct antenna and confirm that the SIM and local network support the modem before sending messages.

Calibrate before setting an alert

1. Install an HX711 library and first run a scale-reading example with the modem disconnected.
2. With the platform empty, tare the scale and record a stable zero reading.
3. Place a known mass on the platform and adjust the calibration factor until the displayed value matches it. Repeat with another mass to check the range.
4. Set the alert threshold in the project code only after the readings are stable. Add a small margin or hysteresis and a resend delay so noise near the limit does not trigger repeated SMS messages.

SparkFun’s HX711 hookup guide explains the breakout connections and calibration process. Calibration depends on the load cell, mounting, and electronics in your build; do not copy a calibration factor from another scale.

Bring-up and troubleshooting

Test one subsystem at a time: check raw HX711 readings over USB serial, then verify that the modem starts and registers on the network with a suitable supply. Combine them only after both work independently. If the weight drifts, check the mounting, wiring, vibration, and zeroing process. If the modem resets when sending, review its supply and wiring before changing the Arduino code.

Note: Treat this as an educational prototype. Verify the load cell’s mechanical rating and the calibration of the assembled scale before relying on its readings.