[] (http://knot.cesar.org.br/) KNoT Network of Things

Scale Application Documentation

This tutorial explains how to hack a weight scale, model EB9322 of Camry Scales ® using a converter (ADC) for weight scales HX711.

Weight Scale Specifications

• Product Name: HEAVY DUTY GLASS SCALES
• Producer: Camry Scales
• Model: EB9322

Link for more details: EB9322

Similar model used in the application: [EB9321] (http://www.camryscale.com/showroom/eb9321.html)

24-Bit Analog-to-Digital Converter (ADC) for Weigh Scales - HX711

HX711 is a precision 24-bit analog-to-digital converter (ADC) designed for weigh scales and industrial control applications to interface directly with a bridge sensor.

Link to the datasheet with more details about the board used: Datasheet

HX711 Library

An Arduino library to interface the Avia Semiconductor HX711 24-Bit Analog-to-Digital Converter (ADC) for Weight Scales.

To use the HX711 lib correctly with this application, follow the instruction:

1 - Download and save the library in the default directory of Arduino

$wget --output-document="$HOME/Arduino/libraries/HX711.zip" https://github.com/bogde/HX711/archive/master.zip && cd $HOME/Arduino/libraries/ && unzip HX711.zip

OBS: If the Arduino folder isn't in the default directory you must change the command to point to the correct directory.

Link to lib: Git

How to hack the Weight Scale

Opening the back of the balance, you can find the controller board. In order to capture the data read by the scale using the pins of the HX711 chip, you need to solder the pins of the HX711 with those of the balance controller board as follows:

HX711

PCB EB9360R11

Doing this now you are able to read the values and use them in your application.

Water Dispenser Application

Constants defined for use in the application

READ_INTERVAL      	1000
BOUNCE_RANGE 		200	// Constant to filter the highest and lowest values read

1 - Set the analogic pins of microcontroller to connect with HX711 chip.

HX711 scale(A3, A2);

2 - Set the microcontroller digital pin number 3 as input of the button of tare.

pinMode(BUTTON_PIN, INPUT);

3 - Wake up the HX711 chip.

scale.power_up();

4 - This part of the function get_weight(times) converts the average of values read from the HX711 chip to a readable value.

mes = scale.get_value(times);
a = ref_w/(k2 - k1);
b = (-1)*ref_w*k1/(k2-k1);
raw_kg = a*mes + b;

5 - Verify if the button to tare is pressed.

if ((button_value_current == HIGH) && (button_value_previous == LOW)) {
    offset = get_weight(TIMES_READING);
}

6 - Save the offset on EEPROM.

EEPROM.put(OFFSET_ADDR, offset);

7 - Calculate the average of the last values reads from HX711 chip and subtract this with the offset (value read of tare).

kg = get_weight(TIMES_READING) - offset;

8 - The function remove_noise(value) uses logic to filter the read data avoiding abrupt data variation.

 if(value > (previous_value + BOUNCE_RANGE) || value < (previous_value - BOUNCE_RANGE)) {
        previous_value = value;
}

9 - Read the values only during the set interval.

currentMillis = millis();
if(currentMillis - previousMillis >= READ_INTERVAL){
	previousMillis = currentMillis;
	kg = get_weight(TIMES_READING) - offset;
}

10 - Convert from kilogram to gram and call the function remove_noise(value).

 *val_int = remove_noise(kg*1000);