Methodology

Below are the block diagram for the project :

Load Sensor

When clothes is placed on the weighing scale, load cell sensor will convert the load into an electronic signal. This signal is in analog voltage. Load sensor work on the principle of piezo-resistivity ; when a force is applied to the sensor, it changes its resistance. This change in resistance leads to a change in output voltage

Amplifier

Analog to digital converter (ADC) on the Microcontroller can read approximately 19mV per step based on the formula :

(𝟓 𝒗𝒐𝒍𝒕𝒔)/(𝟐𝟓𝟔 𝒔𝒕𝒆𝒑) = 19 mV per step

For the scale, total voltage different is less than 19mV when force is applied. Hence, we need to amplify the signal using INA125P amplifier

MCU System

Microcontrollers can only differentiate between HIGH or LOW level on input pins. Most of the modern MCUs have an Analog to Digital Converter (ADC). ADC will convert the analog voltage from the weighing scale to a digital value so that it can be processed by the Microcontroller. PIC16F88 is 8bit Microcontroller meaning it has the ability to detect 256 discrete analog levels. ADC ports on PIC16F88 are labelled with AN0 – AN6

Transmitter and Receiver

In order to transmit and receive data from the Microcontroller, we use HC11 wireless serial port module. It is a half-duplex data transmission meaning that only one end transmits at a time, the other end receives. Transmitter is connected to the MCU whereas receiver is connected to the USB to UART converter. After ADC in the MCU convert the signal from analog to digital, transmitter will transmit the data to the receiver

USB to UART Converter

Most serial interface from Microcontroller to laptop is done through USB. Level shifter is necessary for UART interface because it converts digital signals from one logic standard to another. For this project, UC00B is used as an USB to UART converter because it offers direct interface with microcontroller and it provides low current 5V supply from USB port

Laptop

A user interface is created using Visual Basic 2010 software. The interface provides the user with :

       I.            1.Weight and total price of the items

    II.           2. Item selections

 III.           3. Barcode on the receipt

 IV.           4. Customer’s database

This is the flow chart of the project (based on interface in Visual Basic Application)