Digital Weighing MachineWeighing machine is very useful product. It helps us in checking our weight aswell as other product’s weight. Moreover, Weighing machine are used in manyindustrial and commercial applications. Without a weight machine it is not possibleto know the exact mass of anything. There are two types of weighing measuringsystem called analog weighing machine and digital weighing machine. The analogweighing system is very erroneous. In digital weighing machine, we don’t have toface any problem. Only viewing the display we come to know the exact weight of the product.In digital weighing machine, we interface with the digital world. In our project, wecome to know the basic principle of digital weighing machine and know how tomake a digital weight machine easily and cheaply. This project basically helps usto compete with Manufacturer Company of digital weighing machine in future

•Load Cell•Stand of iron to set up Load Cell•Microcontroller( PIC16F877A)•Op-Amp (AD620 & LM358)•Bread Board, Printed circuit Board(PCB)•7-segment display•Three power supply•Concomitant: Resistors, Capacitors, Connecting Wires.

 

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MPLAB software to do the codeBrief descriptions of important usable components•Load cell : Load cell is used as force measuring component.We collected a load cell of 3KG.As a result in our project weonly could measure till 3 KG. There are four or two straingauges in load cell depending on the capacity of load cell. It  works like a Wheatstone bridge.Fig: Block diagram of load cell When we don’t give any load to it, it remains balanced; there is no flowing of current as well as corresponding voltage.

  Whenever we hang on a load on to it, imbalance situation is occurred. Stress iscaused by external force . S t ra in gauge conver ts the deformat ion to e lec t r ica l signals. The electrical signal output is typically in the order of a few mill volts.•

Op-amp AD620 &LM358: It is an instrumentation amplifier which is atype of differential amplifier   . They are mainly used to amplify very smalldifferential signals from strain gauges. It has got very lowDCoffset, lowdrift, lownoise, very high gain. We made our gain 100 using AD620. We also used op-amp LM358 to amplify our AD620 output further more. •Bread board:breadboardi s a c o n s t r u c t i o n b a s e f o r a o n e - o f - a - k i n d electronic circuit . I t ’ s the solder less breadboard which does not requi re soldering. Moreover is reusable.Fig: breadboard 

 

•Microcontroller (PIC16F877A   ) :P I C 1 6 F 8 7 7 A i s a s m a l l p i e c e o f   semiconductor in tegra ted c i rcui t s . The package type of these in tegra ted c i r c u i t s i s D I P p a c k a g e . W e c o u l d u s e a n o t h e r m i c r o c o n t r o l l e r .

B u t P I C 1 6 F 8 7 7 A h a s s o m e a d v a n t a g e s . . T h i s p a c k a g e i s v e r y e a s y t o b e soldered onto the strip board. PIC16F877A is very cheap. Moreover, it isa l s o v e r y e a s y t o b e a s s e m b l e d . S o w e u s e t h i s c h i p . T h i s I C c a n b e reprogrammed and erased up to 10,000 times .Therefore it is very good for new product development phase. •Pr in ted c i rcui t board: A p r i n t e d c i r c u i t b o a r d o r P C B i s u s e d t o mechanically support and electrically connectelectronic componentsusingconductivepathways , t racks or s ignal t races etchedfrom copper sheets laminatedonto a non-conductive substrate. We set up our microcontroller inthe PCB.

 

 Fig: Printed circuit board Components that we need to make our IC (PIC16F877A) work are just a 5V  p o w e r s u p p l y a d a p t e r , a 2 0 M H z c r y s t a l o s c i l l a t o r a n d 2 u n i t s o f 2 2 p f   capacitors. So we set up the components in PCB board. We also set up someLED’s for the 7 segment in the PCB.•

7 segment display: A seven-segment display, or seven-segment indicator, isa form of e lec t ronicdisplay devicefor displayingdecimal numerals. A 7 segment display consists of 7 individual LEDs with one end of each LEDc o n n e c t e d t o g e t h e r . W e u s e d c o m m o n c a t h o d e m e a n s t h e m i n u s e s a r e connected together 

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Project description

Hardware DescriptionHardware part is done in a breadboard. We constructed our circuit

diagram on bread board. Isolate four wires are coming out of the load cell. Two wires are

usedas input in our amplifier and other two are used as and Ground. We connectedit with the AD620;

supplied (+ve) 7.5 volt and (–ve) 7.5 volt separately using two power supply in the AD620 op amp.

We measured the change in voltage as theweight flexes via the green and white wires. We measured

across a resistor of 1

 

k.ohm and connected a capacitor parallel to resistor, so that we can avoid the noise.For one KG it was giving us only 1.8 volts whereas we supposed to get 5 volt for one KG. S o t o a m p l i f y o u r v o l t a g e w e p a s s e d t h e o u t p u t v o l t a g e ( ) t o o p - a m p LM358.Using a variable resistor of 17.2k.ohm we amplify our previous voltage 18times using following formula:= ( 1 + )Fig: circuit diagram for hardware part After that, we got the 5volt for one 1KG and 3 volt for 1/2 KG.Software description Whenever we got the analog input from the hardware part, we need to pass itthrough the microcontroller to the 7-segment display.

  S o f t w a r e p a r t i s m a i n l y t h e c o d i n g o r p r o g r a m m i n g . W e d i d o u r c o d e u s i n g MPLAB software. As our input was analog and we wanted to show it on a digitalform. The analog to digital conversion was completely done in our code. This partwas little bit difficult to as we had to check the code many times. Port A is used asout input of the analog input. We used port B of microcontroller for the 7-segmentdisplay and used port D for the data selector. We converted the input from binaryto 10-bit BCD which displayed in 7-segment in a corresponding decimal value. Aswe use 10 bit, we could show from 0-1023.We burn the code into microcontroller using a burner and then set up the chip in aPrinted circuit board.Fig: Digital weighing machine

  After that we connected the PCB board with the 7-segment display. When we din’tgive any load, the display was showing an offset value. For 1/2 kg it was showingus decimal 588 and for one kg it was 1023.Problem faced during doing our project Mainly we faced problem in hardware part.•By checking the load cell wires again and again we had to find out which of the two are inputs.•

Making of stand of iron was also very difficult part.•When we use LM358, the output was highest fixed on 3.87 volt. It was little bit difficult to find out the reason behind it. After some analysis we foundout that because of the less power supply of 5 volt we were not getting theexact amplified voltage. When we increase the voltage, we got our outputfrom the LM358.•Moreover we could not make sub tractor circuit for avoiding the leakagevoltage only because of time shortness.•We a lso burned one of our microcontro l le rs because of excess ive power  supply. As because PIC16F877A is very sensitive we have to very careful of  Further Improvements can be done in following ways•O u r w e i g h t m a c h i n e w a s g i v i n g u s a h u g e o f f s e t v a l u e w h i c h i s r e a l l y undesirable. So to avoid that offset value we could make a sub tractor circuitwhich would peter out the offset almost.•If we want than we can develop this weighing machine by many ways suchas we can use keypad like if we keep any product on the machine than it willshow the cost, weight and total cost of that product on the display. For doingthis we also have to develop the code.Weighing machine i s very useful for us . Without weighing machine i t i s not  possible to know exact mass of anything. It becomes a part of our daily life. Theo r i g i n a l c o s t o f a w e i g h i n g m a c h i n e i s v e r y h i g h b u t o u r i n v e n t e d w e i g h i n g machine is very low cost. We spend almost 1500 taka for this weighing machine but the original price of weighing machine is approximately 4000-5000 taka.D o i n g t h i s p r o j e c t w a s v e r y i n t e r e s t i n g . F r o m t h i s p r o j e c t a c q u i r e l o t s o f