This is a kitchen scale with a maximum weight of 2.5kg and an accuracy of 10g. Exceeding the range is indicated by an acoustic signal and an LED. Weight is displayed on a four-digit LCD display. The weight also includes a weight-zero reset button. The power is solved by a battery whose discharge is below the set limit indicated by the LED.Then the battery needs to be recharged, otherwise the voltage drops below the operating level and the wrong weight information appears in the display. In order to avoid having to switch off the balance for unnecessary battery discharge, after 2 minutes of inactivity (only if there is no weight on the weight) it will turn itself off.

Block diagram :

Block diagram

Block diagram analysis: 
By weighing the weighing platform with force on the sensor, it deforms and at the same time changes the resistance of the 4 strain gauges attached thereto. These strain gauges are bridged. The output voltage of the sensor is of the order of magnitude, so it needs to be amplified for further processing, so I used the AD620 instrument amplifier. The amplification is set to 150. The amplified signal can now be converted to a digital value using the TLC549 8-bit serial A / D converter. The core of the full scale where the measured value is converted from the A / D converter to the correct weight reading displayed on the 4-digit LCD is the AT89C2051 μ-processor. If the weight is exceeded, the overload indicator is activated and —- appears on the display. The reset button is used to reset the current weight from which it will be read, for example: put the bowl on the scale and reset, only what we place on this bowl will be weighed. If we cut the bowl, —- will appear, we must reset the weight again, this time without a load. The balance is powered by a Ni-Cd battery that can be recharged if required by a built-in charger. When charging, the power for the rest of the balance is disconnected. The balance is switched on and off with the On / Off button (press on / off to turn off). To avoid unnecessary discharging of the battery, after 2 minutes of inactivity, the balance will turn off only if there is no weight on it. The low battery indication indicates that we should recharge the balance as soon as possible, otherwise weighing errors occur due to a low power supply, and incorrect weight information will be displayed on the display.

Control section diagram
Control section diagram
Diagram of the imaging section
Diagram of the imaging section 

More detailed activity: Only IO4, transistor emitter T2, C11 and On / Off button are powered directly from the battery. Because the IO4 is made by CMOS technology, its take-off is minimal and hence the stand-by take-off of the whole balance when it is turned off. IO4 is connected as a T flip-flop (divider two). C11 and R14, after connecting the battery, sets output Q to log. H, the transistor T1 is closed, thereby interrupting the supply to the other part of the circuit. After pressing the button, CLK clock input is logged. H, the circuit is overturned, the transistors T1 and T2 are closed and the power balance of the rest of the balance is switched on. OZ3 is connected as a voltage comparator that monitors the battery voltage. If it drops below the set threshold by the trimmer P4, it switches on the transistor T3, which has an LED indicator in its collector and indicates a weak battery. For other circuits, we need a voltage + 5V, which is obtained from the IO5 stabilizer, C12 – C14 only serves to filter the voltage. The strain gauge sensing sensor is in the bridging circuit and at its output is a signal in the order of mV. This signal is too small for further processing, so integrated IC amplifier IO2 is connected to the circuit, its gain is 150 and is given by the Trimmer P3, which is set to 330. The amplified signal is further superimposed on the constant voltage (approximately 1 , 0 V), this is obtained from a multiple voltage divider and impedantly separated by the OZ2A operational amplifier. This amplified signal is applied to the A / D input IO3 converter. The range of the converter can be set by bringing two reference values to its REF + (about 2.7V) and REF- (about 1.1V) inputs. Both voltages are derived from the same voltage divider and impeded by OZ1. The size of the reference values can be set by the P1 and P2 trimmers to determine the lower and upper limits for the converter. According to the converter manufacturer, the difference between the reference values must always be greater than 1V. The amplified and converted signal is processed by the IO1 processor (more). The processor is in standard wiring, the C9 and R8 components ensure that when the processor reset is turned on, the Q1 crystal taps it at 24MHz.Output from the processor goes through the four 8-bit IO6-IO9 sliding registers that drive DIS1 segments on the LCD display. The common display terminal is connected directly to the processor port. Display segments need to be triggered by an AC signal with a frequency of 20 to 150 Hz (40 to 300x / s change). This means that the common outlet is set to the log. H and display the segments displayed in the log. L and vice versa the next time you broadcast. If the 2.5kg weight is exceeded (regardless of possible reset), the processor switches the transistor T5 and starts the piezo-siren SIR and the DSIR LED indicating overload. When the zero key is pressed, the processor port P3.2 is grounded, which is the input of the external interrupt and the processor remembers the current weight on the scale. If no weight is placed on the weight and is idle for 2 minutes, the transistor T4 switches on, which transmits the IO4 log to the CLK input. H, it flips and interrupts the power supply for the processor and other components except IO4. Weighing in operation is about 40mA.

Design of printed circuit boards – control:
Upper side – staff:
Design of printed circuit board (90 x 57 mm).

Bottom side – toolbar:
Bottom side - toolbar

Design of printed circuit board (90 x 57 mm).

Design of printed circuit board


Design of PCB – Display:
Upper side – staff:
Upper side - staff

Design of printed circuit board (104 x 56 mm).
Design of printed circuit board
Design of printed circuit board (1)


Program function analysis
Program function analysis: After resetting the processor, the values are initialized and a timer 0 is started, which determines when to be sent to the display and timer 1, which determines when the new value from the A / D converter should be read. After setting, the value from the converter is read and the program starts. First, read from the read value is the value stored at zero (currently 0) and compares if it is not in negative values when it is displayed on the display —-. If it does not find out if the weight is zero, then it starts counting for 2 minutes, which turns off if the zero weight is not changed. Now it compares the value of the A / D converter to see if the weight of 2.5kg is exceeded, otherwise the overload indication will be switched on. It also uses a division to find thousands, hundreds, tens, and units. If there is something else 0, it compares the others and excess zeros are not displayed on the display. The number of thousands, hundreds, tens and units needs to be converted to a combination of bits to display the desired number on the display. The weight data is processed and ready for broadcasting, so the program detects whether it has already timed out and does not transmit on the display, otherwise it compares if the new value of the A / D converter should not be read, if not compare the transmission to the display. .
The program has about 300 lines and you can download it here – it’s complete with a lot of comments.


Scheme, design and charger description :

This is a simple charging connection for a Ni-Cd battery. This connection is not the most appropriate, but it is sufficient for occasional charging. I’ve chosen this simple connection to get the charger into the U-KPZ3 adapter box. The mains voltage is fed via a fuse to a transformer with a secondary voltage of 12V. This voltage is thrust across the M1 bridge, filtered through C1, C2 and applied to the LM317T regulated voltage stabilizer. With the R3 and R4 resistors the output voltage is set to 11.5V according to the formula U out = 1.25 (1 + R4 / R3). By R2, a constant charging current is set to 60 mA according to the formula I out = 1,25 / R2. This strain is filtered using C3, C4. The D1 LED serves only to indicate that the charger is on the network. The D2 protects the stabilizer against back-up by disabling the charger before disconnecting the rechargeable battery. It is advisable to place a small cooler on the IO1. The Ni-Cd battery is about to charge about 125% of the battery voltage, which in my case is 12V and current approximately 1/10 of the battery capacity, that is 60mA. I deliberately lowered the charging voltage to avoid potential battery damage.

Charger connection diagram:
Charger connection diagram
Design of printed circuit boards (48.5 x 75 mm) and layout of components:



List of used components:
  C1 100n
  C2, C3 22p - 2x
  C4-C8, C10, C12,
  C13, C15 - C19 100n - 13x
  C9 22M / 16V
  C11, C14 10M / 25V - 2x

  R1 1k8
  R2 - R4, R6, R7,
  R9, R15, R18, R22 1k - 9x
  R16 1k SMD 1206
  R5 560
  R8 5k6 SMD 1206
  R10 - R12 4k7 mini - 3x
  R13, R14, R17 10k - 3x
  R19 1k5
  R20 1k2
  R21 680
  R23 390
  P1, P2 PM19K001 - 2x
  P3 PM19E500
  P4 PT-6VK005

  DZ1 BZX83V056
  DZ2 BZX83V033
  T1 BC560
  T2 BD140
  T3 BC546
  T4 BC556
  T5 BC327

  IO1 AT89C2051
  IO2 AD620
  IO3 TLC549
  IO4 4013
  IO5 78L05
  IO6 - IO9 4094 - 4x
  OZ1, OZ2 LM358 - 2x
  OZ3 TL071
  Q1 24MHz
 DIS1 LCD3906
 DSIR LED 3mm red
 POD LED 3mm red

 Strain gauge DF2S-3 / 5kg
 ZERO, ON / OFF P-0SRB - 2x
 BAT ARK500 / 2
 CIDLO ARK500 / 2 - 2x
 CON1, CON2 MLW14G - 2x
 PFL14 connectors - 2x
 flat cable AWG28-14 (15cm)
 B-8F600AA battery
 power connector K3716A
 display frame AR1950

 Charger :
 C1 1000M / 25V
 C2, C4 100n - 2x
 C3 4M7 / 50V

 R1 1k5 / 2W
 R2 15 / 2W
 R3 220
 R4 1k8

 D1 LED 3mm green
 D2 1N4007
 IO1 LM317T
 M1 B250C1500
 TR1 TRHEI304-1x12

 IN, OUT ARK500 / 2 - 2x
 POJ1 KS20-01 footrest
 tube fuse 200mA

 adapter box U-KZ3
 power connector SCP-2009B
 cable to connect the charger to the balance


Finally some photos completed and FUNCTIONAL scales.

Source: Digital Scales project Alternative link:at89c2051-digital-scales-circuit-atmel.RAR


About The Author

Ibrar Ayyub

I am an experienced technical writer holding a Master's degree in computer science from BZU Multan, Pakistan University. With a background spanning various industries, particularly in home automation and engineering, I have honed my skills in crafting clear and concise content. Proficient in leveraging infographics and diagrams, I strive to simplify complex concepts for readers. My strength lies in thorough research and presenting information in a structured and logical format.

Follow Us:

Leave a Comment

Your email address will not be published. Required fields are marked *

Scroll to Top