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 :
More detailed activity:
Design of printed circuit board (90 x 57 mm).
Program function analysis:
The program has about 300 lines and you can download it here – it’s complete with a lot of comments.
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.
Design of printed circuit boards (48.5 x 75 mm) and layout of 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 SIR KPE242 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