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I’m a software guy so please excuse any silliness in this question.

I need a scale which will not shut off on its own and continuously show changing weight values.

Although I may have no choice but to learn about load cells and arduino/raspberry interfaces, I figured I’d start with cheap digital bathroom scales.

Thr bathroom scale I bought shuts off after some time of inactivity. Worse, even if I’m standing on it, after a few seconds it locks in the weight.

enter image description here

Back side and the lcd: enter image description here

As far as I can tell, they use a custom chip/board. I was hoping it would be a standard hx711 with the shutoff feature bolted on, which I could somehow disable.

Is there anything I can do to get the scale to continuously read values, until I turn it off?

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    \$\begingroup\$ It's embedded code. But if you add haptic vibration , it will stay on with ext power. \$\endgroup\$ Apr 5, 2022 at 4:19
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    \$\begingroup\$ (1) Yes, almost all bath room scales use HX711. (2) The scale usually consist of two parts (a) A "Wheatstone Bridge, (b) An ADC (Analog to Digital Converter). (3) You can use any MCU (Arduino, Raspberry Pi, ESP12/32, STM32 etc) to to control the HX711, and display the weight/results on a LCD/LED display. (3) Ref: Reading old weight scale using HX711 - forums.rpi.com forums.raspberrypi.com/… \$\endgroup\$
    – tlfong01
    Apr 5, 2022 at 4:24
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    \$\begingroup\$ I use this dual beam balance scale where the bars flip over in order to work in kg or lb. It just works when you want. Lasts forever. Looking at your picture, I'd say that you have only a finite number of permutations of re-wirings to try out (may need to also include jumpers to that mix.) If there is a finite, non-zero chance one of them will get you where you want to be, then you can get there from here that way. As a programmer, I'm sure you can compute out the permutations available. I've no real clues from that picture to offer, otherwise. \$\endgroup\$
    – jonk
    Apr 5, 2022 at 4:27
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    \$\begingroup\$ Most of that board is capacitors and 0Ω resistors. All the connections top and right are LCD controls. SOT is in vicinity of battery and may supply power to U2. I'd investigate behaviour when weight is applied around SOT. Otherwise I'd scrap board and use sensors. \$\endgroup\$ Apr 5, 2022 at 12:35

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Almost certainly you have no easy access to what would be needed to make that change.

There will be an MCU, maybe hidden under a blob of black epoxy close to the LCD display (because of all the required connections). The MCU may be locked (firmware cannot be read, even with the proper equipment, because the security bits are set) and quite possibly OTP (one-time programmable), and likely of an random type from an obscure manufacturer eg. like this one (and would likely require an emulator type setup to debug).

Note also that if you keep it 'on' it will drain the batteries in short order, mostly because of the current draw from the load cells. It also goes through an auto-tare routine at power up that deals with the drift in the load cells (and that means that load cell "creep" is not an issue).

Chances are both the MCU and the HX711 are powered continuously and the MCU issues a shutdown instruction to the HX711, which puts it into a low power state. Similarly the MCU would go into a low power mode and blank the display while waiting to be triggered.


Edit:

Based on the additional photo it looks like they've integrated the HX711 function into the MCU U2 (they're always innovating to trim costs in this kind of mass market). If it's a similar circuit, the SOT23 part will likely be marked "2TY". Maybe you can find the part number on the MCU. The programming would be done via the test pads on the PCB, including the Vpp pad.

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    \$\begingroup\$ Battery is a simple enough thing to bypass in this setup. \$\endgroup\$
    – Passerby
    Apr 5, 2022 at 6:50
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    \$\begingroup\$ @Passerby Sure. Just added for completeness. The 'zero' function is more fundamental. \$\endgroup\$ Apr 5, 2022 at 6:50
  • \$\begingroup\$ Useful! I added a picture of the back and the lcd. There are only two cables going into the lcd so I’m assuming there is no mcu hidden there. There is no blob on the pcb. Where is the mcu? \$\endgroup\$
    – Shahbaz
    Apr 5, 2022 at 7:20
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    \$\begingroup\$ U2 is the MCU which includes the LCD driver and analog and mixed signal (ADC etc.) functions, it looks like. \$\endgroup\$ Apr 5, 2022 at 7:29

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