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How does this weight scale power on? (step-on)

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CharlesK:
Hello,

I've been puzzling about how to get one of my projects to power up. I've made a weight measurement scale using a load cell, hx711 module and an arduino. This scale is powered using a 9 volt battery. It is powered on by using a soft latch power switch circuit.
()
The scale powers off after 2 minutes of inactivity.

On some digital scales, the scale powers on by putting a small load on the load cell. Here is an example of such a scale ()
The scale in the example video uses a button cell battery. How does this scale measure changes and power on without draining its battery? I thought it might be using a comparator, however I do not see anything similar in the simple circuit shown in the video.  :-//

Regards,

IanB:
One guess would be to have the sense circuit AC coupled with a capacitor. Combining this with a MOSFET as the switching element it would draw next to no current in standby because the capacitor blocks DC. Placing a load on the load cell could send a pulse through the capacitor and trigger the MOSFET which would latch on.

As a side note, I have a cheap digital caliper powered by a button cell. It also turns on automatically when you operate it. However, if you leave the button cell installed it drains to empty within a few months or less. Do you know for sure your example scale doesn't in fact drain its battery while in standby?

james_s:
I have one of those calipers too and it drives me nuts. It would have been far better to just have a conventional switch, or at least a button to apply power that could latch on.

mc172:

--- Quote from: IanB on April 05, 2020, 09:59:41 pm ---As a side note, I have a cheap digital caliper powered by a button cell. It also turns on automatically when you operate it. However, if you leave the button cell installed it drains to empty within a few months or less. Do you know for sure your example scale doesn't in fact drain its battery while in standby?

--- End quote ---

They don't turn on when you operate them as they were never "off", they just turn off the display after a set amount of time and leave it off until there is any activity. They still do the measurement activities in the background - you can see this as some of the really cheap ones have about a 1 second delay before the display turns on. If you open them from fully closed really fast, you can see that the measurement is reproduced faithfully, despite having been "off".

james_s:
Isn't the sensor on those absolute using capacitance?

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