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

[IDEngineer]

I suppose they could have a magnet and a Hall effect sensor (or a shorting contact in the pattern, etc.) at the fully-closed position. That could yield an absolute zero, at the expense of relative zeroing (where you use the calipers to measure difference from a reference).

[CharlesK]

Quick question, I've managed to initiate power down mode using 5v from the arduino (led pin 13). Only thing is, it only works if going through my multimeter. The multimeter running is in series measuring 5v dc going from pin 13 to the SCK pin on the hx711 module. If I connect pin 13 directly to the SCK pin, sleep mode crashes and the screen shows random characters. I assume this is because pin 13 is messing with the digital signal coming from the module.

Could it be that the circuit is missing a resistor? I don't have any close right now so I can't try unfortunately. How many ohm should the resistor be?

[CharlesK]

Ok, I couldn't wait and drove to my house to pick up some resistors. I tried a couple between 6k and 30 something k. basically they all worked. Now I need a diode to make the power only go one way (into the hx711 module). I measured the power consumption and its between 10 and 16 mA measured at 9v. The lcd board and the arduino nano have some on board LED lights that I should take off. It will save some power too. About 2mA per LED if I measured it correctly. That should bring down the average current to below 10mA. I also read that the USB chip consumes quite a significant amount of power which stays on during sleep mode. I now have a couple of arduino pro mini's on the way which should further reduce the current consumed. I wonder how low I can make it go and thus how long it will last on a 9v battery.

[SiliconWizard]

The ones that "power on" when loaded are actually on all the time, the microcontroller just turns the display off and reduces the measuring frequency in "off" mode, maybe once a second or less. Then when a change is detected it increases the frequency and turns on the display.

That is how I would do it.  Most of the power draw is from excitation to the load cell so periodically apply excitation for a few milliseconds, and make a quick low resolution measurement.

That's likely how most of them work. You can get the average consumption down to a couple µA this way with the appropriate design with not too much effort. Then even a CR2032 cell would last several years this way. With a more involved design, you could get down to a few tens of nA I guess, but I'm sure very few vendors bother with that.

[xani]

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?

I also have cheapie but mine have on/off button. "Turning it off" reduces power usage by about 20%. Like, what's the fucking point  of "turning off"

[SiliconWizard]

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?

I also have cheapie but mine have on/off button. "Turning it off" reduces power usage by about 20%. Like, what's the fucking point  of "turning off"

Ahah, yeah they're not all created equal. I've owned a cheap digital caliper that would just deplete batteries in a few weeks when off.
OTOH, I have a Mitutoyo one that I've bought a few years ago. Original battery is still good and I haven't changed it yet.