Craptastic Kitchen Scale: Button Sensor Tweaks

The four control “buttons” on the SmartHeart kitchen scale are copper-foil tabs that sense the presence of your finger though about 5 mm of white plastic and glass:

SmartHeart 19-106 Kitchen Scale - top view
SmartHeart 19-106 Kitchen Scale – top view

The main failure mode seemed to come from the microcontroller locking up and refusing to recognize any of the buttons, most annoyingly the On/Tare button, while continuing to measure whatever weight was on the scale with whatever zero point it chose. Recovery involved waiting until the thing timed out and shut itself off.

The two buttons on the left select Kilocalories for any of the various foods arrayed around the display. Depending on how it jammed during startup, it might display the Kilocalorie value for, say, sugar, while ignoring all button presses. As the manual does not mention any way to return to weights after activating the Kilocalorie function, other than turning it off, it’s not clear recognizing the other buttons would be much help.

Because we have no use for those functions, I unsoldered the wires to those sensor pads and it no longer jams in that mode:

SmartHeart 19-106 Kitchen Scale - PCB detail
SmartHeart 19-106 Kitchen Scale – PCB detail

The alert reader will note the PCB legend says I have unsoldered the ON/OFF and UNIT wires. If one believes the silkscreen, the PCB dates back to 2015, so it now carries a reprogrammed microcontroller with functions that no longer match the silkscreen.

The overall soldering quality resembles mine on a bad day.

With those out of the way, the scale still jammed and refused to recognize the remaining two buttons. I wondered if it was somehow sensing ghost fingers over both sensors and waiting for one to vanish, so I added a shield ring around the power tab:

SmartHeart 19-106 Kitchen Scale - shielded sensor
SmartHeart 19-106 Kitchen Scale – shielded sensor

That reduced the sensitivity of both sensors to the point where they pretty much didn’t work, without reducing the number of jams.

So I tried increasing the sensitivity of the power tab by replacing it with a larger copper foil sheet:

SmartHeart 19-106 Kitchen Scale - larger sensor
SmartHeart 19-106 Kitchen Scale – larger sensor

That definitely got its attention, as it will now respond to a finger hovering half an inch over the glass, as well as a finger on the bottom of the case: it can now turn on and jam while I pick it up.

More tinkering is in order, but it’s at least less awful in its current state than it was originally, so I can fix a few other things of higher priority.

3 thoughts on “Craptastic Kitchen Scale: Button Sensor Tweaks

  1. And then there is our digital kitchen scales with an alarm timer built in. Like your finely crafted scales, ours will randomly start the timer often as it is put away. Minutes to hours later, we, or our visitors, will hear periodic faint beeps coming ghostlike from the depths of the kitchen cabinets. Helpful in reducing the battery life of the four AA cells.

    1. So it has a special Poltergeist mode? Most excellent!

      Hey: four AA cells? You could replace them with two lithium coin cells! :grin:

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