The health plan I use pays $100 toward the year’s over-the-counter healthcare stuff, although with a caveat: you can only buy the stuff from a specific website. As you might expect, what’s available consists of no-name generic products with absurdly high sticker prices and, just to rub it in, the hundred bucks gets paid in quarterly use-it-or-lose it installments.
Seeing as how it was free, I got a kitchen scale:
It has two catastrophically bad design features:
- Terrible battery life
- Overly sensitive controls
It runs from a pair of series-connected CR2032 non-rechargeable lithium coin cells. Which would be fine, except that the blue LED backlight stays on for 30 seconds after each button touch and draws about 10 mA.
The battery lifetime is best measured in days.
The four control “buttons” on either side of the backlit LCD are touchless sensors using copper foil stickers:
The alert reader will spot those the empty CR2032 coin cell contacts over on the left and a pair of NP-BX1 batteries in the middle.
I figured there was no need to keep feeding it coin cells while I played with it, so I conjured a holder from the vasty digital deep. Normally, that would be an OpenSCAD solid model suited for 3D printing, but in this case the lithium cells exactly filled the space between the PCB and the bottom of the case, so it became a 2D design neatly suited for laser cuttery.
I planned to stick the orange cutout (in 1.5 mm acrylic) as a stabilizer around the pogo pins making contact with the cell terminals from the red cutout (in 3 mm acrylic), but just melting the pins into the acrylic seemed sufficient for the purpose. Strips of adhesive sheet saved from the margins of previous projects affix the holder (not the cells!) to the scale’s upper glass layer.
As far as I can tell, the scale is perfectly happy running on 7.4 V, rather than 6.0 V. The PCB has two terminals marked +3V and +6V, so it probably depends on which LEDs they use for backlights:
The alert reader will notice a peculiarity concerning the sensor pad connections along the top edge.
More on that second bad design decision later …
The Smell of Molten Projects in the Morning 
I thought all kitchen scales of this general variety ran on a couple of AA or AAA batteries.
How I imagine the boardroom meeting:
Muhahaha, they’ll have to buy a replacement sooner.
While my brain works more like this:
I definitely won’t buy a SmartTheArt next time. (Also why did I buy it? Bad brain, bad!)
I retrieved the previous scale from the darkness of the Basement Shop, where it was awaiting use for epoxy measurement, and restored it to a place of honor on the kitchen counter. It has its own character flaws, but the pair of AAA cells inside power it for well over a year of daily use.
Poking fun at the new scale is the most useful thing I can do and definitely worth its price …
My first digital scale was a Brookstone, powered by a single non-replaceable CF2032. It’s over a quarter century old, and has been retired from active duty for 5 years, but it still works. No backlight and it’s bog-slow at starting up, but that’s keeping the current draw to reasonable values. I haven’t bothered to find tabbed CR2032 cells and am reluctant to try soldering to any cell case. Not sure if there’s room for a larger battery.
The current scale is an Oxo, powered by two AAA cells. Lifespan is about a year on the batteries, but the main button (on/zero/set tare) is one of two membrane switches, and it’s dying. The control head is glued together, so I’m resisting any temptation to try a fix.
The Oxo was fairly inexpensive (no backlight, total capacity circa 5 pounds, with values 0 to 2 pounds or the metric equivalent). If the Kroger affiliate has a duplicate in stock, I’ll go with it.
Er, “single CR2032, soldered in place”.