How do digital scales work?
They use strain gauges to measure the weight. An electrical strip on the surface of a flexible backing. The slight bending of the gauge when a load is applied distorts the conductor, changing its electrical resistance. The resistance is measured by the circuit and displays the result digitally.
Walk across a board bridging across a creek.
When you stand in the middle the board bends down. When your little sister stands in the middle, the board bends down less because she’s lighter.
The inside face of the board compresses and the outside stretches. That means when a load is applied, the outside face gets longer.
With math and some knowledge of the materials and span length we can guess at how heavy the load on the board is by how much it bends down or how stretched the outside face is.
Strain Gauges are a common tool that does this. We run electricity through some wire, and measure the resistance. When the wire stretches, the resistance changes in a predictable way.
So, you glue some bits of wire to the “board” of your scale, and when you stand on the board, the wire’s resistance will change.
We know how big the board is, what it’s made out of, and where the gauges are. Since we know everything about the wire and the input current, we can calculate how much it’s stretched when you stand on the scale.
Take all those inputs and feed them into the scale’s calculator and you have a reasonably accurate weight estimation. Most ~$20 household scales are within a pound or so. Definitely enough that it’s not the scale’s fault you’re not getting the results you want.
Naturally, better materials and construction make for more accurate weights. Generally speaking, for every decimal point of accuracy, you can expect to add a 0 to the cost.
The digital part is just the display mechanism. From a functional standpoint, it isn’t really a part of the scale at all.
The digital scale ultimately works the same way as other scales, in that it has a spring in it, and by measuring how far the spring is compressed, it can tell how much force is being applied to the spring. On a purely physical scale, the spring is hooked up directly to a set of levers that move the display lever. But in a digital scale, the level of compression gets sent to a set of circuits that determine what numbers to display.