Building a digital scale from scratch


[Raivis] was given a particular task at his university – find a way to measure how many Duplo bricks are stacked together. There are a number of ways to do this, everything from computer vision to using a ruler, but [Raivis] chose a much more educational method. He built a digital scale from scratch out of a strain gauge and a Wheatstone bridge. The build log is immensely educational and provides some insight into the challenges of weighing things digitally.

A strain gauge is a simple piece of equipment, just a small force sensitive resistor. When attached to a metal bar and a force is applied, the resistance inside the strain gauge changes, but not by much. There’s only a few micro Ohms difference between the minimum and maximum of [Raivis]’ load cell, so he needed a way to measure very slight changes in resistance.

The solution was a Wheatstone bridge, or four resistors arranged in a square. When one of the resistors in the bridge is replaced with a strain gauge, very small changes in resistance  can be measured.

With a custom ‘duino amplifier shield, [Raivis] can measure the resistance of his load cell with 10-bit resolution, or a maximum weight of 1.32 kg with a resolution of two and a half grams. A single duplo block weighs about 12 grams, so we’ll call this one a success.

12 thoughts on “Building a digital scale from scratch

  1. He states that “superglue doesn’t work”… this is wrong. I’ve used superglue many times, it’s on my blog and in my last and current job — the difference is in surface preparation, using an accelerator on the gages, and you apply they in a specific way. Vishay has a tech note on it…. their cyanoacrylate is called m-bond 200, it’s “superglue” just a highly tested consistent quality glue. I’ve used it on hundreds of strain gages for 4 years now. Epoxy is the better long term solution, but cheap off the shelf epoxies have their own issues — don’t use one with filler!

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