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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.

Paper accelerometers for pennies in the works

Cheap paper accelerometers? Put us down for a dozen to start. They’re not quite ready for mass production yet but it looks like they’re on the way.

[George Whitesides] led a team to develop the new technology that uses simple manufacturing methods to produce the sensor seen above. Graphite and silver inks were screen printed onto heavy paper. The single limb sticking out from the body of the sensor is a separate piece of paper that bends the carbon area when force is applied. This changes the carbon’s resistance which is measured using a Wheatstone bridge constructed by gluing resistors to the device.

It sounds unsophisticated compared to most of the accelerometer modules we’re used to, but if you need a sensor that detects sudden motion this sounds like the perfect part. Now who wants to be the first person to replicate this in their basement?

[Thanks Fabien]


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