Digital displays are useful for quick and accurate readout, but lots of people prefer the physical motion of a needle moving along a dial. For instance, many smartwatch users choose an analog face to show the time, and modern cars with digital dashboards often default to showing an analog speedometer. Following this trend, [Miro Pavleski] built a digital weighing scale with an analog display that not only looks neat, but also serves as a good demonstration of the way that modern scales work.
Inside, the device is built up like a typical electronic scale: the heart of the instrument is a load cell that supports the platform and bends in proportion to the weight applied. This bending motion is sensed by a set of strain gauges wired up in a Wheatstone bridge configuration. An HX711 readout chip measures the resulting voltage and converts it to a digital code that is sent to a microcontroller, in this case an Arduino Nano.
Whereas a typical scale would then simply show the resulting number on an LCD display, [Mirko] decided to use a moving coil meter driven by the Arduino’s analog output. That meter was originally designed to show currents, so [Mirko] printed a new background image using kilograms instead.
As with any analog sensor, calibration and nulling are essential to obtain a reliable measurement. When the instrument is powered up, it asks the user to place a 100 gram weight on the scale, which will then be used as a reference to control the voltage-to-weight scaling factor. You can also subtract the tare weight of any containers you might want to use by simply placing them on the platform before flipping the power button.
If you’re planning to design your own digital weighing scale, this older project is a great place to start. This teardown of a mass-produced scale is also full of interesting details. We’ve even seen a home-built scale that automatically weighs cats and feeds them based on ID.