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.
What’s annoying about digital scales is the timeout that shuts the power and resets it to zero if you don’t keep adding stuff continuously. If you pause for 15-30 seconds it’s all gone – so don’t go hunting for that second packet of sugar, or answer the phone, or anything. If you tare the scale with an empty cup and then add something, then look away for too long, the reading is gone and re-starting the scale forgets how much the cup weighed so you have to empty it and begin again.
With the analog scale, all the functions could be realized without the Arduino, with two op-amps and a precision voltage regulator. Taring the scale would not be as automatic, but you could dial it to any arbitrary value you want – and it would remember where you left off.
I think a “hidden” reason for the cheap digital scales turning off quickly is because of cheap circuitry – they drift a lot!
I don’t see an immediate reason why they would. The standard load cell is temperature compensated by construction, and the next thing in line is a feedback controlled amplifier, and the signal is basically ratiometric to the input voltage. They’d have to be doing something completely silly to have it drift all over the place.
Methinks its because they’re zealously conserving the battery, because they keep having those flashy blue LEDs and beepers and whatnot that consume too much power versus a simple reflective LCD and nothing else.