Sunday, 1 September 2019

Empty roll warning buzzer

This was a commissioned project that went nowhere, but could easily be re-purposed for many different use cases, so I thought I'd share it.

The aim is to create a range finder device that can be mounted and aimed at a roll of material on an assembly line, so that the range finder can be used to determine the amount of material left on the roll, and sound a buzzer when the supply falls below a set amount, informing the machines operator so that they can replace the roll and minimize downtime.

The sensor is a Sharp GP2Y0A21YK0F. The datasheet shows that the voltage drops as the distance increases. In our particular use case, this means that as the material runs out, the voltage will drop, so we want to flag a warning below a set voltage.

Initially I thought about using a low battery circuit similar to the one I modified to use in the Keyboard Pedal project. However, I later realised that would be over-complicating it, and that the same result can be achieved using a simple comparator circuit.

The chip used is the LM358 op-amp, and the circuit is similar to the one found on page 6 of this PDF.

The key differences being:
  • The voltage provided by the resistor and zener diode in their diagram is replaced by a potentiometer in mine, so that the 'trigger' value can be adjusted.
  • The potentiometer in their diagram is replaced by the IR sensor in mine.
  • The resistor and LED output in theirs, replaced by a buzzer.
  • The voltage of my circuit is 5V so it can run off a USB charger.
With all those changes taken into account, the resulting circuit looks like this.

Note that RV2 is the IR distance sensor - I didn't have the symbol for it in KiCad so used a potentiometer symbol as that's essentially the behaviour it exhibits.

The finished circuit, fit neatly into it's enclosure.

The case

The circuit needed to be neatly enclosed for it's purpose. Often for electronics projects I'll end up re-purposing existing containers and boxes as enclosures, but as it was a commissioned project something more professional was in order.
The whole enclosure is 3D printed, based off a simple lidded box design with cut outs for controls and the IR sensor itself.

I also took the opportunity to turn the box into a simple parametric template so I can recycle it for other projects. the SCAD source code can be found on GitHub.

All that's needed is to supply the internal dimensions that are required and it'll create a thin-walled box meeting those criteria, using only stock OpenSCAD commands - no other modules required.

The lid fits snug - on my (admittedly by this point quite old) 3D printer, a nice unintended side effect of the rough resolution is that the ridges create a nice friction fit, allowing the box to be closed tight, but also popped back open with relative ease should maintenance be required.

The controls and buzzer

The sensor - the glue-filled part was to allow flexibility in angling the sensor, but was later found to be not required


The test

As I didn't have access to the factory where this was intended to be used, I whipped up a quick test rig using kitchen roll as the stunt double for the actual material.



The short buzzing before the alarm fully triggers is driven by the 'wobble' of the roll on the holder. In production use the material rolls are much more securely held, and so this effect would be minimised.

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