Laser cut casing
In this section we'll show you how to construct the casing based on the hardware in the previous sections. There are many options to make a case for the electronics behind this project, from a tupperware box to 3D printing. Given the nature of device, and its need to have various holes for internal and external mounting, as well as holes for the inlet and outlet of air for the PM sensor - laser cutting seemed to be the obvious route for the quick and easy production of a large amount of cases needed for this project.
If you do not have a laser cutter, you are welcome to download the plans, print them to scale, and use any other means to cut the box.
On our GitHub, you will find the relevant .dxf files, which can be used to make the CamBike Sensor box on a laser cutter. The files have been designed in mind for 3 mm thick materials, and a "kerf width" of 0.15 mm (to take into account the thickness of the laser beam). The internal dimensions of the cut box are meant to be 90 * 90 * 60 mm. Below, left, shows the overall layout for the laser cut case, and on the right, explains the relevant holes and engravings which make the casing.
The chosen material for the majority of the box was "LaserPly", which is made of birch plywood glued with a waterproof glue. The lid was chosen to be made of clear acrylic, such that you can see the electronics within the box.
There are multiple laser cut files on GitHub, one file per side of the box, and three files that each have all the sides together (two without the lid for when you're cutting just wood, and one of those contains a different button design). We were using an 80 W laser cutter controlled with LaserCut 5.3.
The settings followed for cutting both the 3 mm LaserPly and the clear acrylic were:
The relevant colours should appear when you import the .dxf files into LaserCut 5.3, or any other .dxf file reader. The total time for cutting each box is about 16 minutes, with the lid taking the longest given the amount of engraving it has to do. It takes approximately 8 minutes for the wooden part, and 8 minutes for the acrylic lid.
The above values may change depending on your cutter's specifications. The green engraving (ideally give 1 mm of depth) is used for three aspects of the box, as you can see in the image above. It is designed to provide the air outlet an enclosed and indirect channel of air for the PM sensor - to prevent water directly getting from underneath the sensor. It is also for the acrylic lid, for it to nicely slot onto the box once it is completed. Lastly, it is used for the SD card, to have a better grasp for pulling out the SD card (sadly it's not a push-spring style SD card reader). The grey engraving is just used for logo, text and images. Blue cutting is for all the holes, and black for cutting out each side of the box. The laser cutter should evaluate the colours in the order described.
The photo below shows when we were mass producing the boxes with square buttons
After laser cutting, all that is left is gluing and varnishing. For gluing, lay out the sides faced down, like in the image below, and then use wood glue to cover all the joints. Assemble the box, and also glue in the air outlet channel cover with the engraved layer faced down on top of the non-engraved layer, it effectively produces a hood (see one photo down for reference). Clean up any excess glue, and secure with elastic bands. Leave to dry for 24 hours.
Once dried, to increase the "waterproofness" of the plywood, it is best to cover all the wood with a varnish of sorts. We recommend yacht varnish. Below shows the final finished wooden part of the build.
To complete the box, feed the cable ties through your chosen mounting side(s), place the constructed electronics inside with the inlet tube going through the hole, secure the electronics with three mounting holes on the bottom, place the relevant button and battery in/on the case. Then, to secure the box, place the lid on top and lock it all together with 70 mm M4 screws (you are welcome to use M6 screws as they are easier to source, but you will have to widen the holes for this).
The box should be complete now. There are multiple ways of mounting, as long as PM sensor's inlet is facing down. Below shows some images of possible mounting methods.
You can improve upon the "waterproofness" using silicone in the engraving for the lid, and rubber washers for the screw mounts if you have the resources.
If you have any problems, please don't hesitate to contact us on email@example.com