Building an Arduino based capacitive touch kitchen timer – Part 4

The fourth part of the Arduino capacitive touch timer series discusses the case design of the project. It also outlines how to assemble the individual pieces of the enclosure and how I plan to improve the design in the near future.

When I planned this project, I tried to keep it as modular as possible. That’s also the reason why I ended up using three PCBs for the electronics. I applied the same strategy to the case design.

This approach allowed me to quickly build prototypes and swap out pieces that I didn’t like in the end. It should also allow you to easily modify the project to add or remove certain features. And I’m unbelievably glad that I chose to design the project this way, as I can now easily create a new revision of the logic board to make it easier to assemble, cheaper to build, and more reliable, without having to re-design the entire case. Anyway, more about the upcoming revision later in this article.

For now, let’s take a look at the enclosure that will house the electronic components. Please note that I might slightly change individual parts of the enclosure to better accommodate the changes that I’ll introduce in the next article.

The top piece

I talked about the capacitive touch ring PCB in the last article. When I published it, I haven’t received the finished round PCBs yet. However, they luckily showed up last week, and they turned out beautifully! Anyway, I started designing the case as soon as I held the new PCBs in my hands, and I decided to split the enclosure into three parts with several individual pieces. The top part of the case consists of four plastic pieces and two PCBs.

It starts with the top cover of the case. It consists of a very thin top layer that allows the seven segment display to shine through and the plastic is also thin enough to allow the electronics to detect a user’s finger and input gestures:

Figure 1: The top cover of the case in tinkercad

Note that the design contains two thin pegs that align with the two holes in the touch ring PCB. These two pegs not only help you align the PCB when assembling the device, they’re also ever so slightly smaller than the holes on the PCB, which ensure that the enclosure holds it in place securely:

Note that the case is slightly larger in diameter than necessary so that variations in size won’t cause problems. Also note that the PCB must sit flush on the thin plastic layer that forms the top of the finished device. There must not be any space between the two components to ensure that the rest of the case fits together properly.

As you can see in figure 4, the plastic piece is slightly taller than the PCB to ensure that the seven segment fit in the case nicely. I created the following piece to compensate for that additional space:

Note how the spacer leaves enough space for wires to run from the cpacitive touch ring to the MCU. Figure 6 shows how the adder spacer forms a flat surface with the other components contained within the topmost piece of the enclosure.

As you can also see, the two white plastic pegs extend beyond the teal-colored spacer. This is on purpose and it allows you to firmly snap the next piece of the case in place:

Figure 7: The next piece of the case snaps into the existing pegs and it contains cutouts for the display and wiring

Note how the next part contains cutouts for the wires coming from the touch ring and the seven segment display that’s present on the main logic PCB. Also note how the spacer, seen above, creates a small gap between the PCB and the blue part seen in figure 7.

Next, the logic board PCB snaps in place:

It actually doesn’t snap in place. Instead, the PCB loosely sits on top of the blue part. The next piece of the case, however, holds the PCB firmly in place. It connects to the existing enclosure components via the two large blue snap-in clips:

Note how the last piece contains ventilation holes for the electronics as well as a hole for the power wires to go through. Besides that, the white plastic piece also contains four screw holes.

The middle section of the case

Luckily, the rest of the case is not as complicated as the top piece. The middle section, for example, consists of a single plastic component:

This part’s only purpose is to add some space where the battery and power supply board will sit. This part connects to the top with four self-tapping five millimeter screws. Unfortunately, I was unable to source the correct screws when writing this article. However, I’ll buy them before I publish the last part of this series.

The bottom of the case

Now comes last part of the case. This is a single plastic piece that holds the PSU board and the 9V battery in place:

Note how the PCB doesn’t sit flat on the plastic part, unlike the touch ring PCB on the top. This is to allow for some air flow in the case. It also allows you to nicely tuck away the wires under the power supply.

Download the 3D printable models

You can download the STL files for 3D printing here. I created a separate file for each part of the case.

The upcoming revision

While assembling and testing the case, I noticed that the board had some reliability issues. If you followed this series, you might have noticed the extra capacitors I added when I tested the firmware in part three of this series. These prevented the MCU to restart when the voltage of the power supply dipped below a certain threshold value due to increased current draw. So, before I can finish this project, I’ll have to address these realibility issues in an updated revision of the circuit.

In this updated revision, I’ll add two capacitors (one close to each IC on the board), and I’ll also move all components, except for the seven segment displays, to the underside of the boards. This change will hopefully make it easier to mount the logic board in the case when assembling the final product. Right now, it can be rather tricky to properly align the PCB in the case. Furthermore, I might switch to SMD components if they are cheaper then the through hole ones. And if I end up switching, I’ll also have to add a programming interface that allows me to flash the firmware. This also means that updating the firmware, in case of bug fixes, will become much easier.

Table of contents

Part 1: Project Idea and the theory behind capacitive sensing
Part 2: The circuit and a custom PCB
Part 3: The software
Part 4: The case design and an upcoming revision (You are here)
Part 5: The finished product, lessons learned

4 thoughts on “Building an Arduino based capacitive touch kitchen timer – Part 4

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