Taking a design from concept to production is no small feat, but careful planning and proper use of prototyping construction methods can greatly accelerate any idea. In this series, we will develop and prototype a simple Wi-Fi IoT temperature and humidity sensor utilising both PCBs and laser-cut parts.
Developing a product or project from an initial concept requires various stages of development, prototyping, and testing. The first steps in any project are the alpha stages which generally use off-the-shelf parts, modules, and rapid prototyping construction methods (such as desktop 3D printers). This stage is designed to confirm the functionality of the design as well as help to realise how the final product may look, but rarely produces a finalised design.
The secondary/beta stage creates a product that mostly resembles the final result. However, this is still a prototyping stage which can see many variations of a product being developed before a finalised design is completed. Such variations will occur as a result of unforeseen problems with a design which include ventilation, heat dissipation, miscalculated mechanical parts, poor performance, and general improvements.
In this 4-part project over the coming months, we will take a simple IoT device from its alpha stage to a beta and early production stage with the help of Ponoko and PCBWay services. While the device is designed in these articles will not be intended for commercial release, the steps that we will encounter, the choice of components, and the method for prototyping enclosures & special parts will be identical to those faced during the development of commercial electronic products.
Ponoko is an on-demand prototyping and production service that offers sheet laser cutting, engraving and bending services on a range of over 200 engineered products including steel, brass, plastics, woods, silicone and foam. Their Bay Area US facility is specifically designed for customers with smaller orders (as little as 1 part), offering same-day quotes and delivery. Larger orders (up to 10,000 parts) are handled by their Bay Area China facility which offers volume discounts of up to 93%, making it ideal for the later stages of beta projects and early stages of production. Ponoko’s laser cutting services can be used on large parts measuring up to 790x384mm, can be used on small parts measuring just 6x6mm, materials with a thickness between 0.1 - 19.1mm, and require dimensional accuracy of ±0.13mm for organic material sheets and ±0.XXmm for metal materials.
PCBWay is a PCB production and assembly service that has made its name in both the maker and professional markets. Based in China, PCBWay is able to manufacture multi-layered PCBs, cut PCBs to any shape, manufacture individual PCBs, and provide advanced manufacturing techniques including flexible PCBs. A more recent service provided by PCBWay is PCB assembly whereby PCBWay will source parts, purchase, and then assemble PCBs fabricated by them. Such a service is highly ideal for modern designs that are reliant on small SMD components.
We will be designing and prototyping a simple IoT device that will measure temperature, humidity, and air pressure.
The heart of our design, provided by PCBWay, will be an ESP32 Wi-Fi module as it offers wireless connectivity, has many I/O pins, and comes with an RF shield to help improve its EMC performance.
Sensor capabilities will come from two devices; the DHT11 and BMP20. The DHT11 is a one-wire device that only requires a single GPIO on the ESP32 making it very easy to connect to. The BMP20 is an SPI device that requires 4 GPIO, and the physical size of the BMP20 is so small that a PCB assembly service is the only reliable method for soldering it.
To keep circuitry simple, we will not be integrating a battery for remote operation, but instead a micro USB connector for providing power. This connector will also allow us to program the ESP32 with the help of a CH340 USB to UART converter. Information about the environmental conditions will be displayed on a 3 x 7 segment display, and several SMD LEDs will provide information on current units (°C or °F), and Wi-Fi status.
The enclosure will be constructed from laser-cut parts provided by Ponoko and will be required to display the current readings from the electronics as well as provide access to any buttons and connectors. As the product will eventually become a commercial device, we will also have to consider its overall aesthetics, how easy it is to construct, and the physical dimensions.
Read More - Creating laser-cut parts from various CAD software tools
The final step in the enclosure development process will be choosing a production method for large-scale production. Such methods include plastic injection moulding, off-the-shelf enclosures, and panel assembly.
Creating the electronics for PCBWay
We will be designing and prototyping a simple IoT device that will measure temperature, humidity, and air pressure.
The heart of our design, provided by PCBWay, will be an ESP32 Wi-Fi module as it offers wireless connectivity, has many I/O pins, and comes with an RF shield to help improve its EMC performance.
Sensor capabilities will come from two devices; the DHT11 and BMP20. The DHT11 is a one-wire device that only requires a single GPIO on the ESP32 making it very easy to connect to. The BMP20 is an SPI device that requires 4 GPIO, and the physical size of the BMP20 is so small that a PCB assembly service is the only reliable method for soldering it.
To keep circuitry simple, we will not be integrating a battery for remote operation, but instead a micro USB connector for providing power. This connector will also allow us to program the ESP32 with the help of a CH340 USB to UART converter. Information about the environmental conditions will be displayed on a 3 x 7 segment display, and several SMD LEDs will provide information on current units (°C or °F), and Wi-Fi status.
The enclosure will be constructed from laser-cut parts provided by Ponoko and will be required to display the current readings from the electronics as well as provide access to any buttons and connectors. As the product will eventually become a commercial device, we will also have to consider its overall aesthetics, how easy it is to construct, and the physical dimensions.
The final step in the enclosure development process will be choosing a production method for large-scale production. Such methods include plastic injection moulding, off-the-shelf enclosures, and panel assembly.
KiCad Schematic
The schematic for this IoT device is incredibly simple and takes advantage of the ESP32 modular form. Power is provided via a micro USB connection that is regulated from 5V to 3.3V using an AMS1117 (U2), and this power circuitry also integrates a special diode (D2) to protect against any voltage spikes on the ESP32 side.
The display uses three 7 segment displays that use transistors (Q1, Q2, Q3), to control which display is on at any time. By cycling through each display at high speed we can connect all the displays to the same I/O pins and drive each display one at a time.
The connection between the ESP32 and USB is bridged using a CH340 USB to UART converter. This allows for a connected computer to program the ESP32 without needing to directly access the flash chip inside the ESP32 module. The remaining components (ESP32, DHT11, and BMP20) do not have any special requirements and are connected as needed.
The PCBWay's PCB
The PCB is where the design gets interesting as this will dictate the design of the enclosure. While the enclosure itself will influence the layout of the PCB, it is the PCB that will determine the dimensions and positioning of features in the enclosure.
The first PCB layout puts all the components on a single PCB with the interactive parts to the left and the computational parts to the right. While this design is highly practical, it is not aesthetically pleasing as the front panel of the design would be empty on one half.
Another potential design is to separate the computational parts from the interactive parts into two separate PCBs. Headers attached to the display portion would connect it to the computation board and allows for a much smaller device that removes the dead space on the front panel.
However, this design would significantly increase the thickness of the overall design. Furthermore, the use of an additional PCB increases the costs while also requiring a more complex enclosure design. If the 7 segment displays were replaced with SMD varieties, then components could be soldered onto both sides of the PCB but this is extraordinarily difficult to achieve.
Overall, we will be using the second design utilising multiple layers as it is a far neater design that uses a smaller enclosure (despite the increase in depth), and is partly modular allowing for changes to be made without needing to replace the entire design.
Choosing a material
To create our enclosure, we will be using Ponoko laser cutting services which means that producing a 3D model of the enclosure is somewhat pointless. This is because laser cutters only produce 2D parts which have a specific thickness. Therefore, we can design our enclosure as a series of various 2D parts that will fit together to create a 3D enclosure.
Before we decide how we will design our enclosure we first need to pick a suitable material. This is where environmental factors become critical as some environments (such as seawater and chemical production) can harm the enclosure. As our device is a simple environmental sensor that could be subjected to wide changes in temperature and humidity, we will stick with a plastic material such as acrylic.
Choosing our construction method
When it comes to creating a design there are multiple methods that we can choose which include a layered design, box method, and a pre-made enclosure with a laser-cut panel.
The layer method involves splicing an enclosure into various layers that are placed on top of each other (similar to 3D printing). However, each layer in our design can be thick (up to 10mm), and these layers can be stacked using guiding pins before being screwed together. While this method uses a lot of layers (as there needs to be a cavity for our PCB to fit), it can be the most aesthetically pleasing.
The box method is a more practical method that is very common in maker projects. Simply put, boxes can be exploded into their individual faces with a jigsaw-like pattern appended to their edges. These individual parts are then glued together so that the jigsaw teeth lock in place. While this is practical for large boxes, small enclosures can be tricky to put together. This method is also far from a practical commercial design meaning that a final prototype would have to use a different construction method.
The pre-made box method simply uses a commonly available project box or enclosure on the market while the faceplate is replaced with a laser-cut design. This design method is a quick option to create a product and can sometimes be the final product (as such enclosures are available in large quantities). However, such a design is far from professional and can make a product look underdeveloped.
Our design
Our design will use the layer method as we can create a compact design that looks similar to the final design. Another advantage of the layer design is that we can use different materials at different layers; in the case of this project, we could make the front panel in aluminium for a metallic face finish (similar to Apple products) while having a plastic backing.
As laser-cut designs are 2D, we can easily design our enclosure in KiCad’s PCB development tool as it is able to export SVG files (easy to upload to Ponoko’s website for instant quoting). Furthermore, we can import other PCBs into our design so we can easily match our enclosure design to our PCB design with ease.
Wrapping things up
With the various designs complete it is time to order our parts and get them manufactured. While we know that the circuit functions from earlier testing (not discussed here), the final design could be met with challenges that have not been currently foreseen (such as incorrect placement of parts or improper trace widths). Furthermore, our PCB may not fit our enclosure correctly, or the use of layers may prove to be too challenging.
This is why creating a prototype is essential as the only way to truly understand if a design will work or not is with a physical product. In the next article, we will look at ordering PCBs and PCB assembly from PCBWay, ordering laser-cut parts from Ponoko, and the challenges that will follow.
