This article is about a small sensor node with a decorative case. It is based on the Raspberry Pi Zero W board with a custom sensor shield on top.
I publish all hardware files for a simple version of the sensor, so you should be able to build this kind of sensor nodes and use it to monitor anything you like. You can also extend/modify the design easily with additional sensors. Nevertheless, the case lid design is based around the Plantower PMSA003 particle sensor. It has all required air vents for this use.
The term “node” is used, because the idea of this sensor is to use a large number of these nodes in a network to monitor location and time based sensor data.
I cannot publish any software at this point, until I have a simplified and redacted version which I can publish under an open source license. Yet, accessing the sensors of this node is dead simple and can be easily done.
The Required Hardware
The simplified design is assemble using the following components:
Raspberry Pi Zero W with soldered headers.
Minimalistic Custom Shield (see description below)
Four short M2.5 screws and nuts to fasten the Raspberry Pi in the case.
Optional four M2.5 10mm long spacer to also fasten the shield.
One or two short M2 screw to fasten the Plantower sensor to the shield.
USB Cable and power supply to power the node.
The Raspberry Pi Zero W
I use the Raspberry Pi Zero W because of the very compact size and computing power. Each node can prepare the sensor data, which takes a lot of load from the central processing unit. Also, using a platform like the Raspberry Pi easily allows to run a whole web server on each node – so one can query the sensor data of each node independently. This makes testing a breeze.
For my current project I searched for a good boost power converter which is able to deliver continuous 400mA power for various sensors.
There are an endless number of good boost converters around, but not many can be hand soldered to a board. I would really like to see some like the TPS61092 with SOIC or similar packages. The biggest problem seems to be the heat transport, why most chips have to be mounted flat on the board.
Before using the chip in my project, I created a small test board. Using this board I can test various things. First I liked to test the performance under load. Next I tested if the chip can be hand soldered and finally I tested the final board layout I will use in my project.
The performance of this chip is really good, producing a very stable output. I designed everything for a load up to 2A with all suggested components from the specification. There will never be a higher load than 0.5A, so I probably could use a smaller coil for the final project.
Running under 0.5A load from 3.3V for over half an hour, the chip stays quite cold. Even in my case, where the chip bottom is not directly soldered to the board, it seems to be able to transfer the heat into the board. This is nicely visible in the thermal image of the board.
The board was produced by OSH Park in a good quality. If you like to experiment with this chip, you can order this board very cheap at OSH Park using the following link.
I designed a very simple power converter for the snow flake decoration. The snow flake boards require 3.3V, a voltage which is not commonly available in a household. Therefore the power converter takes 5V USB power and convert it into the required 3.3V.
There is no fancy circuit inside of this box, lazily I simple used a Traco Power TSR2-2433 DC/DC power converter. It is a single component containing everything required. No external components like capacitors or resistors needed.
Today I show you a little bit longer video about the assembly of the component side of the snow flake panel.
Compared with the very small 0402 LEDs (1mm x 0.5mm) on the front side, the component side contains just regular 0805 resistors and capacitors.
The first part of the video shows how I apply the solder paste using a metal stencil. I use a system called “eC-stencil-fix” from Eurocircuits. This is basically a set of plates with metal knobs to perfectly align the board with the stencil. With a tick in a checkbox, they produce panels which perfectly work using this stencil fix system, which is really convenient.
Today prototype boards from Eurocircuits arrived. The quality of this boards is outstanding. I do not know any other board house which delivers this incredible quality of boards. They really thrive for perfection.
Recently I finished a small cat feeding device. Meanwhile I added some minor missing parts, like an USB port which can be used to charge the device. I also added a larger battery to keep the device running for months without charging.
I finished a very simple and quick project recently. It is a programmable cat feeder device. The device is just made for one single portion, but it uses this slide which produces the distinctive sound if the pellets hit the bowl. It is no replacement for the usual feeding of the cat, but some flexible addition to give food out of the regular times.
The whole build was done in roughly four days, using a very quick prototyping and build method.
Design and Build Process
First I had to make a decision about the final size of the device. I used Autodesk Fusion360 to create a model of the device and try to fit all required parts into the device.
This is one of the last designs I made until I had the right size. I used a different mechanism as shown here, the important part was to get the right size.
Next I bought plywood in the right sizes for the outer shell of the case. I also got some additional panels fitting inside of the shell for the internal structure.
I created the shell with four sides, left the top and front side open. To work quickly I fastened the panels with hot glue in place. This has the benefit that you can remove panels using a hot air gun in the prototyping stage.
There was a long delay with no update about my projects. The reason for this is, I focused on other non electronic related projects. I promised to publish all files and information about the plant watering sensor project in August, and here are all the files.
First some Important Notes
Even with these files, it is a very advanced project to do. To produce the PCB, you need a board house which is used to precise and high quality PCBs, like Eurocircuits. Also all pads are prepared to reflow soldering, and not hand soldering. It may be possible to solder the resistors in place using a regular solder iron, but it will be hard to do.
This is the sixth part of the meta-tutorial, where I talk about designing a cheap plant watering sensor. If you did not already read the first, second, third, fourth and fifth part please do it now. These parts contain a lot information which lead to this point of the tutorial.
The fifth part ended with step 24, where I talked about calculating the total bill of materials. This part will focus on preproduction of a small batch of sensors to solve some final details.
Just note, I obviously do not follow these steps in a perfect sequential way. Often I start with some tasks earlier and things are running in parallel. There are various dependencies and it would make no sense to wait with some task just to follow a strict sequence. 🙂
If you follow my blog you may already read some details about ongoing tasks. I will just briefly talk about them in this article. You will find more details in the other blog posts.
Step 25: Build an Alpha Series
Everything looks very promising, so its time to build a small batch of the final devices to see if they work as expected. This is also a test to see how a larger number of these devices can be produced and what kind of tools are needed for this task.
Order the Components
First I order the components. This is very important, because the availability of electronic components changes all the time. It is nice to have all required components, so you can order the boards with the correct footprints. If you order the boards first and are unlucky, an important component is suddenly unavailable and you have lots of boards with wrong component footprints on it.