Build a Raspberry Pi Vehicle Interior Monitor – Overview

Posted by Graham Smith on June 20, 2017No Comments (click here to comment)

Over the past year or so I've been teaching myself whole new areas of learning based around the Raspberry Pi, including Linux, GPIO programming, basic electronics and Windows 10 IoT Core. I'm now at a point where I'm ready to build something that might be half useful, and I thought it might be helpful to someone if I blogged about my fledgling maker journey.

For my first project I'm going to build a Raspberry Pi Vehicle Interior Monitor—PiVIM. The idea is that on the odd occasion when we need to leave our dogs in the car for a few minutes, PiVIM will provide extra reassurance that all the ventilation and safety measures we've provided (windows partially open, tailgate open but secured with a Ventlock Tailgate Lock type device, reflective windscreen shade and so on) are actually working, through some sort of messaging to our mobile phones.

Important notice: The aim of PiVIM is only to provide extra reassurance on top of an already very cautious approach to reluctantly leaving dogs in the car for very short periods. Dogs die in hot cars!

With the sombre stuff out of the way, sure you can buy something but where's the fun in that? Making something from scratch offers an opportunity to learn a whole new set of skills, and in a new series of blog posts I'm planning to share my journey building PiVIM. In this first post I'm setting out the big picture—the features I hope to incorporate in to PiVIM and the developer tools I'll be using.

PiVIM Features

Here's a list of potential features that I'm considering for this project:

  • Temperature measurement. The key requirement for this project is to monitor the temperature of a vehicle's interior. A popular component for temperature measurement is the DS18B20. This comes as a small three-pin unit that looks like a transistor and also a waterproof version with the sensor embedded in a metal tube at the end of an attached wire. The waterproof version looks most useful for my project due to ruggedness and flexibility of being on the end of a wire.
  • Mobile connectivity. Since PiVIM will need to work in remote locations it will need a mobile internet connection. There's a cost to this of course, and I want to keep costs as low as possible. One of the problems with most mobile broadband plans is that they are based on a monthly data allowance and at the end of each monthly period any unused allowance is lost. Given that PiVIM might be used a lot in summer and very little in winter such a plan would likely be wasteful and uneconomic. Happily the Three network have a PAYG SIM where the data allowance lasts for as long as it isn't used. I'm planning to partner this SIM with either the ZTE MF730 3G USB dongle or the ZTE MF823 4G USB dongle, and both, if Google searches are anything to go by, should work with the Raspberry Pi.
  • Data access. Related to mobile connectivity is how to access the data that PiVIM generates. In addition to sending SMS alerts, the options that I'm considering are to store all data locally and make it accessible via a website running on the Pi, or to upload it to somewhere like Microsoft Azure and access it from there. Lots to research needed here, not least because although I have plenty of experience with Microsoft Azure right now I have no idea if it's possible to host a website on a Raspberry Pi that's accessible via a mobile broadband connection.
  • Battery powered. Although PiVIM could use a vehicle's 12V power supply via a USB adaptor the cabling would be messy and a dedicated battery feels more suitable. Tests with a RAVPower 22000mAh portable charger and a Raspberry Pi Model B with camera attached showed that the RAVPower could keep the Pi going for at least 36 hours (I stopped the test before the RAVPower was fully drained) so a unit like this feels like it will be a good choice. It would also be useful to have some power management system to monitor the battery's charge status.
  • Onboard display. I want to be able to see some basic information about PiVIM whilst it's running—mobile broadband signal strength, current temperature and so on. I've seen the Pimoroni Display-O-Tron HAT used for this purpose and was impressed, so that will probably be my starting point.
  • Power button. Raspberry Pis don't come with a power button and if left connected they will also gradually drain a battery even when powered down so I'll want some sort of solution to these problems.
  • Camera pictures. More of a nice to have rather than a necessity, but since the Raspberry Pi has a very handy camera module available as an accessory I might try and see if it's viable to access pictures over mobile broadband.
  • Robust case. The PiVIM internals will need to be well protected so some sort of robust case will be essential. It will need to be able to house the battery as well as the Pi, ZTE USB dongle and the Display-O-Tron. Current thinking is an electrical junction box such as the one here might be a good starting point, with the Display-O-Tron screwed to the exterior surface of the lid and connected to the Pi with something like the Pimoroni Mini Black HAT Hack3r.
  • Raspberry Pi model. I'll be prototyping on a Pi 3 Model B but might switch to a lower-powered board when it comes to building something that will be used out in the field.

Development Environment and Tools

I'll be starting off coding in Python, however a developer friend has very good things to say about developing with Kotlin for the Raspberry Pi so I'll probably try my hand at a Kotlin port once I have a Python version working.

In an ideal world I'd do all development directly on the Pi since there will be quite a lot of Python libraries that are talking directly to Pi hardware or to hardware attached to the Pi. In practice though I find that the development experience on the Pi doesn't give me what I want either in terms of performance or in the coding tools I want to use. Since I do a lot of work with Microsoft technologies my current development workstation is running Windows 10 and I use scp to push code out to the Pi which is running in headless mode on my local network. My configuration is as follows:

  • Windows 10 Pro with the Windows Subsystem for Linux (WSL) installed and a registry setting to ‘Open Bash window here‘.
  • I used to go to the trouble of giving my Pis fixed IP addresses so I could always be certain which one I was connecting to. I don't bother now and instead have Bonjour Print Services for Windows installed so that I can remote to a Pi using the hostname.local format. This works a treat in applications such as FileZilla and PuTTY. Unfortunately there is currently a bug in WSL which stops this from working. WSL is still in beta so hopefully this will be fixed soon.
  • I do find it's worth configuring SSH to use certificate authentication to avoid having to deal with passwords, and have the same certificate set up for both Windows 10 and WSL.
  • Python obviously needs to be installed—I just go for the latest version from the website here which also installs pip.
  • One of the issues with Python development is that if you don't do anything about it packages are installed globally. This creates problems if you need to create or edit Python code that needs a specific version of a package, or indeed Python itself. The solution to this is to use virtual environments courtesy of Virtualenv and (on Windows)  virtualenvwrapper-win. There's a great guide to configuring and using virtual environments on Windows here.
  • I'm using Git for version control and the Python version of PiVIM is on my GitHub site here.
  • My lightweight code editor of choice is Visual Studio Code. It's free and Python is fully supported with the help of Don Jayamanne's Python extension. The best way to start Visual Studio Code if you are using virtual environments is from the command line of a virtual environment using code . (make sure you don't miss off the period). Whilst you are at the command line make sure you install pylint (pip install pylint) in to your virtual environment and any other packages your code needs.
  • My heavyweight IDE of choice is Visual Studio. A free version is available and it's got a huge amount of support for Python via the Python tools. Whilst I don't use it on a daily basis for Python development it's great for remote debugging using the ptvsd package. Anyone who's used Visual Studio to develop .NET applications will love and appreciate the debugging experience and there are details on how to set up this awesomeness here.
  • I have FileZilla and PuTTY installed and have them configured to connect to my Raspberry Pi devices using SSH and certificate authentication. I have a bash script under version control on my Windows 10 workstation file system which I run from WSL (one of the handy things about WSL is that it can see the Windows 10 file system). The bash script uses scp to copy Python files to the Pi, after which I switch to PuTTY to run the code. A bit clunky but it works. (UPDATE: I've stopped using the bash script as it was too cumbersome. I now clone my code from GitHub to the Pi and and then in a PuTTY connection to the Pi—after having pushed code to GitHub—I run a command such as git pull && python3 module_to_run.py).

That's it for now! Watch out for my next post in this series where I'll be getting stuck in to the details.

Cheers—Graham