The first thing to do on the electronics side is too get a proof of concept working. This is important to confirm that the idea is viable and figure out if any parts will drive the enclosure design. I can do this based on general ideas of what will work but in this case it will be best to layout what I want to accomplish with this project. This will help you understand why I chose the parts that I did and give me a checklist to reference during the development process.
- This project will be a failure if other people do not replicate this project. Because of this, I must try and make sure parts are available, easy to assemble, or made available by myself. The software must be accessible and everything well documented.
- What is affordable differers from person to person, but the spirit is this goal is to make sure BOM cost is a factor in the design from the beginning. I would be happy if the first design can be made for $100 or under, in quantities of one.
- This device will be be outside and subjected to a variety of weather. In my city temperatures range from -20C to 40C with a decent amount of rain, sleet, and snow. Ideally the goal is to work from -40C to 50C ambient temperatures and have a IP67 rating. Starting off I am aiming at 0-30C and IP65 rating.
- Low Power
- Since the first design will only have a battery, a person will be responsible for charging the device will it is still attached to a pole. This person will likely be using that bus station to get to work so the minimum interval would be 3-4 days. This would allow them to charge when they go to work in the morning and not have to worry about it during a typical two day work weekend. I would be happy will a week long battery life, but obviously I will try for more if it doesn't add significant cost. Future, large scale designs, will need to be self sustainable, but I will leave that requirement to another version.
I want to use cellular data on this project, specifically Hologram, for a couple reasons. The biggest reason is that I don't think I can rely on WIFI or BLE. I like very close to my local bus stop but I still can't see my home WIFI network. There is a coffee shop nearby, but not everyone is going to have that convenience. With cellular I know that most people will at least have signal.
Hologram is going to be the choice to start mainly because I don't know enough about how Particle differs, they have a free tier, their headquarters is in Chicago, and I know people who work there.
I picked the Adafruit Feather 32u4 Fona because Adafruit has a large reach to the community I am targeting. This could be a custom PCB, but that would take time to develop and make things more complicated to start. I want to test a system right now and the more know working components I can get the faster I can get to testing system level functions. This was also the cheaper option than some of the other modules on the compatibility this here.
E-ink offers two advantages in this application. It is easily readable in direct sunlight and only requires power when changing states. They have been being used more often in hobby projects because the cost has come done and they have Arduino library support. Waveshare seems to be the de-facto brand at this time and should have enough documentation to get everything glued together. I bought a couple of the small sizes and the three color options so I can see which works best.
I am using the Atmel 32u4 because that's what the Fona board uses and it is Arduino compatible. If/when I make a custom PCB, I will need to think more about optimizing for cost, power, price, etc. but this will work for now.
I want to work from a battery early in the project but I won't know what size I need until I do power calculations. Once there is functioning code, I can measure power for different events and make a spreadsheet to calculate scenarios based on a variety of refresh rates.