A frequently touted promise of IoT is to make your life easier by making everyday objects smarter. This has led to a slew of connected devices targeted at the consumer.
I don’t know about you, but many of these products seem to be targeting a need that I didn’t realise I had, rather than being genuinely useful.
Instead of developing yet another solution in search of a problem, I decided to address a very real need in our household. Due to busy schedules, I often find myself in the supermarket on the way home from work, trying to remember what food needs buying. It seems that I am not the only one experiencing this situation. Compounding this problem is that my teenage sons can consume a LOT, but what they are consuming at any point in time is erratic and difficult to predict.
As an example, we recently went through a (relatively healthy) phase of consuming Greek yoghurt. To keep up with demand, we standardised on buying a 2kg (4.4lbs) tub – but only one tub at a time because of the space it occupies in the fridge. Unless I have recently eaten some, it becomes guesswork as to whether or not to replenish. And, in case you’re wondering, forget about getting my sons to proactively flag that anything is running out.
Time to hack together something to automatically give me that information, whenever and wherever I needed it. Yes, I could wait for a product (most likely at substantial cost) to be available like the smart mat demonstrated at CES 2016… but what would be the fun in that?
Fortunately, with a NodeMCU, some clever (at least I think so) Arduino code, the Adafruit IO cloud platform and a weight sensor, we have all the ingredients to build a device that will let us check the current yoghurt level on a smartphone at any time, and also send a text alert if supplies hit a critical level, all at a very low cost (excluding your time of course).
You can experience a live version of my yoghurt monitor here.
As you read on, keep in mind that this approach could be generalised to monitor any container. Also, the strategy would be to have multiple of these or similar devices. With many devices, it would be annoying to receive alerts every time something needs replenishing; instead, I imagine that they would update an automated shopping list, ready for when you are in the store.
I therefore came up with a simpler alternative – using an analog switch ic. I was pleased to discover that they are now available in extremely small form factors. Actually, my choice turned out to be rather too small – at a size of 1 mm x 2 mm, soldering directly to each pin (without the benefit of pcb mounting) proved more of a challenge than I was looking for.
I settled on the 
Whatever the cause of disruption, the MQTT architecture provides for the broker to store any important messages (i.e. those flagged with high qos or “quality of service”) to be later delivered to a client that had been previously subscribed, also with high qos, but is at that point in time disconnected.
