At some point I though about building the smallest PCB for a sensor node that I could. Hence, the ZEN was born. The PCB is small enough to fit on a holder of 2 AA batteries.
I have only build a few of these. Here is one reading the soil moisture sensor on basil.
Here is another one chilling in the fridge, reading a temperature sensor.
And here is one replacing the alarm on a door sensor, so it becomes the door sensor. I thought these would look better, but the quality of the casing leaves a lot to be desired, hence I will not use them anymore and will probably 3D print a case on my new 3D printer.
Is it useful?
So far the only case where the small size mattered is for the door alarm, where I wanted a PCB to fit in the original device, but there is not need to do that. Generally, I prefer the normal type where the PCB is under the battery more. There is more space for connections and the PCB design is faster.
Now, I never found an application where I needed a very very small sensor, something that will be powered by a coin cell around the house…