I started the idea with imagining how the user would like to interact with the device and what would their experience be.
Based on a mental idea of how I would like to use it
1) I expect the device to work twice a day for a month, without needing to charge.
2) I expect to know the exact temperature my beverage is at.
3) I should be able to clean the device easily and with running water.
4) It should not be heavy at all, and should weigh roughly about a pencil.
5) It should have the form factor of a stirrer.
6) It should be able to adapt to every known kind of tea/coffee mug available around me.
Some of these were easy to meet( based on experience), but some were big question marks. Nevertheless, I started to order parts and put together a basic working circuit i could test and refine my code.
I initially thought of not putting a Li Ion Battery because of export restrictions and certifications I would require to go through. I planned my design around a CR2032 battery.
The battery ran for quite a few days before it drained and was rejected as the product size was starting to get cumbersome.
My initial prototype was also with a Red, Yellow and Green discrete LED tied to the I/O pins of the Attiny85.
I got better and better information about the behavior of the system, which brought confidence to go ahead and attempt the Low Power code for the Attiny85.