How do I know when filter coffee machine is finished by using any sensor?
Just two ideas, both non-invasive:
The machine moves water from the reservoir at the back to the coffeepot at the front of the machine, so its weight distribution will change while making coffee.
If you put a thin pressure pad (like this one, or something similar) under the whole machine at the back you should see the pressure at that point become smaller while coffee is being made.
If the effect isn't pronounced enough, you could make the whole machine tilt forward while making coffee by putting a narrow and thin strip of plastic under its centre from left to right so it can swivel forward and back just a little depending on the location of the bulk of the water.
If the pressure on the pressure pad at the back stops dropping, the coffee is ready. You can also detect when the coffee can is empty again this way, or maybe even how many cups of coffee are still available in the coffee can.
Or maybe you could detect with a contact microphone when the machine goes silent, assuming it makes sufficient noise when making coffee. Mine does.