To sense the pressure i chose a small sensor:

It has a range from 0-200g and a 8MR to 2kR resistance depending on the pressure.

This datasheet here said it hat about 2.9kR when under 200g pressure.

So i came up with a quit circuit based on a 741 op amp i had still around from another project:

The circuit uses the 741 as comperator. R2 and R3 give me a reference voltage, R5 is sized it a way that together with R4 the voltage on in 3 will be higher than the reference voltage when more than 200g are on the sensor.

Its a bit chaotic on the breadboard but i am happy it works, heres a first test:

The LM741 isnt a very good choise for the long run tho.
It needs at least 4.5V to work right....others make doe with 1.8V.

It also isnt very good in therms of energy saving.
If i realy wana make this device be powered with a supercap and not a battery i will need something like the MAX480CPA....