Pressure

Small aneroid barometers - for hanging on a wall - can be picked up cheaply in a jumble sale, car-boot sale or junk shop. The mechanism inside the barometer will remove easily.

Here is an example:
 

Quality of the mechanism will vary from good to poor.	Remove the dodgy  mechanical bits ...	... pointer, spring, pivot, chain ...
... to leave just the pressure- operated bellows and its  mounting plate.	Fasten the mechanism to a circuit-board on M3-size pillars.	Solder a small, strong, magnet  to the arm.

The magnet can be a piece from a floppy-disc-drive motor, glued on a shaped paper-clip.
A floppy-drive magnet will break apart easily with a hammer, but cover it first with a cloth to trap flying particles.

       Completed PIC-Barometer ...

... and it doesn't need tapping.

The tiny movements of the magnet change the inductance of a coil which varies the frequency of an oscillator. The barometer is very very sensitive !  It will easily detect a door opening, as well as a thunderstorm. With re-arrangement, it will also act as a seismometer.

Two consecutive 12-hour Pressure charts logged by a MessagePad 2100 in landscape mode.
Plot 1 starts at 8:52am and ends at 8:52pm:

Plot 2 starts at 8:52pm and ends at 8:51am:

The prototype barometer had not been calibrated so the Y-axis units are just a measure of frequency.
Range of the Y-axis is barely 10 milliBars. If plotted on a typical Barometric scale, the line would be almost horizontal.

More details to follow ...