The Blue Peter vario. .
(Varios at the time were about a weeks wages! ).
Take a container about a pint and wrap it in tinfoil for insulation.
Stick a bit of white card on the lid with a pin hole in it and in the lid. Stamp some very fine parallel lines onto this card. I made a rubber stamp from (I think) an old hot water bottle.
Get some cling film type stuff and stamp the exact same very fine parallel lines onto this and make a tiny tiny pinhole. Overlay the film onto the card so the parallel lines line up exactly.
This thin diaphragm detects tiny increases in air pressure between the inside and outside of the pot and the parallel lines magnify this movement using something called the Moiré effect.
This only detected if you were going up or not going up and it was to make my fortune.
Instead when I showed it to (Snowy I think or one of the Suffolk boys) they laughed out loud so I hid it away.