a single sensor mounted on a bed support beam
As part of a sleep phase monitoring alarm clock, i was looking at different ways to track my movement during the night. The sensor i finally settled on, was a homebuilt pressure sensor made from aluminium / tin foil, an anti-static bag and foam rubber. There are similiar sensors already described on various pages. However these are either rigid (using copper board) or use conductive fabric, which can be hard to find. The nice thing about this sensor is, that it is rather flexible, thin and has a wide range of resistance. Distadvantages are the large drift over time and variance between different sensors.
Theory
The sensor itself consists of one layer of anti-static material sandwiched between two layers of tin foil. When presure is applied, the contact area between the tin foil and the anti-static material increases and the resistance between the two tin foils decreases. The change in resistance is roughly related to the pressure exerted. A layer off foam rubber is added on each side, to protect the tin foil.
from left to right: black foam rubber, tin foil, black anti static material, tin foil, white foam rubber
Construction
The tin foil is glued on to the foam rubber sides. A small tab is left over on each foil, to provide a contact point for the electrical connection. The anti-static material is cut to a square large enough to cover all of the tin foil.
the different sensor layers
As far as my experience goes, only the black anti-static bags work, pink, silver or clear bags don’t work. The anti static material should have a resistance of a few kiloohms at any two points (close) on its surface.
Tin foil can be difficult to connect to a circuit, since it is rather fragile and won’t solder very well. To attach wiring to the sensors i use snap fasteners. These have a ring of sharp teeth on one side and a hollowed rim on the other. The tin foil get pressed between both sides and since the teeth penetrate the foil, the connection is decent enough.
the top side of the sensor with the two connector leads
on the bottom side of the sensor, there is only one connector visible
The 5-layer sandwich can than be closed by either sewing or stapling the top and bottom foam rubber layers together.
Practical application
The sensor has a resistance between ~20 kOhm unactivated and 1 kOhm under pressure. To test the usability I arranged 6 sensors on the bed base below the mattress and connected them to an arduino via a simple voltage divider.
the six sensors in the base of my bed
While the absolute results from the sensors are mostly useless, the changes over time (especially the second order derivative) seem usefull to identify sleep phases.
values of the six sensors over the course of one night
