What is the pressure/force sensor used in the Beddit?

Question

The Beddit sleep-tracking device (Beddit and @indiegogo) has a pressure sensor that looks like a thin (~0.3mm?), flexible tape about 2ft long. The tape seems to be constructed of several layers internally. If bent and pressed flat it seems to form permanent wrinkles, but still functions about the same. It has a two-pin electrical connector at the end. The sensor is usually placed under a person lying on a bed (0.2-0.5 psi pressure) but can detect tiny, fast variations in pressure/force (best guess from device performance: << 0.001 psi with > 20Hz bandwidth).

Can anyone identify what type of sensor that is? How can I try to test it? Where can I get one like it?

Answer 1

It's likely a flex sensor, or force sensitive resistor. These are long devices that change resistance according to the amount of flexure (flex sensor) or force applied (force sensor) along the tape. If this is the case, you can measure the resistance, and should find that it changes as you bend or put pressure on the tape. Breathing and movements aren't very quick actions, so there's more than enough bandwidth available in such a sensor, and they are very inexpensive and fairly durable. Flex sensors are typically more stiff, and only respond in one bending direction, probably requiring two for this type of application (unless the instructions have a distinct "up" side for the sensor placement), so it's more likely a force sensitive resistor.

Here is a force sensitive sensor that closely matches your description:

Interlink model 408 FSR with 1/4-inch x 24-inch sensing region: