A cheap way to flex sensor. It will be very useful for your projects
Step 1: Things Needed :
LDR(light dependant resistor)
Step 2: Join It
Fix one of the tube with LDR and another end by LED.
Supply power to LED.
when you bend the tube the resistance changes at LDR with respect to luminance of LED.
Step 3: Interfacing to Arduino
It gives voltage as output thus it is connected to analog pin.