Used in the construction of mobile robots for collision detection mechanisms or moving joints, but also in other various physics experiments and applications and even in the gaming industry (a good example of this is the Power Glove, released by Nintendo in the late ’80s), flex sensors are basically devices that change their resistance when bent. Some flex sensors also react to pressure changes and can be used as pressure sensors. Some of them can be bent in either direction (bi-directional flex sensors).
Such is the case with this one, a small home-made bi-directional pressure sensitive flex sensor 3/8″ wide and 5″ long. The sensor presented here is built using copper foil laminate, acetate, heat shrink tubing and some resistive material (you can find precise measurements in the link). The copper foil actually has copper only on one side and plastic material on the other. You need to cut the foil into two pieces of the same size and then solder wires to one end of each strip of copper in different end corners.
The resistive material mentioned earlier is a strip of black plastic poly bag that is commonly used for storing electrical components (it’s made of carbon-loaded polyethylene). This resistive material is placed between the two copper strips with the copper facing the material and the wires on the same side. Then the acetate is added to the sandwich, so the sensor returns to its original position after it’s bent. All of this gets inserted into the heat shrink tubing.
The last part is testing and requires a multimeter to measure the resistance. The nominal resistance for this sensor is around 20K Ohms and this values decreases as the sensor is bent or pressed. An easy way to make a flex sensor using procurable materials, this flex sensors’ size can be adjusted to fit your needs.