There are many sensors that go into devices, such as temperature sensors, pressure sensors or even hemoglobin oxygenation sensors. Many of the sensors are made by engineers to be very precise and also very linear in their response. However, they are expensive and most hobbyist look for alternatives to the sensors. Sometimes, it is possible to make a sensor by common materials that would not usually be thought of as sensor material. So consider the flex sensor. It is sold by many companies and one example is by a company called Sparkfun and as sensors go it is relatively cheap. A flex sensor changes the resistance in response to a change in shape or a change in bend.
However, many people have created video’s to show how it is possible to make a really cheap flex sensor. These home-made sensors can be made for pennies and they are relatively stable. They may not be perfectly linear but if all that you are doing is to prove a concept or make a prototype for yourself then this would work perfectly well.
Take a look at the video of some home-made flex sensors.
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Or follow the link below to create another type of flex sensor.