Please, I need some help since I need a kind of sensor able to measure preasure, for example, with a hand grip when you close the grip the sensor send a signal able to turn on a led, as more you close the grip the light will become brighter and as less you close the grip the light will become ligther; I am doing some kind of experiment from my high school and I need one sensor able to measure preasure in this way since I need to show as how far you can close the grip and show it with light; thanks a lot!!!..
I think the first question to ask is whether or not you really intend to measure pressure, as opposed to force.
It sounds like you actually want to measure force and that you already have a force-to-position converter (the spring in your hand grip), so all you really would need is a potentiometer (linear or rotary) to measure how much a person can close the grip. The only real challenge is then to install the potentiometer in some unobtrusive way.
You can make a force sensor that will respond to the grip of robot fingers from the black conductive foam used to protect integrated circuits from static electricity.
Take two small pieces of thick copper foil, or bits of circuit board (say 1 inch x 1 inch), with wires soldered to them. Place a piece of conductive foam in between to make a sandwich, with the copper in contact with the foam.
The foam will conduct electricity, more so depending on how much pressure is applied. If the sandwich has large enough area it will pass enough current to light an LED. The brightness will then depend on the force applied.