Although many people have created their own touch sensor with aluminium foil, doorknobs and so on - they can only detect touch/no-touch situations. However by using a new system based on swept-frequency capacitive sensing - you can not only detect a touch, but the strength of touch. Using an Arduino that generates a sweeping frequency while measuring the sensor, values are returned that relate to the strength of the touch. See the video below for an explanation and demonstration:
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