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January 11, 2012
about 3 years ago
The simplest way to think about it is you are “pulling” the output to ground, or 0 volts, so that will be the default reading. You use a large resister so that the effect is not very strong and can easily be overridden by the rest of the circuit. Also, using a large resister limits the amount of current being drawn / power being wasted.
I got one of these in an Arduino inventors kit. One of the projects uses it (as a voltage divider, reading the output from the middle pin) to control the color of an RGB LED. The sensor was not acting at all the way I thought it would when I was not touching it.
I realized that when nothing is touching the sensor, the voltage is at about 33% of the voltage range. Touch the sensor near the connections, and the voltage drops to nearly 0 or ground. Touch it near the top (away from the connections) and the output voltage rises to whatever the high pin is.
This makes it impossible to tell when the sensor is being touched and when it is not. I put a resister (10K) between the ground pin and the output pin, and now the output is exactly what I want. I can look for a voltage of 0 to see that it is not being touched, any value above zero means it is being pressed. The higher the voltage, the higher on the strip it is being pressed.
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