The REV Robotics Touch Sensor (REV-31-1425) is a digital sensor that can be used as a button input or as a basic mechanical limit switch. The touch sensor is similar to a keyboard button, when the button is pressed the touch sensor notifies the Robot Controller and an action in the code is triggered. Sometimes this action may stop the motors or reset the encoder angle, depending on the use case.
Like all digital sensors, the Touch Sensor acts on a binary. When the button is not pressed, the LED light remains unlit and the value read by the Expansion Hub is 3.3V (high) and when the button is pressed the LED will light and the Expansion Hub will read 0V (Low