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Knock Sensor
Designed by Mark Little
 
The KY-031 is knock sensor that outputs a momentary digital output when it detects a knock or tap,

The device is configured as a normally open switch with a pull up resistor holding it normally high. When there is a knock or tap is detected, the switch activates, pulling the output low.
While this sensor can be used to monitor for any kind of shock, as the name implies, it is suitable for detecting knocks on the door, allowing the knock to trigger the door bell. This can be just what you need when you have a doorbell that extends around the house, but people still persist in knocking on the door.