Supplies Needed

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PIR Motion Sensor

Arduino and USB Cord

LED 

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What is this sensor? It is a PIR Motion Sensor. 

PIR =Pyroelectric IR (infrared). Pyroelectrics make voltage when heated or cooled. IR light is really good at heating. It detects if incoming IR light, at the same wavelengths body temperature emits, is rapidly changing .

Yes, you emit light, even in the dark! This sensor detects changes in that light. Here is a picture from an IR camera. 

The white dome is a lens to focus light coming from all frontal directions to a central IR pyroelectric crystal. 

IR Light penetrates thin plastics 

Person in a rain coat IR camera image at night. 

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The Code and Wiring

The sensor has three pins. Here is how to connect them for this code. 
1) VCC (+)  goes to the 5V on the Arduino 
2) Out (signal line)  goes to Arduino pin 2
3) GND (ground or -)  goes to an Arduino GND

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Code with comments explaining each part. 

Save your code, verify it, and upload to the board.  

Test if it is working. Set the sensor down and don't move. Try moving by it and see if your light comes on. Also, open up the Serial Monitor and see if the "motion detected" prints out ever time you move. 

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How to Adjust the Sensor's Sensitivity. 

There are variable resistors (knobs) under it. 
A mini screwdriver to adjust these. 
The right knob adjusts the detectable distance. 

The left one changes how long the sensors state stays HIGH after it senses motion. ​

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Image sources: ​
https://nitrocdn.com/FamnJWKPilluExxZNRhJjJibvSMjKfnK/assets/static/optimized/rev-7714f53/wp-content/uploads/2017/02/26192329/Best-Thermal-Imaging-IR-Cameras-for-Ghost-Hunting.jpg

https://media.wired.com/photos/5932644eb8eb31692072f207/master/w_2560%2Cc_limit/irimage1.jpg