ESP8266/PIR
< ESP8266
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Passive Infrared Sensor (PIR sensor)
https://en.wikipedia.org/wiki/Passive_infrared_sensor
HC-SR501[edit]
This is a cheap "Human Sensor Module Pyroelectric Infrared" device that can detect motion, particularly people, with a range of around 20 feet.
Data Sheet https://www.mpja.com/download/31227sc.pdf
Modification to run off 3.3v, rather than 5v http://lowpowerlab.com/motionmote/#pirsensor
ESP8266 specific integration, with battery optimized considerations http://www.esp8266.com/viewtopic.php?f=11&t=4458
2760347[edit]
This is Radio Shack's DIY PIR Sensor available at most stores for 10 bucks. Works with 5v or 3.3v
http://www.radioshack.com/radioshack-passive-infrared-sensor/2760347.html