ESP8266/PIR

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Passive Infrared Sensor (PIR sensor)

https://en.wikipedia.org/wiki/Passive_infrared_sensor

ESP8266#IoT_X'ample

HC-SR501

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

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