What remains is then filtered and regulated to 9VDC. [brmarcum] is using a Parallax PIR sensor which requires 4.5V. Therefore, the 9V signal is then lowered once more using a voltage divider circuit.

As an example, [Circuit Valley] has an IKEA Vallhorn motion sensor for a teardown, it’s as you might expect, a passive infrared sensor (PIR) sensor coupled with a Zigbee interface.

This motion detector alarm can detect a moving person from a distance of 1 meter. It uses a dual IR Transmitter-Receiver module HOA1405. When the sensor detects the reflected IR rays, alarm will sound ...

The sensor does have some limits. It can only detect motion up to about 16.5 feet away. And while Philips will give you the option of reducing that — that is, only turning on lights when motion ...