I found the Motorola design reference manual on Passive Infrared Unit it's a PDF file.
Motorola Design Manual
I hopes this helps you.
As several people have already said there PIR, ultrasound, radar, and laser ranging, also there vision systems.
The PIR detects motion through a sensor window like you walking in front of a window and someone sees you go by the person is the sensor.
The ultrasound units, radar and laser ranging all emit a signal to be reflected back to the unit. These units all have there advantages and disadvantages based on there means.
And visions units can be visible light, IR, and other means. They usual use some method of computers but not always. It could look for a change from light to dark,
the visual field could be broken up in to grids and the computer monitor the sections or even the pixels for changes.
This is just a general break down in some of the methods used.
I hopes this will lead you in the direction to the info you want.
Also you can Google your search with "diagram", "schematics", in it to help find drawings to circuits examples.
"Persistence prevails, like a stream that is temporarily blocked by boulders and then collects force enough to overflow onward." -- Vernon Howard