Movement sensor

Hi friends, I need to detect if a person is unconscious (no movement) or not in a room to activate an alarm. What kind of Movement sensor I can use?

just a simple one e.g. http://www.ebay.nl/itm/PIR-Sensore-Infrarosso-Movimento-Pyroelectric-Infrared-Motion-Sensor-HC-SR501-/141129329467?pt=Componenti_elettronici_attivi&hash=item20dbf6ab3b&_uhb=1

Thanks robtillaart I'm not looking only for the presence but also for the mouvement, I mean if the person is moving or not, is it the right sensor?

These sensors are meant to switch on the light if you walk to your garage for example. Given the price I think they are worth to investigate.