Step 1

We're going to use something called a 'Passive InfraRed' (PIR) motion sensor. You've probably seen these before: they're often used in security alarm systems. Sensors placed in the corners of rooms are usually PIR sensors. 

Almost all objects give off heat energy, called infrared radiation. Infrared is not visible to humans, but it can be detected by the electronics inside one of these sensors.

Step 2

Using a search engine, find two examples of products that use PIR sensors.

Step 3

The sensor is called 'passive' because it doesn't send out any signals in order to detect movement. It adjusts itself to the infrared already in the room and then watches for any changes. Any object moving through the room will disturb the infrared radiation, and will cause a change to be noticed by the PIR sensor.

Using a search engine, investigate a disadvantage of using a PIR sensor (hint: they sometimes have problems with objects that have similar temperatures).