The purpose of this video is to demonstrate how to design and build two PIR motion sensor circuits that illuminate an LED when an object passes in front of the sensor.
Every object that has a temperature above absolute zero (-273.15°C or zero on the Kelvin scale) emits some heat energy in the form of electromagnetic radiation. The hotter the object, the more radiation it emits. This is invisible to the naked eye because it radiates at infrared wavelengths, which we can’t see, but it can be detected by purpose-built electronic devices. So, if an object passes in front of one of these devices, the device is able to identify that there has been a change in the radiation detected.
A Passive Infrared (PIR) sensor is able to detect movement that takes place within a certain range of the sensor (typically within a few meters).
A PIR sensor consists of two main parts: a Pyroelectrical sensor and a Fresnel lens, which is a special type of lens that focuses infrared signals onto the sensor.
The pyroelectrical sensor has two rectangular slots in it that allow infrared radiation to pass through. Behind each slot is an infrared sensor electrode; one electrode produces a positive output and one produces a negative output. The way that the electrodes are wired up is such that they cancel each other out. If one electrode detects more or less infrared radiation than the other, which is what happens when an object passes through the detection area, the output signal swings either high or low. When there is no movement, the same amount of infrared radiation passes through both slots and there is no output signal.