We’re now going to learn how to detect motion and we will switch ON an LED when motion has been detected. Just as in the case of the previous circuit, we will use a sensor to detect motion and the output of this sensor will be connected to a transistor.
The simplest way to detect motion is by using something called a passive infrared or PIR sensor. A PIR sensor can detect changes in the amount of infrared radiation falling on it.
In the absence of motion, the PIR sensor forms “an image” of the surroundings.
When a person walks in front of the PIR sensor, there is a change in the amount of infrared radiation and the PIR can detect this change.
There are different types of PIR sensors and the one we will be using is called the AM312 sensor which gives us a digital output. It has three terminals – VCC, GND and SIGNAL and works on a maximum of 3.6V. The PIR sensor itself has a very narrow field of view which means that it cannot detect longer distances or greater angles. We use something called a Fresnel lens to increase this range and sensing angle or cone. BBox 1 contains 2 different lenses – the larger one giving you the longest range and field of view.
The circuit diagram for this is extremely simple. We use a current limiting resistor between the PIR sensor and transistor. Please ensure that the PIR sensor is connected the right way around and you can use the tab that’s present on the casing to orient it correctly.
Let’s use the breadboard layout to build the circuit.
The LED switches ON when motion is detected and stays ON for a few seconds. It then switches OFF and stays OFF for as long as no motion is detected.
We can use the PIR sensor to build some very interesting circuits including a motion activated night light and intruder alarm. Let’s move on to the next project.