Passive Infrared Sensor (PIR) Part - 01: How It Works

What does a sensor do actually? The simple answer is, "They sense the environment around them". But why sensing is important? Actually, rather than sensing the surrounding, it is more important to detect the changes in the surroundings and then act accordingly. Like any other living being, a machine or robot needs sensors to take decisions. Think about Sophia, the humanoid, what do you think how she communicates with the audience? 

Now curious minds may say security installations also require sensors. Such a kind of sensor is Passive Infrared Sensor (PIR), resembling CCTV. This sensor detects the motion of an object on the basis of emitted Infrared (IR) rays by that object. We have a mini version of this sensor module namely HC-SR501 PIR Motion Sensor Module that uses a BISS0001 microchip onboard. It can cover about 120 degrees and 7 meters.

Structure:

First of all the question arises, why it is called 'Passive'? Active components are those which emit some signal or rays like Sonar sensor. But PIR receives IR from objects instead of sending it. Therefore it is called passive. A huge plastic cover appears at the first glance that covers the Pyroelectric sensor. Therefore the apparent parts are;

1	Fresnel Lens	Round	Array of lenses	Widens the range of the sensor
2	Output pins	Vcc	Input pin	4.5V~12V (Typically 5V)
		Dout	Digital Output	High (3.3V)		Motion detected
				Low (0V)		No motion
		GND	Ground	Connects to system ground
3	Protection Diode			Limits the input current to the voltage regulator
4	DTA	Adjusts the delay time range
5	STA	Adjusts the sensitivity of the sensor
6	3V DC Regulator	Stabilizes the input voltage
7	BISS001 Chip	Processes the signal
8	Retrigger Select	Repeatable Mode (H)			Dout will be high, once the object passes through
		Non-repeatable Mode (L)			Dout will be high as long as the object is within the range

Working Principle:

Each object emits some kind of IR. A moving object produces a variation of IR due to the heat generated by the friction of the object and air. The pyroelectric sensor has two slots in it. When the object passes through the first half, the state of it becomes greater than the second half and creates a positive trigger. Agan when the object goes away from the coverage area, similarly the second half creates a negative trigger. This triggers, after processing, appear in the Dout pin.

Circuit Implementation:

PIR sensor can only give the output as either High (3.3V) or Low (0V).  Therefore it can be used as a trigger to any security system. A simple implementation of the sensor can be an alarm circuit.

  

This is the circuit diagram of a PIR alarm system. Te shaded area represents the coverage area of the sensor and a BC547 NPN transistor has been used to amplify the alarm. The PIR sensor can be used with any kind of microcontroller like Arduino, Raspberry Pi, PIC, etc.

So, this was the working principle of the PIR sensor module. To know about the module circuitry and signal processing operation, visit Passive Infrared Sensor (PIR) Part-02: Inner Circuitry & Control 

Image source:

* Wikipedia

* indiaMART

* ozcott.com

* Wikimedia.com

* Tinkerdesk