PIR is the acronym for Passive Infra Red. The term "passive indicates that the sensor does not actively take part in the process, meaning it does not itself emit the referred infra red signals, rather passively detects infrared radiations emanating from warm blooded animal in the vicinity.
The detected radiations are converted into an electrical charge proportional to the detected level of the radiation. This charge is then further enhanced by the built-in FET and fed to the output pin of the device which becomes applicable to an external circuit for further amplification and for triggering the alarm stages.
The image shows a typical PIR sensor pin out diagram. It's quite simple to understand the pinouts and one may easily configure them into a working circuit with the help of the following points:
As indicated in the following diagram, PIN#3 of the sensor should be connected to the ground or the negative rail of the supply.
Pin#1 which corresponds to the "drain" terminal of the dvice should be connected to the positive supply, which must be ideally a 5V DC.
And pin#2 which corresponds to the "source" lead of the sensor must be connected to ground via a 47K or 100K resistor. This pin also becomes the output pin out of the device and the detected infrared signal is carried forward to an amplifier from pin#2 of the sensor.
Making a Practical PIR Sensor Based Motion Detector Circuit
In the above section we learned the datasheet and the pinouts of a standard PIR sensor now lets' move on and study a simple application for the same:
The circuit diagram of a motion detector unit is shown above. A practical implementation of the explained pin-out details can be witnessed here.
In the presence of a human IR radiation, the sensor detects the radiations and instantly converts it into minute electrical pulses, enough to trigger the transistor into conduction, making its colector go low.
The IC 741 has been set up as a comparator where its pin#3 is assigned as the reference input while pin#2 as the sensing input.
The moment the collector of the transistor goes low, the potential at pin#2 of the 741 IC becomes lower than the potential at pin#3. This instantly makes the output of the IC high, triggering the relay driver stage consisting of the another BC547 transistor and a relay.
The relay activates and switches ON the connected alarm device.
The capacitor 100uF/25V makes sure that the relay remains ON even after the PIR is deactivated possibly due the exit of the radiation source.
The PIR device should be suitably enclosed inside a frensel lens cover so that its efficiency is adequately enhanced.