PIR Motion Sensor Tutorial




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Pyroelectric ("Passive") InfraRed sensors:

'''What is a PIR sensor?'''

PIR sensors allow you to sense motion, almost always used to detect whether a human has moved in or out of the sensors range. They are small, inexpensive, low-power, easy to use and don't wear out. For that reason they are commonly found in appliances and gadgets used in homes or businesses. They are often referred to as PIR, "Passive Infrared", "Pyroelectric", or "IR motion" sensors.

PIRs are basically made of a pyroelectric sensor (which you can see above as the round metal can with a rectangular crystal in the center), which can detect levels of infrared radiation. Everything emits some low level radiation, and the hotter something is, the more radiation is emitted. The sensor in a motion detector is actually split in two halves. The reason for that is that we are looking to detect motion (change) not average IR levels. The two halves are wired up so that they cancel each other out. If one half sees more or less IR radiation than the other, the output will swing high or low.

Along with the pyroelectic sensor is a bunch of supporting circuitry, resistors and capacitors. It seems that most small hobbyist sensors use the BISS0001 ("Micro Power PIR Motion Detector IC"), undoubtedly a very inexpensive chip. This chip takes the output of the sensor and does some minor processing on it to emit a digital output pulse from the analog sensor.

For many basic projects or products that need to detect when a person has left or entered the area, or has approached, PIR sensors are great. They are low power and low cost, pretty rugged, have a wide lens range, and are easy to interface with. Note that PIRs won't tell you how many people are around or how close they are to the sensor, the lens is often fixed to a certain sweep and distance (although it can be hacked somewhere) and they are also sometimes set off by house pets. Experimentation is key!

Some basic stats

These stats are for the PIR sensor in the Adafruit shop which is very much like the Parallax one. Nearly all PIRs will have slightly different specifications, although they all pretty much work the same. If there's a datasheet, you'll want to refer to it

More links!

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Step 1: How Does It Work?

PIR sensors are more complicated than many of the other sensors explained in these tutorials (like photocells,FSRs and tilt switches) because there are multiple variables that affect the sensors input and output. To begin explaining how a basic sensor works, we'll use the rather nice diagram below (if anyone knows where it originates plz let me know).

The PIR sensor itself has two slots in it, each slot is made of a special material that is sensitive to IR. The lens used here is not really doing much and so we see that the two slots can 'see' out past some distance (basically the sensitivity of the sensor). When the sensor is idle, both slots detect the same amount of IR, the ambient amount radiated from the room or walls or outdoors. When a warm body like a human or animal passes by, it first intercepts one half of the PIR sensor, which causes a positive differential change between the two halves. When the warm body leaves the sensing area, the reverse happens, whereby the sensor generates a negative differential change. These change pulses are what is detected.

The PIR sensor itself

The IR sensor itself is housed in a hermetically sealed metal can to improve noise/temperature/humidity immunity. There is a window made of IR-transmissive material (typically coated silicon since that is very easy to come by) that protects the sensing element. Behind the window are the two balanced sensors.

Check out the images for more details:

Step 2: Lenses

PIR sensors are rather generic and for the most part vary only in price and sensitivity. Most of the real magic happens with the optics. This is a pretty good idea for manufacturing: the PIR sensor and circuitry is fixed and costs a few dollars. The lens costs only a few cents and can change the breadth, range, sensing pattern, very easily.

In the diagram above, the lens is just a piece of plastic, but that means that the detection area is just two rectangles. Usually we'd like to have a detection area that is much larger. To do that, we use a simple lens such as those found in a camera: they condenses a large area (such as a landscape) into a small one (on film or a CCD sensor). For reasons that will be apparent soon, we would like to make the PIR lenses small and thin and moldable from cheap plastic, even though it may add distortion. For this reason the sensors are actually Fresnel lenses (see image below).

OK, so now we have a much larger range. However, remember that we actually have two sensors, and more importantly we dont want two really big sensing-area rectangles, but rather a scattering of multiple small areas. So what we do is split up the lens into multiple section, each section of which is a fresnel lens.

The different faceting and sub-lenses create a range of detection areas, interleaved with each other. That's why the lens centers in the facets above are 'inconsistent' - every other one points to a different half of the PIR sensing element

Step 3: Connecting to Your PIR

Most PIR modules have a 3-pin connection at the side or bottom. The pinout may vary between modules so triple-check the pinout! It's often silkscreened on right next to the connection. One pin will be ground, another will be signal and the final one will be power. Power is usually 3-5VDC input but may be as high as 12V. Sometimes larger modules don't have direct output and instead just operate a relay in which case there is ground, power and the two switch connections.

The output of some relays may be 'open collector' - that means it requires a pullup resistor. If you're not getting a variable output be sure to try attaching a 10K pullup between the signal and power pins.

An easy way of prototyping with PIR sensors is to connect it to a breadboard since the connection port is 0.1" spacing. Some PIRs come with header on them already, the ones from Adafruit don't as usually the header is useless to plug into a breadboard.

By soldering in 0.1" right angle header, a PIR is easily installed into a breadboard!

Step 4: Testing Your PIR

Once you have your PIR wired up its a good idea to do a simple test to verify that it works the way you expect. This test is also good for range testing. Simply connect 3-4 alkaline batteries (make sure you have more than 3.5VDC out but less than 6V by checking with your multimeter!) and connect ground to the - pin on your PIR. Power goes to the + pin. Then connect a basic red LED (red LEDs have lower forward voltages than green or blue so they work better with only the 3.3v output) and a 220 ohm resistor (any value from 100 ohm to 1.0K ohm will do fine) to the out pin as shown. Of course, the LED and resistor can swap locations as long as the LED is oriented connection and connects between out and ground

Now when the PIR detects motion, the output pin will go "high" to 3.3V and light up the LED!

Once you have the breadboard wired up, insert batteries and wait 30-60 seconds for the PIR to 'stabilize'. During that time the LED may blink a little. Wait until the LED is off and then move around in front of it, waving a hand, etc, to see the LED light up!

Step 5: Retriggering

Once you have the LED blinking, look on the back of the PIR sensor and make sure that the jumper is placed in the L position as shown below.

Now set up the testing board again. You may notice that when connecting up the PIR sensor as above, the LED does not stay on when moving in front of it but actually turns on and off every second or so. That is called "non-retriggering".

Now change the jumper so that it is in the H position. If you set up the test, you will notice that now the LED does stay on the entire time that something is moving. That is called "retriggering"

For most applications, "retriggering" (jumper in H position) mode is a little nicer. If you need to connect the sensor to something edge-triggered, you'll want to set it to "non-retriggering" (jumper in L position).

Step 6: Changing Pulse Time and Timeout Length

There are two 'timeouts' associated with the PIR sensor. One is the "Tx" timeout: how long the LED is lit after it detects movement. The second is the "Ti" timeout which is how long the LED is guaranteed to be off when there is no movement. These are not easily changed but if you're handy with a soldering iton it is within reason.

First, lets take a look at the BISS datasheet again (see image below)

Determining R10 and R9 isnt too tough. Unfortunately this PIR sensor below is mislabeled (it looks like they swapped R9 R17). You can trace the pins by looking at the BISS001 datasheet and figuring out what pins they are - R10 connects to pin 3 and R9 connects to pin 7. the capacitors are a little tougher to determine, but you can 'reverse engineer' them from timing the sensor and solving!

For the sensor in the Adafruit shop:

Tx is = 24576 * R10 * C6 = ~1.2 seconds
R10 = 4.7K and C6 = 10nF


Ti = 24 * R9 * C7 = ~1.2 seconds
R9 = 470K and C7 = 0.1uF

You can change the timing by swapping different resistors or capacitors. For a nice tutorial on this, see Keith's PIR hacking page

Step 7: Project Examples

A USB-powered singing and blinking Mario mushroom (there's a video on the site!)

Rain Umbrellas

Testing a PIR sensor for interfacing to Max/MSP for an interactive garden

A home-made security system using PIR sensors (which is built into a Start Trek panel!)

PIR sensor + Arduino + Servo = automatic cat door!

A PIR-based remote camera trigger (also by Lucky Larry!)

Step 8: Reading PIR Sensors

Connecting PIR sensors to a microcontroller is really simple. The PIR acts as a digital output so all you need to do is listen for the pin to flip high (detected) or low (not detected).

Its likely that you'll want reriggering, so be sure to put the jumper in the H position!

Power the PIR with 5V and connect ground to ground. Then connect the output to a digital pin. In this example we'll use pin 2.

The code is very simple, and is basically just keeps track of whether the input to pin 2 is high or low. It also tracks the state of the pin, so that it prints out a message when motion has started and stopped:

/* PIR sensor tester*/

int ledPin = 13; // choose the pin for the LED
int inputPin = 2; // choose the input pin (for PIR sensor)
int pirState = LOW; // we start, assuming no motion detected
int val = 0; // variable for reading the pin status

void setup() {
pinMode(ledPin, OUTPUT); // declare LED as output
pinMode(inputPin, INPUT); // declare sensor as input


void loop(){
val = digitalRead(inputPin); // read input value
if (val == HIGH) { // check if the input is HIGH
digitalWrite(ledPin, HIGH); // turn LED ON
if (pirState == LOW) {
// we have just turned on
Serial.println("Motion detected!");
// We only want to print on the output change, not state
pirState = HIGH;
} else {
digitalWrite(ledPin, LOW); // turn LED OFF
if (pirState == HIGH){
// we have just turned of
Serial.println("Motion ended!");
// We only want to print on the output change, not state
pirState = LOW;

Don't forget that there are some times when you don't need a microcontroller. A PIR sensor can be connected to a relay (perhaps with a transistor buffer) without a micro!



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82 Discussions


Question 11 months ago

i have a problem that my pir sensor always generate HIGH output it is not detect any motion how to solve this problem please help me......

2 answers

Answer 20 days ago

yeah,I had a same problem. its detected only for motion. we have establishing for the PIR sensor projects for inplant training students.


Answer 10 months ago

hi, thought this might help. I do had the same problem. All posts and videos says the same story and sketches with slightly different coding. Nothing helped. So I hooked up the PIR to an analog input. In my case A0. The LED on any pin works.

Int pir = A0

Int led = 13

Int t = 5000

Void setup()

Pinmode(13, OUTPUT)

Pinmode(A0, INPUT)

Void loop()

Int val= analogread(pir)

If (val>580){







This is the basic thing. Im typing on my iPad, so symbols are missing. Pls add them. Don’t paste this code.

Here i used a lcd to read the analog value. I did not add that part. You can use the serial monitor too. In my case i got readings above 580 when motion was detected and other below readings while nothing moving in front the pir. I guess that is why the led lights up even if no movement when the ” digitalread “ functoin is given.

Mine works fine.


2 years ago

how to combination 4 pir sensor sir? please help

1 reply

Reply 2 months ago

It's easy to do with relay.... Connect the output each relay to the common of next..


8 months ago

Its not a Fresnel lens, rather a set of small lenses each of which focusses a heat image onto the detector. As someone walks past, a series of (hot) images cross the detector (with gaps in between). The circuit relies on a series of hot-cold-hot-cold signals before it registers. So it's a "motion sensor". The lens is polyethylene becaust it transmits IR radiation. (Glass absorbs IR) Slow signals (sunrise etc ) will not trigger it.


Question 9 months ago

I want to increase delay time of my solar
light time it just off with 10 seconds plz help. The centre encircled is PIR sensor


Question 1 year ago

please can someone explain this for me

PIR security systems are not able to detect stray motions (from pets and birds etc) from actual threats.)


1 year ago

actually my problem is that the led stays on when there is no motion but blinks when it detects motion and again stays on.what could be the possible problem?


1 year ago

Hi, I'm working on a lightning system with a PIR 8 module (https://goo.gl/images/ctikQH). My goal is to make the light on when something is passing by, and when there is nothing moving around, the light is off.

So the PIR 8 appears to be the right module for that.

I have a little constraint because, I have installed this system in a place where there is no electricity (My garage). I have test two kind of battery, a old motorbike battery and a USB portable battery. My problem is the old motorbike battery is now dead, the system was fully working with this battery. I am now using the usb battery but after some minutes, the usb battery turns to sleep mode, so when the light is off, it stay off. I don't understand why. Could you please help me on finding a solution to use a PIR with a battery system (portable system).

Thank you,


Sayed sajeed

2 years ago

Would u like to send me the complete instruction of your project my email address is akheeq@gmail.com


2 years ago

i am not using led for output . so how i can use pir sensor and display its output in serial of arduino IDE thanks


2 years ago

my pir sensor is giving so random output. E.g. i have connected one LED with load of 220ohms and it is giving me output so anonymous like LED glows for some time then after shuts off and again after some times it glows with out any motion in front of the PIR sensor. I dont understand what ids the problem? can you help.

1 reply

Reply 2 years ago

try to read about repeatable and non repeateable modes of the PIR sensors and the delays. you may find your answer. goodluck


2 years ago

I need help on a project, I'm trying to make a soccer ball that lights up whenever you kick it can you help by any chance?


2 years ago

it is HC sR-501


2 years ago

Would a PIR sensor still work if it was attached to a moving object?


2 years ago

Not sure it originated here but take a look at this URL: