{"id":52313,"date":"2026-11-01T10:50:56","date_gmt":"2026-11-01T10:50:56","guid":{"rendered":"http:\/\/siratek.net\/index.php\/tutorials\/pir-motion-sensor\/"},"modified":"2026-11-01T10:50:56","modified_gmt":"2026-11-01T10:50:56","slug":"pir-motion-sensor","status":"publish","type":"siratek_tutorial","link":"https:\/\/siratek.net\/index.php\/tutorials\/pir-motion-sensor\/","title":{"rendered":"Detecting People with a PIR Sensor"},"content":{"rendered":"

<\/p>\n

What it does<\/h2>\n

A passive infrared sensor watches for changes<\/strong> in the infrared light falling on it. Everything warm glows in the infrared, and when a person moves across the sensor\u2019s view the pattern shifts, which the module reports as a single digital HIGH on one pin.<\/p>\n

\u201cPassive\u201d is the important word. It emits nothing and measures nothing; it only notices change. That one fact explains almost all of its behaviour, including the part people find most frustrating: a person who stands perfectly still disappears.<\/strong> The sensor is not broken. There is no change for it to see.<\/p>\n

It is also the easiest sensor in this library to read, because the module does all the work and hands you a HIGH or a LOW.<\/p>\n

Before you begin<\/h2>\n

Required parts and tools<\/h3>\n
    \n
  • An Arduino Uno or compatible board.<\/li>\n
  • One PIR motion module. This is Siratek product 2855, the common HC-SR501 type.<\/li>\n
  • Three female-to-male jumper wires.<\/li>\n<\/ul>\n

    What is on the board<\/h3>\n

    Under the white plastic dome is a sensor the size of a shirt button. The dome itself is a Fresnel lens<\/strong>, moulded into many small facets that divide the view into a fan of separate zones. Movement across the boundary between zones is what the sensor actually detects, which is why walking across<\/strong> the field triggers it far more reliably than walking straight at it.<\/p>\n

    On the back of the board:<\/p>\n

      \n
    • A three-pin header<\/strong>.<\/li>\n
    • Two orange-topped trimmers<\/strong>: sensitivity, meaning range, and time delay.<\/li>\n
    • A yellow jumper<\/strong> with two positions, which sets the retriggering mode.<\/li>\n
    • A voltage regulator, which is why the module takes 5 V but its output is a lower voltage.<\/li>\n<\/ul>\n

      Read your own pin order<\/h3>\n

      The three pins are VCC<\/code>, OUT<\/code> and GND<\/code>, and the order differs between batches of this module<\/strong>. Some put VCC<\/code> on the left, some on the right.<\/p>\n

      The silkscreen is usually printed on the board underneath the white dome. The dome lifts off: it is a push fit, not glued. Take it off, read the three labels, put it back.<\/p>\n

      Guessing here reverses power across the module, so it is worth the thirty seconds.<\/p>\n

      The output is about 3.3 V<\/h3>\n

      The module has its own regulator, and its HIGH is roughly 3.3 V rather than 5 V.<\/p>\n

      An Uno reads anything above about 3 V as HIGH, so this works. But the margin is small, and it is the reason this sensor is more sensitive than most to a long wire or a poor connection. If a reading seems erratic, measure the OUT<\/code> pin with a meter while you wave at it, rather than assuming the sensor is faulty.<\/p>\n

      The two trimmers<\/h3>\n

      Both turn with a small screwdriver, and both are worth setting deliberately rather than leaving at whatever the factory chose.<\/p>\n

      Sensitivity<\/strong> sets the range, roughly three to seven metres. Start it at minimum. A sensor at maximum range in a small room triggers on everything, including warm air moving, and feels hopelessly unreliable.<\/p>\n

      Time delay<\/strong> sets how long OUT<\/code> stays HIGH after a trigger, from a few seconds to several minutes. Start it at minimum too, so you can see each detection separately while testing. Turn it up only when you know what you want.<\/p>\n

      Which trimmer is which varies by board. Turn one a little, test, and label it with a marker.<\/p>\n

      The jumper, which decides what happens during a long event<\/h3>\n
        \n
      • Repeat, sometimes marked H<\/code>:<\/strong> while movement continues, the timer keeps restarting, so OUT<\/code> stays HIGH the whole time and goes LOW once the movement stops and the delay runs out.<\/li>\n
      • Single, sometimes marked L<\/code>:<\/strong> OUT<\/code> goes HIGH, stays HIGH for the delay, then goes LOW regardless of whether the movement is still happening, and only then can it trigger again.<\/li>\n<\/ul>\n

        Repeat is what you want almost always.<\/strong> A light that should stay on while someone is in the room needs it. Single mode is for counting discrete events.<\/p>\n

        Give it a minute to settle<\/h3>\n

        When first powered, the module needs thirty seconds to a minute<\/strong> to establish what the room normally looks like. During that time it reports nonsense, often a long HIGH.<\/p>\n

        This is normal and it catches everyone, because the natural thing to do is upload the sketch and stare at the output immediately. Power it, wait a minute, then start testing.<\/p>\n

        Pin reference<\/h2>\n\n\n\n\n\n\n\n
        Pin or label<\/th>\nConnect to<\/th>\nPurpose<\/th>\nNotes<\/th>\n<\/tr>\n<\/thead>\n
        VCC<\/code><\/td>\nUno 5V<\/code><\/td>\nPower<\/td>\n4.5 to 20 V; the regulator handles it<\/td>\n<\/tr>\n
        OUT<\/code><\/td>\nUno D2<\/code><\/td>\nMotion signal<\/td>\nHIGH on detection, around 3.3 V<\/td>\n<\/tr>\n
        GND<\/code><\/td>\nUno GND<\/code><\/td>\nGround<\/td>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
        \"A
        Three wires, and three settings on the board that matter more than the code.<\/figcaption><\/figure>\n

        Wiring steps<\/h2>\n
          \n
        1. Unplug the USB cable.<\/li>\n
        2. Lift the white dome off and read the three pin labels. Put the dome back.<\/li>\n
        3. GND<\/code> to GND<\/code>.<\/li>\n
        4. VCC<\/code> to 5V<\/code>.<\/li>\n
        5. OUT<\/code> to D2<\/code>.<\/li>\n
        6. Set both trimmers to minimum and the jumper to repeat.<\/li>\n
        7. Plug the USB cable back in, then leave it alone for a minute before judging anything.<\/li>\n<\/ol>\n

          Software and libraries<\/h2>\n

          No library is required. The module gives you a plain digital signal, so this is digitalRead()<\/code> and nothing more.<\/p>\n

          Use plain INPUT<\/code>, not INPUT_PULLUP<\/code>. The module drives the pin both ways, so an internal pull-up would be fighting it.<\/p>\n

          Minimal working example<\/h2>\n
          \/\/ Siratek: reporting motion, printing only when the state changes.\n\nconst int pirPin = 2;\nconst int ledPin = 13;\n\nint lastState = LOW;\n\nvoid setup() {\n  pinMode(pirPin, INPUT);\n  pinMode(ledPin, OUTPUT);\n\n  Serial.begin(9600);\n  Serial.println(\"Siratek: warming up, about a minute.\");\n  delay(60000);\n  Serial.println(\"ready.\");\n}\n\nvoid loop() {\n  int state = digitalRead(pirPin);\n\n  if (state != lastState) {\n    lastState = state;\n    digitalWrite(ledPin, state);\n    Serial.println(state == HIGH ? \"motion\" : \"clear\");\n  }\n}<\/code><\/pre>\n
          Printing only on a change, rather than every pass, is what makes this readable. A loop that prints the state every time floods the serial monitor with thousands of identical lines and hides the moment you care about.<\/p>\n
          The onboard LED on pin 13 gives you a response you can see without looking at the laptop, which matters when you are across the room waving at the sensor.<\/p>\n
          A light that stays on while someone is there<\/h3>\n
          The module\u2019s own time delay could do this, but doing it in software gives you a number you can change without a screwdriver.<\/p>\n
          \/\/ Siratek: hold an output on for a while after the last movement.\n\nconst int pirPin = 2;\nconst int lampPin = 8;\n\nconst unsigned long holdFor = 15000;  \/\/ milliseconds after the last motion\n\nunsigned long lastMotion = 0;\nbool lampOn = false;\n\nvoid setup() {\n  pinMode(pirPin, INPUT);\n  pinMode(lampPin, OUTPUT);\n  Serial.begin(9600);\n  delay(60000);\n  Serial.println(\"Siratek: ready.\");\n}\n\nvoid loop() {\n  if (digitalRead(pirPin) == HIGH) {\n    lastMotion = millis();\n    if (!lampOn) {\n      lampOn = true;\n      digitalWrite(lampPin, HIGH);\n      Serial.println(\"on\");\n    }\n  }\n\n  if (lampOn && millis() - lastMotion >= holdFor) {\n    lampOn = false;\n    digitalWrite(lampPin, LOW);\n    Serial.println(\"off\");\n  }\n}<\/code><\/pre>\n
          Set the module\u2019s own delay to minimum when using this, so the two timers do not fight. The hardware reports events; the software decides how long they last.<\/p>\n
          Expected result<\/h2>\n
          After the warm-up, OUT<\/code> goes HIGH when you move in front of the sensor and returns LOW a few seconds after you stop. With the trimmers at minimum you should need to be within a couple of metres.<\/p>\n
          Wave your hand across<\/strong> the sensor\u2019s view rather than towards it, and you will see it trigger far more readily. That is the Fresnel lens doing its job.<\/p>\n
          Troubleshooting<\/h2>\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n
          Symptom<\/th>\nLikely cause<\/th>\nCheck or fix<\/th>\n<\/tr>\n<\/thead>\n
          Triggers constantly at first<\/td>\nStill warming up<\/td>\nWait a minute after power-up<\/td>\n<\/tr>\n
          Triggers at random afterwards<\/td>\nSensitivity too high, or a draught<\/td>\nLower sensitivity; keep it away from vents and sunlight<\/td>\n<\/tr>\n
          Never triggers<\/td>\nPower reversed, pin order assumed<\/td>\nLift the dome and read the labels<\/td>\n<\/tr>\n
          Stays HIGH for ages<\/td>\nTime delay trimmer<\/td>\nTurn it down<\/td>\n<\/tr>\n
          One long HIGH instead of separate events<\/td>\nJumper in repeat, which is usually right<\/td>\nUse single mode only if counting events<\/td>\n<\/tr>\n
          Misses a person standing still<\/td>\nWorking as designed<\/td>\nPIR sees change, not presence<\/td>\n<\/tr>\n
          Misses someone walking towards it<\/td>\nFewer zone crossings<\/td>\nMount it across the path, not along it<\/td>\n<\/tr>\n
          Erratic over a long wire<\/td>\nOutput is only about 3.3 V<\/td>\nShorten the wire; measure OUT<\/code> with a meter<\/td>\n<\/tr>\n
          Triggers when a lamp switches on<\/td>\nInfrared from a hot filament<\/td>\nReposition, or lower sensitivity<\/td>\n<\/tr>\n
          Works indoors, useless outdoors<\/td>\nSunlight and moving warm air<\/td>\nPIR is an indoor sensor<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
          Next steps<\/h2>\n
            \n
          • Easier first:<\/strong> Read a Push Button<\/em>, which is the same digitalRead()<\/code> with no settings.<\/li>\n
          • Related products:<\/strong> an active buzzer for an alert, or a relay to switch a real lamp.<\/li>\n
          • Next project:<\/strong> combine it with the relay tutorial for a light that comes on when someone enters, using a low-voltage lamp.<\/li>\n<\/ul>\n
            Sources and attribution<\/h2>\n
            All text, wiring guidance and code in this tutorial are original Siratek work.<\/p>\n
              \n
            • HC-SR501 module documentation for the warm-up period, the adjustable range and delay, and the retriggering jumper.<\/li>\n
            • The two trimmers, the jumper and the onboard regulator described here were read from the Siratek product photographs for item 2855.<\/li>\n<\/ul>\n
              Pin order varies between batches of this module, so this tutorial tells the reader to lift the dome and read their own silkscreen rather than giving a fixed order. Range and delay figures are typical values that depend on the trimmer settings. No third-party tutorial text, diagrams, photographs or code has been reproduced here.<\/p>\n
              <\/p>\n<\/body>","protected":false},"excerpt":{"rendered":"
              A passive infrared sensor watches for changes in the infrared light falling on it. 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