{"id":52273,"date":"2026-11-01T10:50:49","date_gmt":"2026-11-01T10:50:49","guid":{"rendered":"http:\/\/siratek.net\/index.php\/tutorials\/ir-tracking-sensor\/"},"modified":"2026-11-01T10:50:49","modified_gmt":"2026-11-01T10:50:49","slug":"ir-tracking-sensor","status":"publish","type":"siratek_tutorial","link":"https:\/\/siratek.net\/index.php\/tutorials\/ir-tracking-sensor\/","title":{"rendered":"A Four-Way Line Tracking Sensor"},"content":{"rendered":"

<\/p>\n

What it does<\/h2>\n

Four infrared reflectance sensors, each looking straight down at the floor. A white surface reflects the infrared back; black tape absorbs it. So each sensor answers one question: is the floor under me light or dark?<\/strong><\/p>\n

Four of those answers, read a hundred times a second, is enough to follow a line. It is the sensing half of a line-following robot, and the reason four is better than one is the whole idea of the part.<\/p>\n

Before you begin<\/h2>\n

Required parts and tools<\/h3>\n
    \n
  • An Arduino Uno or compatible board.<\/li>\n
  • One four-way tracking sensor set. This is Siratek product 2943.<\/li>\n
  • Black electrical tape and a sheet of white paper or card, to make a line.<\/li>\n
  • A small screwdriver for the trimmers.<\/li>\n
  • Six jumper wires.<\/li>\n<\/ul>\n

    What is in the set<\/h3>\n

    The Siratek photographs show two kinds of board<\/strong>, and this is the thing that makes this part nicer than it first looks:<\/p>\n

    One main board<\/strong>, red, carrying a comparator chip, four blue trimmers<\/strong> and four indicator LEDs<\/strong>, with an output header along one edge.<\/p>\n

    Four separate sensor heads<\/strong>, each a small red board with an infrared emitter and a receiver<\/strong> and a three-pin connector, joined to the main board by a ribbon of coloured leads.<\/p>\n

    Because the heads are separate, you can space them to suit your chassis<\/strong>. A fixed four-in-a-row board makes you build the robot around the sensor; this one lets you put the sensors where the geometry wants them. That is worth more than it sounds when you come to mount it.<\/p>\n

    Telling the emitter from the receiver<\/h3>\n

    Each head has two LED-shaped parts, marked on the silkscreen:<\/p>\n

      \n
    • DT<\/code>, the clear one: the transmitter.<\/strong> It emits infrared, which your eye cannot see.<\/li>\n
    • DR<\/code>, the dark one: the receiver.<\/strong> The dark package is a filter that blocks visible light so only infrared gets through.<\/li>\n<\/ul>\n

      A phone camera sees infrared where your eye does not, so pointing one at the clear LED while the board is powered is a quick way to prove the emitter is working.<\/p>\n

      Height matters more than anything<\/h3>\n

      These sensors have a narrow working range. Too far from the floor and the reflection is too weak to read; too close and the emitter\u2019s light floods the receiver whatever colour is underneath.<\/p>\n

      Somewhere around 5 to 15 millimetres<\/strong> is usual, and the right figure is the one that works on your surface with your trimmer setting. Mount the heads so the height is fixed and the same for all four<\/strong>, because a sensor sitting two millimetres lower than its neighbours behaves like a differently adjusted one.<\/p>\n

      If one channel refuses to behave no matter how you set its trimmer, check its height before anything else.<\/p>\n

      Digital out only, and what that costs<\/h3>\n

      Each channel gives one digital output: light or dark, with the threshold set by its trimmer. There is no analog pin, as on the flame sensor.<\/p>\n

      That makes the wiring and the code simple, and it means the trimmers are the calibration<\/strong> and you have to set them by watching the LEDs. The procedure is below.<\/p>\n

      For line following it is enough. For anything that needs to know how<\/em> dark, an analog reflectance sensor is the part to look for.<\/p>\n

      Pin reference<\/h2>\n\n\n\n\n\n\n\n\n\n\n
      Pin<\/th>\nConnect to<\/th>\nPurpose<\/th>\n<\/tr>\n<\/thead>\n
      VCC<\/code><\/td>\nUno 5V<\/code><\/td>\nPower for the board and the emitters<\/td>\n<\/tr>\n
      GND<\/code><\/td>\nUno GND<\/code><\/td>\nGround<\/td>\n<\/tr>\n
      D1<\/code><\/td>\nUno D4<\/code><\/td>\nLeftmost sensor<\/td>\n<\/tr>\n
      D2<\/code><\/td>\nUno D5<\/code><\/td>\nCentre left<\/td>\n<\/tr>\n
      D3<\/code><\/td>\nUno D6<\/code><\/td>\nCentre right<\/td>\n<\/tr>\n
      D4<\/code><\/td>\nUno D7<\/code><\/td>\nRightmost<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

      Which physical head is D1<\/code> depends on how you plug the ribbon in, and the first sketch below settles it rather than assuming.<\/p>\n

      \"A
      The heads are separate, so they can be spaced to suit the chassis rather than the other way round.<\/figcaption><\/figure>\n

      Wiring steps<\/h2>\n
        \n
      1. Unplug the USB cable.<\/li>\n
      2. Connect the four sensor heads to the main board with the supplied leads.<\/li>\n
      3. GND<\/code> to GND<\/code>, VCC<\/code> to 5V<\/code>.<\/li>\n
      4. D1<\/code> to D4<\/code>, D2<\/code> to D5<\/code>, D3<\/code> to D6<\/code>, D4<\/code> to D7<\/code>.<\/li>\n
      5. Plug the USB cable back in. All four indicator LEDs should light or go out as you pass something light or dark under the heads.<\/li>\n<\/ol>\n

        Software and libraries<\/h2>\n

        No library is required. Four digitalRead()<\/code> calls.<\/p>\n

        Setting the trimmers<\/h2>\n

        Do this before writing any logic. It takes five minutes and nothing works properly without it.<\/p>\n

          \n
        1. Lay out your line: black tape on white card is ideal.<\/li>\n
        2. Hold the sensors at the height they will be mounted at, over the white surface<\/strong>.<\/li>\n
        3. Turn each trimmer until its indicator LED just changes state, then back it off slightly so it sits reliably on the \u201cwhite\u201d side.<\/li>\n
        4. Move that head over the black tape. Its LED should change.<\/li>\n
        5. Move it back. It should change back.<\/li>\n
        6. Repeat for all four, and check them all again at the end, because adjusting one does not affect the others but your hand position will have drifted.<\/li>\n<\/ol>\n

          The run below makes this easier by printing all four states together.<\/p>\n

          Minimal working example<\/h2>\n
          \/\/ Siratek: what each channel sees, and which physical head is which.\n\nconst int sensorPins[4] = {4, 5, 6, 7};\n\nvoid setup() {\n  for (int i = 0; i < 4; i++) {\n    pinMode(sensorPins[i], INPUT);\n  }\n  Serial.begin(9600);\n  Serial.println(\"Siratek: cover each head in turn to find out which is which.\");\n}\n\nvoid loop() {\n  Serial.print(\"[ \");\n  for (int i = 0; i < 4; i++) {\n    \/\/ On most of these boards the output is LOW over black. Confirm yours.\n    Serial.print(digitalRead(sensorPins[i]) == LOW ? \"#\" : \".\");\n    Serial.print(\" \");\n  }\n  Serial.println(\"]\");\n\n  delay(150);\n}<\/code><\/pre>\n
          Cover one head at a time with your finger and watch which position in the display changes. That tells you the physical order, which is not necessarily left to right.<\/p>\n
          Then slide the whole assembly across your line and watch the pattern move: . # . .<\/code> becoming . . # .<\/code> and so on. Seeing that is the moment the part makes sense.<\/p>\n
          Check the polarity while you are there.<\/strong> If covering a head makes the display show .<\/code> rather than #<\/code>, your board reads the other way round, and the one comparison in the sketch is where to change it.<\/p>\n
          Turning four readings into a position<\/h3>\n
          The useful abstraction is a single number: how far off-centre the line is.<\/p>\n
          \/\/ Siratek: a position from -3 (hard left) to +3 (hard right), or no line.\n\nconst int sensorPins[4] = {4, 5, 6, 7};\nconst int weights[4] = {-3, -1, 1, 3};      \/\/ leftmost to rightmost\n\nconst int NO_LINE = 99;\n\nint linePosition() {\n  int total = 0;\n  int seen = 0;\n\n  for (int i = 0; i < 4; i++) {\n    if (digitalRead(sensorPins[i]) == LOW) {   \/\/ dark: the line\n      total += weights[i];\n      seen++;\n    }\n  }\n\n  if (seen == 0) {\n    return NO_LINE;\n  }\n  return total \/ seen;\n}\n\nvoid setup() {\n  for (int i = 0; i < 4; i++) {\n    pinMode(sensorPins[i], INPUT);\n  }\n  Serial.begin(9600);\n}\n\nvoid loop() {\n  int position = linePosition();\n\n  if (position == NO_LINE) {\n    Serial.println(\"no line\");\n  } else {\n    Serial.print(\"position \");\n    Serial.println(position);\n  }\n\n  delay(100);\n}<\/code><\/pre>\n
          Three things this gives you that four separate readings do not.<\/p>\n
          One number to steer by.<\/strong> Negative means the line is to the left, positive to the right, zero means centred. A motor sketch can use it directly.<\/p>\n
          It handles two sensors seeing the line at once<\/strong>, which happens constantly when the line is wider than the gap between heads. Averaging the weights puts the position between them.<\/p>\n
          It reports losing the line separately.<\/strong> NO_LINE<\/code> is not zero, and the difference matters: a robot that treats \u201cno line\u201d as \u201ccentred\u201d drives confidently off the table.<\/p>\n
          What to do when the line is lost<\/h3>\n
          The honest answer is that losing the line is normal, especially on corners, and the behaviour you choose defines how the robot feels.<\/p>\n
          The simplest useful rule: remember which way you were last turning, and keep turning that way<\/strong>. If the line vanished to the left, it is probably still to the left.<\/p>\n
          int lastSeen = 0;\n\nvoid loop() {\n  int position = linePosition();\n\n  if (position != NO_LINE) {\n    lastSeen = position;\n    \/\/ steer according to position\n  } else {\n    \/\/ steer hard in the direction we last saw it\n  }\n}<\/code><\/pre>\n
          A time limit belongs here too: if the line has been missing for a second, stop rather than continuing to spin.<\/p>\n
          All four dark means something<\/h3>\n
          Four sensors reading dark at once is not a line. It is usually one of:<\/p>\n
            \n
          • A junction or a crossing<\/strong> in the course.<\/li>\n
          • The end of the line<\/strong>, marked with a wide band, which is a common convention.<\/li>\n
          • The sensors lifted off the surface<\/strong>, so nothing reflects.<\/li>\n<\/ul>\n
            Handle it deliberately rather than letting the averaging return zero and the robot drive straight on.<\/p>\n
            Expected result<\/h2>\n
            Four indicator LEDs following light and dark under their heads, and a serial display whose pattern moves as you slide the assembly across a line.<\/p>\n
            Over plain white, all four should read light. Over the tape, the one or two over it should read dark.<\/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
            One channel never changes<\/td>\nHeight wrong for that head<\/td>\nCheck all four sit at the same height<\/td>\n<\/tr>\n
            One channel never changes, height fine<\/td>\nTrimmer past its range<\/td>\nTurn it back through the whole range<\/td>\n<\/tr>\n
            All four always read the same<\/td>\nToo close to the surface<\/td>\nRaise them; the emitter is flooding the receiver<\/td>\n<\/tr>\n
            All four always read light<\/td>\nToo far from the surface<\/td>\nLower them<\/td>\n<\/tr>\n
            Readings inverted<\/td>\nBoard reads the other way<\/td>\nChange the one comparison<\/td>\n<\/tr>\n
            Unreliable in sunlight<\/td>\nInfrared in daylight<\/td>\nShade the sensors; they are indoor parts<\/td>\n<\/tr>\n
            Works on paper, not on a shiny floor<\/td>\nSpecular reflection<\/td>\nMatte surfaces work best<\/td>\n<\/tr>\n
            Pattern jumps about<\/td>\nLoose lead, or a wobbling mount<\/td>\nFix the height mechanically<\/td>\n<\/tr>\n
            Robot drives off on a corner<\/td>\n\u201cNo line\u201d treated as centred<\/td>\nReport it separately and act on it<\/td>\n<\/tr>\n
            Adjusting one trimmer upsets another<\/td>\nIt does not; your hand moved<\/td>\nRecheck all four at the end<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
            Next steps<\/h2>\n
              \n
            • Easier first:<\/strong> Flame Detection with an Infrared Sensor<\/em>, which is one sensor of this kind.<\/li>\n
            • Related products:<\/strong> the geared motor and wheel set, and the L298N driver, which turn a position into movement.<\/li>\n
            • Next project:<\/strong> a line following robot, which is this position number driving two motors.<\/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
              • The main comparator board with four trimmers and four indicator LEDs, the four separate<\/strong> sensor heads on leads, and the DT<\/code> and DR<\/code> silkscreen marking the transmitter and receiver were read from the Siratek product photographs for item 2943.<\/li>\n<\/ul>\n
                Working height depends on the surface and the trimmer setting, so a range is given with a procedure for finding the right value. Output polarity varies between boards of this kind, so the tutorial has the reader confirm it. No third-party tutorial text, diagrams, photographs or code has been reproduced here.<\/p>\n
                <\/p>\n<\/body>","protected":false},"excerpt":{"rendered":"
                Four infrared reflectance sensors, each looking straight down at the floor. A white surface reflects the infrared back; black tape absorbs it. 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