Fig. 11 · Robotics Lab · Eight IR sensors that read by timing

QTR-8RC line follower

Arduino Nano

Loading the figure…

Bench

No reading yet

Line position (0-7000)

No reading yet

Current draw

0 mA

Build and break

1BuildNext up
Strip to the bare PCB
Take every part off the red board. You will put it back the way it is made: the SMD sensors, resistors, capacitors and the MOSFET first, then the header.
2
Pick and place the SMD parts, then reflow
3
Fit the 11-pin header on the back
4
Solder the header
5
Wire the array to the Nano
6
Wire the TB6612FNG
7
Wire the motors
8
Wire the LiPo to VM and GND
9
Slide the test card under the array
10
Power the bench
11
Upload Raw RC times
12
Upload Line position
13
Upload Follow the line
14
Power the array from 3.3 VBreak it
15
Bridge 3.3V BYPASS on 5 VBreak it
16
Leave a cold joint on OUT1Break it
17
Leave out R9Break it
18
Put the bench in the sunBreak it
19
Lift the array 10 mmBreak it
20
Swap the left and right motorsBreak it
21
Run the motors from the Nano's 5 VBreak it
22
Exploded view

Break it on purpose

Each fault is one that catches real builders. Trigger it, read the symptom on the bench, then fix it.

Array on 3.3 V without the bypass
wrong voltage
Why it happens
Symptom: Every sensor reads dark, white card included; the position jumps or sticks in the middle.
Each LED string is two LEDs of about 1.2 V and 141 Ω. From 5 V that leaves 2.6 V for the resistors: about 18 mA. From 3.3 V only 0.9 V: about 6 mA, a third of the light. The phototransistor gets so little IR back that even white falls slowly, and after calibrating on 5 V everything counts as black.
Fix: Power VCC from 5 V. On a 3.3 V board, bridge the 3.3V BYPASS pads so the LEDs get their current back.
Bypass bridged on 5 V
overcurrent
Why it happens
Symptom: Sensors 5 and 6 read black over white card and stay that way; the board pulls about 220 mA and the LEDs get warm.
The bypass pads short out the two 47 Ω resistors every string shares. On 5 V each string then takes (5 - 2.4 V) / 47 Ω, about 55 mA, over the LEDs' 50 mA rating. The LEDs are in series pairs, so when one burns open, its partner goes dark with it.
Fix: Remove the solder bridge and replace the burnt pair's sensors. Bridge the bypass only on 3.3 V.
Cold joint on OUT1
cold solder joint
Why it happens
Symptom: Sensor 1 jumps to 2500 (black) now and then, and the position flicks toward the left.
The RC read needs the pin to charge the node and then watch it fall. Through a joint that makes and breaks, the Nano sometimes sees nothing come back and times out at 2500: the reading of a sensor over black.
Fix: Reheat the joint with fresh solder until it flows into a shiny cone around the pin.
R9 missing
missing part
Why it happens
Symptom: Sensors 3 and 4 read 2500 over white and black alike; the position leans toward them.
The LEDs of sensors 3 and 4 are in series through R9. Without it there is no path, so neither LED lights. A sensor that sends out no IR gets none back and reads as black.
Fix: Fit R9 (47 Ω) and reflow it. Two sensors dead together usually means their shared LED string.
Sunlight on the floor
wrong setting
Why it happens
Symptom: Every sensor reads fast, black tape included: the array cannot find the line.
Sunlight carries far more infrared than the LEDs. It reaches every phototransistor whatever is under it, so the outputs fall fast everywhere and black looks white.
Fix: Shade the array, or read twice with the LEDs on and off and subtract (QTRReadMode::OnAndOff in QTRSensors).
Array 10 mm above the floor
wrong setting
Why it happens
Symptom: White reads about 780 µs instead of 120, black times out; the position wanders and the robot weaves.
The LED's light spreads out on its way down and back. Pololu measured 120 µs over white at 3 mm and 780 µs at 10 mm: a sixth of the light. Calibrated on the low setting, white now looks grey to every sensor.
Fix: Mount the array 3 mm above the floor (Pololu's best distance, 9.5 mm at most) and calibrate at the height you drive.
Left and right motors swapped
wrong setting
Why it happens
Symptom: The robot turns away from the line, loses it and spins.
With the line on the left the sketch slows the left wheel. But that wheel is now on the right side, so the robot turns right and the line moves further left: the correction makes the error bigger.
Fix: Swap the motor wires back, or swap PWMA and PWMB in the sketch.
Motors on the Nano's 5 V
brown-out
Why it happens
Symptom: The wheels jerk, the Nano restarts over and over and calibration never finishes.
A TT motor takes 200-300 mA running and over 1 A to start. Through USB and the Nano's 500 mA fuse that pulls 5 V down below what the ATmega needs, so it resets, the motors stop, the voltage recovers and it starts again.
Fix: Power VM from the LiPo. Keep the logic and the motors on separate supplies with a common ground.

Meet the bench

build the array, read the line, then steer

The QTR-8RC is a red 75 mm strip with eight infrared reflectance sensors in a row, 9.5 mm apart. Each sensor is an IR LED and a phototransistor side by side, looking at the floor 3 mm below. White reflects a lot of IR, black tape almost none. The RC version reads by timing: the Nano drives each output HIGH, lets go, and counts how long the line takes to fall, about 120 µs over white and past the library's 2500 µs timeout over black. Eight of those times become one number from 0 to 7000: where the line is under the array. Build the array, wire it to a Nano with a TB6612FNG and two gear motors, slide a test card under it and make the robot steer back onto the line. Then break it: too little voltage, a bridged bypass, a cold joint, sunlight, the array too high, the motors swapped.

Sources

On the bench

Selected

Sketch uses 4,410 bytes of program storage

Code: Raw RC times

// QTR-8RC: eight RC sensors timed by the Nano
#include <QTRSensors.h>

QTRSensors qtr;
const uint8_t SENSORS[8] = {A0, A1, A2, A3, A4, A5, 11, 12};   // OUT1-OUT8: any digital pin can time an RC output
uint16_t values[8];

void setup() {
  Serial.begin(9600);
  qtr.setTypeRC();
  qtr.setSensorPins(SENSORS, 8);
  qtr.setEmitterPin(2);          // LEDON: the library turns the IR LEDs on for each read
  Serial.println("raw: OUT1-OUT8 in us, 2500 = timeout");
}

void loop() {
  qtr.read(values);              // eight fall times in µs: about 120 over white, 2500 (the timeout) over black
  for (int i = 0; i < 8; i++) {
    if (i > 0) Serial.print(' ');
    Serial.print(values[i]);
  }
  Serial.println();
  delay(1000);
}

Practice tasks

0/2 tasks

Write the code, then run the checks. The run uses a virtual clock and a scripted world, so the same code always gives the same result.

Steer back onto the line

Arduino C++

The starter reads the line but drives both wheels at 150 whatever it sees. Make it steer: error = position - 3500, then left = 150 + error / 20 and right = 150 - error / 20. With the line under sensor 2 the left wheel must slow and the right speed up; under sensor 7 the other way round; centred, both at 150.

Checks

Not run yetRuns for 4.5 s
Not run yetReads the line with readLineBlack()
Not run yetSTBY (D9) is HIGH: the driver is awake
Not run yetCentred: the left wheel (D5) at about 150/255
Not run yetCentred: the right wheel (D6) at about 150/255
Not run yetLine under sensor 2: the left wheel slows
Not run yetLine under sensor 2: the right wheel speeds up
Not run yetLine under sensor 7: the left wheel speeds up
Not run yetLine under sensor 7: the right wheel slows
Not run yetCentred again: back to 150

Hints

readLineBlack() returns an unsigned number; cast it before subtracting: int error = (int)pos - 3500;
error / 20 is the same as error x 0.05 without floating point: that is Kp.
motors() already clamps each wheel to 0-255 with constrain().

Code for Steer back onto the line

// Follow the line: slow the wheel on the side the line is on
#include <QTRSensors.h>

QTRSensors qtr;
const uint8_t SENSORS[8] = {A0, A1, A2, A3, A4, A5, 11, 12};   // OUT1-OUT8: any digital pin can time an RC output
uint16_t values[8];
const int PWMA = 5, AIN1 = 7, AIN2 = 8, STBY = 9, BIN1 = 10, BIN2 = 4, PWMB = 6;
const int BASE = 150;

void motors(int left, int right) {   // both wheels forward, speed from 0 to 255
  digitalWrite(AIN1, HIGH); digitalWrite(AIN2, LOW); analogWrite(PWMA, constrain(left, 0, 255));
  digitalWrite(BIN1, HIGH); digitalWrite(BIN2, LOW); analogWrite(PWMB, constrain(right, 0, 255));
}

void setup() {
  Serial.begin(9600);
  qtr.setTypeRC();
  qtr.setSensorPins(SENSORS, 8);
  qtr.setEmitterPin(2);          // LEDON: the library turns the IR LEDs on for each read
  for (int p = 4; p <= 10; p++) pinMode(p, OUTPUT);
  digitalWrite(STBY, HIGH);
  for (int i = 0; i < 100; i++) {   // sweep the array over the tape now: each sensor learns its own white and black
    qtr.calibrate();
    delay(20);
  }
}

void loop() {
  uint16_t pos = qtr.readLineBlack(values);
  // work out the error from pos and steer with it instead of driving straight
  motors(BASE, BASE);
  delay(20);
}

Spin to find the line

Arduino C++

On a sharp bend the line ends up under the outermost sensor and steering alone is too slow. When the position is under 1000, spin left on the spot: left wheel backwards, right forwards, both at 110. Over 6000, spin right. In between, steer as before.

Checks

Not run yetRuns for 4.3 s
Not run yetLine far left: AIN1 (D7) is LOW
Not run yetLine far left: AIN2 (D8) is HIGH, the left wheel turns backwards
Not run yetLine far left: BIN1 (D10) is HIGH, the right wheel forwards
Not run yetCentred: the left wheel forwards again
Not run yetLine far right: BIN1 (D10) is LOW
Not run yetLine far right: BIN2 (D4) is HIGH, the right wheel backwards
Not run yetLine far right: the left wheel forwards
Not run yetSpinning left at 110: the left wheel's duty is about 43%
Not run yetSpinning right at 110: the right wheel's duty is about 43%

Hints

wheel() takes a signed speed: a negative one sets IN1 LOW and IN2 HIGH, so the wheel turns backwards.
if (pos < 1000) { motors(-110, 110); } else if (pos > 6000) { motors(110, -110); } else { ... }
Keep the steering line from the first task in the last branch.

Code for Spin to find the line

// Spin on the spot when the line is under an end sensor
#include <QTRSensors.h>

void wheel(int in1, int in2, int pwm, int speed) {
  digitalWrite(in1, speed >= 0 ? HIGH : LOW);
  digitalWrite(in2, speed >= 0 ? LOW : HIGH);
  analogWrite(pwm, constrain(abs(speed), 0, 255));
}

void motors(int left, int right) {
  wheel(AIN1, AIN2, PWMA, left);
  wheel(BIN1, BIN2, PWMB, right);
}

void loop() {
  uint16_t pos = qtr.readLineBlack(values);
  // add the two spins: under 1000 spin left, over 6000 spin right
  int turn = ((int)pos - 3500) / 20;
  motors(BASE + turn, BASE - turn);
  delay(20);
}

Build and break

1BuildNext up

Strip to the bare PCB

Take every part off the red board. You will put it back the way it is made: the SMD sensors, resistors, capacitors and the MOSFET first, then the header.

Pick and place the SMD parts, then reflow

Fit the 11-pin header on the back

Solder the header

Wire the array to the Nano

Wire the TB6612FNG

Wire the motors

Wire the LiPo to VM and GND

Slide the test card under the array

Power the bench

Upload Raw RC times

Upload Line position

Upload Follow the line

Power the array from 3.3 VBreak it

Bridge 3.3V BYPASS on 5 VBreak it

Leave a cold joint on OUT1Break it

Leave out R9Break it

Put the bench in the sunBreak it

Lift the array 10 mmBreak it

Swap the left and right motorsBreak it

Run the motors from the Nano's 5 VBreak it

Exploded view

Meet the bench

build the array, read the line, then steer

On the bench

Selected

Sketch uses 4,410 bytes of program storage

Code: Raw RC times

// QTR-8RC: eight RC sensors timed by the Nano
#include <QTRSensors.h>

QTRSensors qtr;
const uint8_t SENSORS[8] = {A0, A1, A2, A3, A4, A5, 11, 12};   // OUT1-OUT8: any digital pin can time an RC output
uint16_t values[8];

void setup() {
  Serial.begin(9600);
  qtr.setTypeRC();
  qtr.setSensorPins(SENSORS, 8);
  qtr.setEmitterPin(2);          // LEDON: the library turns the IR LEDs on for each read
  Serial.println("raw: OUT1-OUT8 in us, 2500 = timeout");
}

void loop() {
  qtr.read(values);              // eight fall times in µs: about 120 over white, 2500 (the timeout) over black
  for (int i = 0; i < 8; i++) {
    if (i > 0) Serial.print(' ');
    Serial.print(values[i]);
  }
  Serial.println();
  delay(1000);
}

Spin to find the line

Arduino C++

Checks

Not run yetRuns for 4.3 s
Not run yetLine far left: AIN1 (D7) is LOW
Not run yetLine far left: AIN2 (D8) is HIGH, the left wheel turns backwards
Not run yetLine far left: BIN1 (D10) is HIGH, the right wheel forwards
Not run yetCentred: the left wheel forwards again
Not run yetLine far right: BIN1 (D10) is LOW
Not run yetLine far right: BIN2 (D4) is HIGH, the right wheel backwards
Not run yetLine far right: the left wheel forwards
Not run yetSpinning left at 110: the left wheel's duty is about 43%
Not run yetSpinning right at 110: the right wheel's duty is about 43%

Hints

wheel() takes a signed speed: a negative one sets IN1 LOW and IN2 HIGH, so the wheel turns backwards.
if (pos < 1000) { motors(-110, 110); } else if (pos > 6000) { motors(110, -110); } else { ... }
Keep the steering line from the first task in the last branch.

Code for Spin to find the line

// Spin on the spot when the line is under an end sensor
#include <QTRSensors.h>

void wheel(int in1, int in2, int pwm, int speed) {
  digitalWrite(in1, speed >= 0 ? HIGH : LOW);
  digitalWrite(in2, speed >= 0 ? LOW : HIGH);
  analogWrite(pwm, constrain(abs(speed), 0, 255));
}

void motors(int left, int right) {
  wheel(AIN1, AIN2, PWMA, left);
  wheel(BIN1, BIN2, PWMB, right);
}

QTR-8RC line follower

The bench

Build and break

Strip to the bare PCB

Pick and place the SMD parts, then reflow

Fit the 11-pin header on the back

Solder the header

Wire the array to the Nano

Wire the TB6612FNG

Wire the motors

Wire the LiPo to VM and GND

Slide the test card under the array

Power the bench

Upload Raw RC times

Upload Line position

Upload Follow the line

Power the array from 3.3 VBreak it

Bridge 3.3V BYPASS on 5 VBreak it

Leave a cold joint on OUT1Break it

Leave out R9Break it

Put the bench in the sunBreak it

Lift the array 10 mmBreak it

Swap the left and right motorsBreak it

Run the motors from the Nano's 5 VBreak it

Exploded view

Break it on purpose

Array on 3.3 V without the bypass

Bypass bridged on 5 V

Cold joint on OUT1

R9 missing

Sunlight on the floor

Array 10 mm above the floor

Left and right motors swapped

Motors on the Nano's 5 V

Live status

Current draw

Check yourself

After: Solder the header

After: Upload Line position

After: Upload Follow the line

After: Run the motors from the Nano's 5 V

End of bench quiz

Meet the bench

Sources

On the bench

Practice tasks

Steer back onto the line

Checks

Spin to find the line

Checks

QTR-8RC line follower

The bench

Build and break

Strip to the bare PCB

Pick and place the SMD parts, then reflow

Fit the 11-pin header on the back

Solder the header

Wire the array to the Nano

Wire the TB6612FNG

Wire the motors

Wire the LiPo to VM and GND

Slide the test card under the array

Power the bench

Upload Raw RC times

Upload Line position

Upload Follow the line

Power the array from 3.3 VBreak it

Bridge 3.3V BYPASS on 5 VBreak it

Leave a cold joint on OUT1Break it

Leave out R9Break it

Put the bench in the sunBreak it

Lift the array 10 mmBreak it

Swap the left and right motorsBreak it

Run the motors from the Nano's 5 VBreak it

Exploded view

Break it on purpose

Array on 3.3 V without the bypass

Bypass bridged on 5 V

Cold joint on OUT1

R9 missing