Lesson 7: Smoothing Noisy Readings

Draw three lines on a live graph - raw, average and median - and see which filter handles a sudden spike best.

One of 9 projects in the Arduino Ultrasonic Distance & Parking Learning Kit — the parts below are from that kit.

Before you start

What you build and learn

• What a filter does to noisy data
• Average (mean) versus median
• Arrays: keeping the last 5 readings
• Drawing live graphs with the Serial Plotter

New words

Filter

Code that cleans up readings, for example by combining the last few.

Average (mean)

Add the numbers and divide by how many there are.

Median

Sort the numbers and take the middle one. One wild number cannot move it much.

Array

A row of boxes with one name, such as history[0] to history[4].

Serial Plotter

Tools → Serial Plotter: draws the numbers the UNO sends as moving lines.

What this project uses

Every item is in the kit — nothing extra to buy.

UNO R3 UNO R3 Development Board ATmega328P Microcontroller with USB Cable, 5V Logic, 14 Digital I/O, 6 Analog Inputs… UNOR3

In the kit

HC-SR04 HC-SR04 Ultrasonic Distance Sensor Module for Arduino, Raspberry Pi & DIY Robotics Projects, 5V Trigger/Echo… DIY-00043

In the kit

◇

a computer with the Serial Plotter

In the kit

◇

your hand or a book

In the kit

Basics 400 Tie Point Solderless Breadboard - Half Size Prototyping Circuit Board with Power Rails, for Arduino & Ele… DIY-238

In the kit

Basics Jumper Wire Assorted Pack 120 Pieces - Male-to-Male, Male-to-Female, Female-to-Female 10cm Ribbon Cables for… DIY-212

In the kit

Build this with

Arduino Ultrasonic Distance & Parking Learning Kit

All 9 parts in one box, with this project and 8 more to build from them.

AED 85.00 See the kit Add kit to cart

Need only these parts? Order them loose · from AED 78.30 ▾

Used in this project tick what you still need

UNO R3 UNO R3 Development Board ATmega328P Microcontroller with USB Cable, 5V Logic, 14 Digital I/O, 6 Analog Inputs… UNOR3 · ×1

AED 26.39

In stock

HC-SR04 HC-SR04 Ultrasonic Distance Sensor Module for Arduino, Raspberry Pi & DIY Robotics Projects, 5V Trigger/Echo… DIY-00043 · ×1

AED 15.31

In stock

Also needed untick if you already have them

Basics 400 Tie Point Solderless Breadboard - Half Size Prototyping Circuit Board with Power Rails, for Arduino & Ele… DIY-238 · ×1

AED 11.58

In stock

Basics Jumper Wire Assorted Pack 120 Pieces - Male-to-Male, Male-to-Female, Female-to-Female 10cm Ribbon Cables for… DIY-212 · ×1

AED 25.02

In stock

Selected parts AED 78.30

Add 4 parts to cart

One of each part. Packs (resistor assortment, jumper wires) already hold more than a single build needs. a computer with the Serial Plotter, your hand or a book: included in the kit, ask us for it loose. Delivery is AED 20, free on orders AED 100+.

Connections

Unplug USB before you change any wire. Pin names are the labels printed on the board.

Step by step

One thing at a time. Tick nothing off until it matches the diagram.

1. Upload the Lesson 7 code. No wiring changes are needed.
2. Close the Serial Monitor, then open Tools → Serial Plotter at 9600 baud.Watch out: Only one of the two can be open at a time.
3. Hold a book still 40 cm away: the three lines lie on top of each other.
4. Wave your hand quickly through the beam: watch which line jumps.
5. Now move the book smoothly from 40 cm to 20 cm: watch which line lags behind.

Sketch for this project

Written for the board in the kit. Copy it into the Arduino IDE; change the pin numbers if you wire it differently.

/*
  DIY KIT - Arduino Ultrasonic Distance & Parking Learning Kit
  Lesson 7 of 9: Smoothing Noisy Readings

  WIRING: the sensor from Lesson 1 (TRIG pin 9, ECHO pin 10).

  WHAT SHOULD HAPPEN
    Open Tools > Serial Plotter at 9600 baud. Three lines follow the target:
      raw     - every single reading (jumpy)
      average - the mean of the last 5 readings (smooth, but slow to react)
      median  - the middle of the last 5 readings (ignores single spikes)
    Wave a hand quickly through the beam and watch which line jumps.
*/

const int TRIG_PIN = 9;
const int ECHO_PIN = 10;

const int WINDOW = 5;                          // how many readings each filter remembers - try 3 or 9
const unsigned long ECHO_TIMEOUT_US = 30000;
const int MIN_CM = 2;
const int MAX_CM = 400;

long history[WINDOW];     // the last WINDOW good readings
int filled = 0;           // how many slots hold a reading so far
int nextSlot = 0;         // where the next reading goes

long readOnceCm() {
  digitalWrite(TRIG_PIN, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);
  unsigned long us = pulseIn(ECHO_PIN, HIGH, ECHO_TIMEOUT_US);
  if (us == 0) return 0;
  long cm = us / 58;
  if (cm < MIN_CM || cm > MAX_CM) return 0;
  return cm;
}

long averageOfHistory() {
  long total = 0;
  for (int i = 0; i < filled; i++) total = total + history[i];
  long average = total / filled;   // whole centimetres
  return average;
}

long medianOfHistory() {
  long sorted[WINDOW];
  for (int i = 0; i < filled; i++) sorted[i] = history[i];
  for (int i = 1; i < filled; i++) {          // sort, smallest first
    long value = sorted[i];
    int j = i - 1;
    while (j >= 0 && sorted[j] > value) {
      sorted[j + 1] = sorted[j];
      j--;
    }
    sorted[j + 1] = value;
  }
  return sorted[filled / 2];                  // the middle one
}

void setup() {
  pinMode(TRIG_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT);
  Serial.begin(9600);
}

void loop() {
  long cm = readOnceCm();
  if (cm > 0) {                                // skip "no echo" readings
    history[nextSlot] = cm;
    nextSlot = (nextSlot + 1) % WINDOW;        // go round in a circle
    if (filled < WINDOW) filled++;

    // "name:value,name:value" is the format the Serial Plotter draws.
    Serial.print("raw:");
    Serial.print(cm);
    Serial.print(",average:");
    Serial.print(averageOfHistory());
    Serial.print(",median:");
    Serial.println(medianOfHistory());
  }
  delay(80);
}

Upload it

1. Click Copy (or Download .ino and open the file).
2. In the Arduino IDE choose File → New Sketch, select everything in it and paste.
3. Check the bottom of the window says Arduino Uno and your port (Start here, step 5).
4. Click Upload (the → arrow) and wait for Done uploading.

How the code works

Change it and see

What you should see or hear

• Three lines - raw, average, median - that sit together while the book is still.
• A quick hand wave: raw jumps, average bumps, median barely moves.
• A smooth move: average and median follow a moment after raw.

If it doesn't work

Small experiment

Numbers 30, 30, 31, 120, 30. Work out the average and the median by hand. Which one is closer to the real 30 cm?

Check what you learned

Next

• Lesson 8 adds the OLED screen, so the distance shows without a computer screen.

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