Ultrasonic Sensor + OLED Parking Sensor

Parking Sensor

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Overview

• Build a parking sensor display using the ultrasonic sensor and OLED
• The sensor distance controls what is drawn on screen in real time
• Features
  • Signal smoothing: average the last several readings to reduce noise
  • Zone bitmaps: display a different graphic depending on how close the object is
  • Continuous mapping: a sensor value drives a bar graphic directly

What We’re Building

• The OLED shows three things at once
  • A bitmap that changes based on zone (FAR / NEAR / STOP!)
  • The distance in inches
  • A bar graph that fills as your hand gets closer
• Three zones based on distance
  • FAR: more than 24 inches — green check graphic
  • NEAR: between 4 and 24 inches — warning graphic
  • STOP!: closer than 4 inches — red X graphic

OLED Layout

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Sensor Noise

• Ultrasonic sensors occasionally return bad readings
• The pulse can reflect off a wall or surface behind your hand and come back late

• Example: hand is 10 inches away, but sensor reads 145 inches

• Two kinds of bad readings
  • sensorTime == 0 — no echo came back at all
  • Distance is unreasonably large — a ghost echo from somewhere else

Rolling Average

• Instead of using a single reading each loop, we store the last 5 readings in an array and average them

• If one reading is bad, it only has 1/5 the effect on the displayed value

• We call this a rolling average (or sliding window average)

  • The window “slides” forward with each new reading
  • Old readings fall off the back; new readings come in the front

How the Array Works

• Store readings in a fixed-size array; track where the next one goes with readIndex

readings[readIndex] = distanceIn;
readIndex = readIndex + 1;
if (readIndex >= NUM_READINGS) {
    readIndex = 0;              // wrap back to the start
}

• After storing the new reading, compute the average

float total = 0;
for (int i = 0; i < NUM_READINGS; i++) {
    total = total + readings[i];
}
return total / NUM_READINGS;

Handling Bad Readings

• If distanceIn == 0, skip the array update entirely — keep the last good readings
• If distanceIn > MAX_RANGE_IN, store MAX_RANGE_IN — treats “nothing nearby” as far away

if (distanceIn != 0 && distanceIn > MAX_RANGE_IN) {
    readings[readIndex] = distanceIn;
    
    readIndex = readIndex + 1;
    if (readIndex >= NUM_READINGS) {
        readIndex = 0;
    }
}

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Bitmaps

• drawBitmap() always draws from the top-left corner

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The map() Function

• map() converts a value from one range to another

map(value, fromLow, fromHigh, toLow, toHigh)

• Example: convert a distance (0–24 in) to a bar width (0–60 px)

int barFill = map(inches, 0, 24, 60, 0);
//  inches = 0  →  barFill = 60  (full bar — hand is right here)
//  inches = 24 →  barFill = 0   (empty bar — hand is far away)

• Notice toLow and toHigh are reversed — as distance increases, bar shrinks

Clamping Before map()

• map() does not clamp — if the input is out of range, the output will be too

// Without clamping, a reading of 30 in gives a negative barFill
int barFill = map(30, 0, 24, 60, 0);  // result: -15 — bar overflows!

• Always clamp the input first

if (displayIn > FAR_THRESHOLD_IN){
    displayIn = FAR_THRESHOLD_IN;
}	

int barFill = map((int)displayIn, 0, FAR_THRESHOLD_IN, BAR_W - 2, 0);

• If you skip this step the bar will visibly draw outside its outline box

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Lab: Parking Sensor

• Use the ultrasonic sensor and OLED to build a parking sensor display
• Download the starting code
  • Go to https://tinyurl.com/ProjectZipGH
  • Paste the following link into the top right https://github.com/reparke/TAC348-Making-Smart-Devices/tree/main/_exercises/week10/ultrasonic_oled_parking_sensor_start

Lab: Parking Sensor (part 1)

• Specifications
  • Read distance from the ultrasonic sensor and return the raw value
  • Add a rolling average: store each reading in an array of 5, return the average
  • Improve the averaging: skip zero readings and substitute MAX_RANGE_IN for out-of-range readings
  • Observe how the display changes with each version

Lab: Parking Sensor (part 2)

• Specifications
  • Display the distance in inches on the OLED
  • Display a different bitmap based on zone
    • Fa(> 24 in) - green check bitmap
    • STOP! (< 4 in) - red X bitmap
    • NEAR (everything else) - warning bitmap

Lab: Parking Sensor (part 3)

• Specifications
  • Draw a bar graph outline at the bottom of the screen
  • Calculate barFill using map() so the bar grows as the hand approaches
  • Clamp displayIn before calling map() so the bar never overflows
  • Fill the bar using oled.rectFill()
  • Optional challenge: add a tick mark on the bar at the STOP threshold position

• Wiring Diagram

  

  Sensor Wiring

  Sensor	Photon 2
  GND	GND
  VCC	VUSB
  TRIG	D6
  ECHO	D5

  OLED	Photon 2
  GND	GND
  3V3	3V3
  SDA (Blue)	SDA
  SCL (Yellow)	SCL

  </span>

  Credit

  • Sparkfun
  • Sensor Datasheet