Ultrasonic Sensor + OLED Parking Sensor Lab

Parking Sensor Lab

TAC 348 - Making Smart Devices

Lab Learning Objectives

By the end of this lab, you will be able to:

• Combine distance measurements and OLED graphics in one device
• Use a rolling average to reduce noise in sensor data
• Use distance thresholds to select text and bitmap graphics
• Use map() and clamping to convert distance into a bar width

Before You Begin

• You should be able to read distance with the ultrasonic sensor
• You should understand how a bitmap is stored and drawn on the OLED
• The starting project includes the basic sensor and OLED setup

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

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: Your hand is 10 inches away, but the sensor reads 145 inches
• Two kinds of bad readings
  • sensorTime == 0: no echo came back
  • Distance is unreasonably large: a reflection came from another surface

Rolling Average

• Instead of using a single reading each loop, store the last 5 readings in an array and average them
• If one reading is bad, it has only 1 / 5 of the effect on the displayed value
• This is 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
}

How the Array Works

• 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 the reading is zero or outside the reliable range, skip the array update and keep the last good readings

Handling Bad Readings: Corrected

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

Bitmaps

• drawBitmap() always draws from the top-left corner
• Use a half-screen bitmap so there is room for the distance and bar graph
• Choose the bitmap after comparing the distance to the FAR and STOP thresholds

The map() Function

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

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

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

int barFill = map(inches, 0, 24, 60, 0);
// inches = 0:  barFill = 60 (full bar; hand is close)
// inches = 24: barFill = 0  (empty bar; hand is far away)

• Notice toLow and toHigh are reversed: as distance increases, the 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

• 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 can draw outside its outline box

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 and return the average
  • Improve the averaging: skip zero and out-of-range readings so the array keeps the last good values
  • 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
    • FAR (> 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

Completion Checklist

• Distance readings are reasonably stable
• The distance is displayed in inches
• The bitmap changes in the FAR, NEAR, and STOP zones
• The bar grows as the object moves closer and stays inside its outline

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

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Credit

• SparkFun
• Sensor Datasheet