Ultrasonic Distance Sensor + OLED Graphics

Ultrasonic Distance Sensor + OLED Graphics

TAC 348 - Making Smart Devices

Learning Objectives

• Explain how an ultrasonic sensor uses sound and echo time to measure distance
• Use pulseIn() and the speed of sound to calculate distance
• Explain how monochrome images are stored as bitmap arrays
• Prepare a bitmap for a 64 x 48 OLED screen
• Describe how sensor data can control text and graphics on a display

Ultrasonic Distance Sensor

Ultrasonic Distance Sensor

• Like SONAR!
• Sends 40 KHz sound pulses (higher than human hearing)
• Waits for return sound wave (“echoes” or bounces off nearby objects)
• Distance to object can be calculated

Uses

• Auto-range finder
• Self-parking cars
• Obstacle avoidance
• Autonomous vehicles
• Fun fact: This is how dolphins and bats navigate

Parameters

• Operating range
  • Officially: 2 cm - 4 m (1 in - 13 ft)
  • More reliable range: 5 cm - 2 m (2 in - 6.5 ft)
• Angle
  • 15 degrees

Sensor Pins

Sensor	Photon 2	Function
GND	GND	Ground
VCC	VUSB	Power (requires 5V, but will work with a 3.7V LiPo battery)
TRIG	Output Pin	Start output pulse sequence
ECHO	Input Pin	Receive reflection response

Timing Diagram

Timing Part 1: Trigger

• Output sequence
  • LOW for 2 microseconds
  • HIGH for 10 microseconds
  • LOW

delayMicroseconds(<<DELAY_VAL>>);

• Sensor sends out 8 sonic pulses

Timing Part 2: Echo

• Sensor “listens” for the sound wave to reflect / bounce off an object
• When the reflection is received, the ECHO pin goes HIGH for the duration of the reflection and then goes LOW
• Use pulseIn() to measure how long the input remains HIGH

Measure Time with pulseIn()

• Syntax

// Measure time in microseconds
// Returns 0 if no signal is received before the timeout
int time = pulseIn(<<PIN>>, <<VALUE>>);

• Example

// Start timing when D2 is HIGH
// Stop timing when D2 is LOW
int time = pulseIn(D2, HIGH);

Calculating Distance

• pulseIn() returns the round-trip time for the sound to reach an object and return
• Speed of sound at 20°C (68°F) = 343.5 m / s
• How do we calculate the one-way distance?

Calculating Distance: Formula

• Speed of sound = 0.03435 cm / microsecond
• Divide by 2 because the measured time includes the trip to the object and back

distanceCm = echoTime * 0.03435 / 2;

distanceIn = distanceCm * 0.393701;

Cautions

• Sound waves can reflect off surfaces in the room and give incorrect readings
• Air conditioning vents and other nearby ultrasonic sensors can cause interference
• There should be a 500 ms delay between readings

OLED Screen Graphics

Screen Parameters

• OLED screen consists of 64 (W) x 48 (H) pixels (3,072 total pixels)
• 3,072 pixels means 3,072 bits (ON or OFF) are needed to display a full image
  • 3,072 bits is 384 bytes (8 bits = 1 byte)
• Each pixel is either ON (HIGH) or OFF (LOW) because there is only one color
• We can display images on the screen in a bitmap format

Pixels

Original Image	Image Closeup to Show Pixels

Bitmap

• Bitmaps are stored as large arrays of bytes
• In a monochrome image, there is one bit per pixel
  • 64 pixel x 48 pixel images = 3,072 bits = 384 bytes
• Example:

const uint8_t heart_bmp[] = {
  0x00, 0x00, 0x00, 0x00,
  0x00, 0x00, 0x00, 0x80,
  0xe0, 0xf0, 0xf8, 0xfc,
  ... };

Creating Bitmaps

• Use a tool to convert an image to a byte array
• Color restrictions
  • Each image should be black and white
  • Color and greyscale can work, but not well
• Image size
  • Use small or actual-size images for better conversion to a bitmap

Example Images

Original Black / White Image	Bitmap on OLED

• White is the color that will be displayed on the OLED

Image Size vs. Canvas Size

• The source image may have different dimensions than the OLED
• The canvas is the complete 64 x 48 pixel area sent to the screen
• The converter scales or positions the source image within that canvas
• Blank areas of the canvas are still represented in the byte array

Settings

• Note
  • The following settings are specific to the SparkFun Micro OLED
  • The SparkFun library requires each bitmap to be specified as 64 x 48 pixels or 384 bytes (canvas size), even if parts of the image are blank
• Image settings
  • Canvas size: 64 x 48
  • Glyph: blank
  • Scaling: Scale fit, keeping proportions

Settings

• Output
  • Code output format: Arduino code, single bitmap (later in the source, change the declaration to const uint8_t <PUT BITMAP NAME HERE>[])
  • Draw mode: Vertical - 1 bit per pixel

Example: Converting an Image with Image2CPP

Image2CPP Settings

Image2CPP Preview

Image2CPP Output

Storing Byte Array

• Create a const uint8_t array (byte array)

const uint8_t heart_bmp[] = {
  0x00, 0x00, 0x00, 0x00,
  0x00, 0x00, 0x00, 0x80,
  0xe0, 0xf0, 0xf8, 0xfc,
  ... };

• Use the library to display the bitmap

Tools for Converting Images to Bitmaps

• Online: Image2CPP
• Windows: LCD Assistant
• Mac: bitmapToC

Connecting Sensor Input to Display Output

1. Trigger the ultrasonic sensor
2. Measure the echo time and calculate distance
3. Use the distance to make a decision
4. Draw the matching text or bitmap
5. Update the OLED

Credit

• SparkFun Ultrasonic Distance Sensor
• Sensor Datasheet