Distance Sensor Sharp 2YOA21 & LCD

The distance sensor produced by Sharp is a popular and relatively low cost solution for measuring distance. The sensor can be used also for measuring the rightness or color (in very limited extent) but in this article we will talk only about measuring distances.
In the current experiment we will use the 2YOA21 model and the characteristics of this sensor are:

1. Minimum Measuring Distance = 10cm;
2. Maximum Measuring Distance = 80cm;
3. Infrared Proximity Sensor;
4. Analog output inversely proportional to distance;
5. Sensor is Ratiometric;
6. Operating Supply Voltage = 4.5V to 5.5V;
7. Average Supply Current – Typical = 30mA;
8. Response Time = 38 ± 10ms.

Obviously there are several versions of sensors which are capable to measure distances shorter or longer, depending on needs.
Examples of sensors are:

• Sharp Distance Sensor GP2D120 (4-30cm);
• Sharp Distance Sensor GP2Y0A21 (10-80cm);
• Sharp Distance Sensor GP2Y0A02 (20-150cm).

The data signal of sensor 2YOA21 are sent in analog way and the connection made with the microcontroller is very simple. The analog output varies from 3.1V at 10cm to 0.3V at 80cm.

Hardware setup:

In the current article I intend to display the measured distance, between sensor and some object, on the LCD with 2x16 characters.
Physical realization is carried out on the breadboard with 2420 dots.

Trough this video I would like to share the situation in real time.

Now, I will share a little bit of theory, theory needed to put things in motion.

Below I present the timing diagram for data signal transmitted on pin 1 of the sensor 2YOA21.

Timing diagram.

The following diagrams are very useful when we want to construct the formula for measuring distance.

The distance vs. output voltage.

Above we have the diagram that specify the relationship between the distance measured and the output voltage of 2YOA21 sensor.

The output voltage vs. inverse number of the distance.

Above we have the diagram that specify the relationship between the output voltage and the inverse number of distance sensor 2YOA21.

Pin description of 2YOA21 distance sensor.

Pinout.

Below we have the diagram block of 2YOA21 distance sensor.

Block diagram.

In the datasheet, the manufacturer recommend us how to align properly, the distance sensor by the measured surface.
Below I will present you trough two pictures how to do that.

Proper Alignment to Surface Being Measured

Proper Alignment to Moving Surfaces.

Notes:
Before we put the toy in operation we must take into consideration the following tips:

• Must be kept the sensor lens clean. The dust, water, oil, and other contaminants can deteriorate the characteristics of this device;
• When using a protective cover over the emitter and detector, ensure the cover efficiently transmits light throughout the wavelength range of the LED (λ = 850 nm ± 70 nm). Both sides of the protective cover should be highly polished;
• Objects in proximity to the sensor may cause reflections that can affect the operation of the sensor;
• Sources of high ambient light (the sun or strong artificial light) may affect measurement. For best results, the application should be designed to prevent interference from direct sunlight or artificial light;
• Using the sensor with a mirror can induce measurement errors. Often, changing the incident angle on the mirror can correct this problem;
• The manufacturer recommends a 10 μF (or larger) bypass capacitor between VCC and GND near the sensor.

For more details, please study the Sharp 2Y0A21YK datasheet.

Circuit Diagram:

Difficulty level of the electronic scheme, is low. The microcontroller used is PIC16F876A because he posses the analog to digital converter, conversion required for the signal processing collected from our distance sensor.
S1 is the master reset button, R1 is the resistor of pull-up button.
Cristal quartz by 8 MHz, is used.
C3 and C4 are used for additional filtering of the sensor 2YOA21.
Trough J1 we ensure the connectivity with distance sensor.
ICSP connector is used to program the microcontroller.
Trough R2 we can adjust the contrast for LCD with 2x16 characters.
R3 adjusts the current through the LED LCD (light intensity of it).

The electronic schematic.

The electronic scheme is built in Eagle Cad, free version.

Software:

The program is written in mikroC Pro for PIC 2011 (version v4.60).
Until this moment, I found 3 ways to extract the formula that converts the sensor output voltage in the distance. I will show below:

1. Based on "typical values" from Sharp, the formula to translate sensor value into distance (the formula is only valid for a sensor value between 80 to 500) is:
   
   
2. Analyzing the diagrams below we can deduce:
   
   
   
   Calibration curves of 2YOA21 sensor.
   
   This equation was derived from the calibration curve:
   
   
   
   
   where “IR Value” is the digital output signal from the sensor.
   The curve was created by plotting output values of the IR sensor versus distances to a stationary, flat object.
   
   Here is the diagram of 2YOA21 sensor values at ranges between 0cm and 150cm:
   
   
   
   Sharp Sensor Data.
   
   
3. And now I will present the formula used in the current article.
   As I said above, the analog voltage is the inverse of distance, so distance can be calculated as:
   
   
   
   then scaled to suit with the datasheet.
   We will load ADC value in 16 bit “math” variable and we will use in this way:
   
   
   
   for scaling, 6050 is a constant.

All three methods are tested by me, so you can have confidence in their accuracy. I chose the latter method because it seemed more interesting, nothing more.
For those who are extremely pretentious can develop a look-up table from data points supplied from the graph, but if you choose this route should be considered the RAM memory of microcontroller, must be big enough to host the entire data table.

Below is my software version:

/*
'*******************************************************************************
'  Project name: Distance Sensor Sharp 2YOA21 & LCD Display
'  Description:
'          In this experiment we will try to display on LCD 2x16 character, 
'          distance helped by IR senzor "Sharp 2Y0A21YK" wich is able to give
'          distance betwen 10cm~80cm.
'  Written by:
'          Aureliu Raducu Macovei, 2011.
'  Test configuration:
'    MCU:                        PIC16F876A;
'    Test.Board:                 WB-106 Breadboard 2420 dots;
'    SW:                         MikroC PRO for PIC 2011 (version v4.60);
'  Configuration Word:
'    Oscillator:                 HS (8Mhz)on pins 9 and 10;
'    Watchdog Timer:             OFF;
'    Power up Timer:             OFF;
'    Browun Out Detect:          ON;
'    Low Voltage Program:        Disabled;
'    Data EE Read Protect:       OFF;
'    Flash Program Write:        Write Protection OFF;
'    Background Debug:           Disabled;
'    Code Protect:               OFF
'*******************************************************************************
*/

// LCD module connections;
sbit LCD_RS at RB0_bit;
sbit LCD_EN at RB1_bit;
sbit LCD_D4 at RB4_bit;
sbit LCD_D5 at RB5_bit;
sbit LCD_D6 at RB6_bit;
sbit LCD_D7 at RB7_bit;

sbit LCD_RS_Direction at TRISB0_bit;
sbit LCD_EN_Direction at TRISB1_bit;
sbit LCD_D4_Direction at TRISB4_bit;
sbit LCD_D5_Direction at TRISB5_bit;
sbit LCD_D6_Direction at TRISB6_bit;
sbit LCD_D7_Direction at TRISB7_bit;
// End LCD module connections;

unsigned int cm,cm10;
unsigned int math;
char *text;
char *digit = "00";

void calc_distance(void)
{
 // from the datasheet the analog voltage is
 // the inverse of distance, so distance can be calculated
 // d = (1 / volts) then just scaled to suit the datasheet
 // load ADC value in 16bit math variable.
 math = ADC_Read(0);
 math = ADRESH;
 math = (math * 255);
 math += ADRESL;
 // now invert it; (1 / volts) use (6050 / volts) for scaling, 6050 is a constant;
 math = (6050 / math);
 if(math >= 2) math -= 2;   // fix linear error (-2)
 if(math < 10) math = 10;   // min limit at 10cm
 if(math > 80) math = 80;   // max limit at 80cm
 // convert from 0-99 to 2 decimal digits, 0-99cm
 cm10=0;
 while(math >= 10)
 {
  cm10++;
  math -= 10;
  }
 cm = math;
}

void main(void)
{
 ADCON0 = 0X01;              // RA0 as Analog Input;
 ADCON1 = 0X0E;              // Reference VDD and VSS, with left justified;

 CMCON |=7;                  // Disable comparator;

 TRISA0_bit = 1;             // Make RA0 as input;
 PORTA = 0x00;               // Initial value of PORTA;

 LCD_Init();                 // Initialize the LCD;
 LCD_Cmd(_LCD_CURSOR_OFF);   // Cursor mode off;
 LCD_Cmd(_LCD_CLEAR);        // Clear the entire LCD;

 text = "Sharp 2Y0A21";
 LCD_Out(1,3,text);
 text = "Distance:";
 LCD_Out(2,1,text);
 text = "cm";
 LCD_Out(2,13,text);

 while(1)
 {
  calc_distance();            // Call the "calc_distance" function;

  digit[0] = cm10 + 48;       // tens digit;
  digit[1] = cm +48;          // unit digit;
  Lcd_Out(2,10,digit);        // display on LCD from column 2, from character 10;
  delay_ms(100);
 }
}