Question

Problem 10.6 A conveyor belt is used for transporting two different products (item A and item B) of different physical sizes (item B is longer than item A). In order to distinguish the items from each other, we can record how long the signal from the photocell stays high. Make the code for a program (or a FB if you wish) that implements this and counts the number of each type. (You can determine for yourself how long the signal from the photocell must be high in order for the product to be interpreted as type B.)

Answer 1

Since no information is given about the platform, I would be writing an ardunio code for the given problem. Property of photocell(or Light Dependent Resistor(LDR)) is that it changes its resistance depending upon the intensity of light falling on it. If the intensity of light falling on the device increases, its resistance decreases resistance and if the intensity of light falling on the device decreases, its resistance increases. Hence, we can say that intensity of light and value of resistance of photocell are inversly proportional. We will use this fact to our advantage.

Hardware description :-

OUTPUT SIGNAL VCC vec REGENDE LIGHT DEPENDENT RESISTOR RESISTOR GND

As you can see, we have created a voltage divider circuit using a resistor and a light dependent resistor(photocell). The value of output signal, Vo will be :-

$Vo = \frac{Ro}{Ro + Rldr} Vcc$

where

Vcc : constant supply voltage

Ro : Value of constant resistance

Rldr : Value of resistance of photocell

From the formula we can see that as the Value of Vo is inversly proportional to the value of Rldr, and Rldr is inversly proportional to the intensity of light. Hence, value of Vo is directly proportional to the intensity of light. This means that the value of Vo will increase as the intensity of light increases and value of Vo will decrease as the intensity of light decreases. We will use this fact while writing the code. All the functions in the code are self explainatory. Comments have been added for the better understanding of the concepts. Still if you nees any help, feel free to ask in the comment section. Hope it helps :)

Code :-

// variables c and p define the state of the photocell, c = state of photo cell in current iteration
  // p = state of photocell in previous iteration. state = 1 means device is illuminated
  // state = 0 means device is not illuminated.
  int c = 0;
  int p = 0;
  void setup() {
    // put your setup code here, to run once:
    Serial.begin(9600);
    pinMode(A3, INPUT); // defining input pin as A3
  }
  
  // In an arduino we have a 10 bit ADC which convert analog values to the digital values
// from 10 bits we get a range of 0 to 1023. This means it maps 0V to 5V to 0 to 1023.
// Hence, when we say Vo = 250, we mean Vo = 250 * (5/1024) V.
  void loop() { // we repeatedly check the status of the photocell.
    // put your main code here, to run repeatedly:
    int Vo = analogRead(A3); // read value at pin A3
    if(Vo < 250) { // 250 is an experimental value and can change depending upon the hardware setup.
      c = 0; // if value of Vo < 250 it means device is not illuminated.
    } else {
      c = 1; // if value of Vo < 250 it means device is not illuminated.
    }

// if p == 0 and c == 1, this means that in last iteration light was not falling on the the device(p=0)
// and in this iteration light is falling on the device(c=1). This means we need to start the timer.
    if(p == 0 && c == 1) { // this means that light has just started to fall on the device
      // we store the timer starting time in s.
s = millis(); // millis() function returns the time that has been passed since the device is on in miliseconds.
    } else if(p == 1 && c == 0) {
// if p == 1 and c == 0, this means that in last iteration light was falling on the the device(p=1)
// and in this iteration light is not falling on the device(c=0). This means we need to stop the timer.
      // we store the timer stoping time in e.
e = millis();
      unsigned long total_time = e - s;
      decoder(total_time); // decoder functions decodes whether the item was A or B.
      s = 0; // reset the values of s and e
      e = 0;
    }
    p = c; // previous states takes the value of current state before the next iteration.
  }

void decoder(unsigned long total_time) {

// these are just random values, they may change depending upon the system setup
unsigned long time_for_A = 1000;
unsigned long time_for_B = 2000;
if(total_time == time_for_A) {
Serial.println("It was item A");
}

if(total_time == time_for_B) {
Serial.println("It was item B");
}
}

For Indentation :-

// variables c and pdefine the state of the photocell, c = state of photo cell in current iteration // p = state of photocell in previous iteration. state = 1 means device is illuminated // state = O means device is not illuminated. int c = 0; int p = 0; void setup() { // put your setup code here, to run once: Serial.begin(9600); pinMode (A3, INPUT); // defining input pin as A3 // In an arduino we have a 10 bit ADC which convert analog values to the digital values // from 10 bits we get a range of 0 to 1023. This means it maps Ov to 5V to 0 to 1023. // Hence, when we say Vo = 250, we mean Vo = 250 * (5/1024) V.

void loop() { // we repeatedly check the status of the photocell. // put your main code here, to run repeatedly: int Vo = analogRead (A3); // read value at pin A3 if (Vo < 250) { // 250 is an experimental value and can change depending upon the hardware setup. c = 0; // if value of Vo < 250 it means device is not illuminated. } else { c = 1; // if value of Vo < 250 it means device is not illuminated. // if p == 0 and c == 1, this means that in last iteration light was not falling on the the device (p=0) // and in this iteration light is falling on the device (c=1). This means we need to start the timer. if (p == 0 && c == 1) { // this means that light has just started to fall on the device // we store the timer starting time in s. s = millis(); // millis() function returns the time that has been passed since the device is on in miliseconds. } else if (p == 1 && c == 0) { // if p == 1 and c == 0, this means that in last iteration light was falling on the the device (p=1) // and in this iteration light is not falling on the device (c=0). This means we need to stop the timer. // we store the timer stoping time in e. e = millis(); unsigned long total time = e - s; decoder (total_time); // decoder functions decodes whether the item was A or B. S = 0; // reset the values of sand e e = 0; p = c; // previous states takes the value of current state before the next iteration.

void decoder (unsigned long total_time) { // these are just random values, they may change depending upon the system setup unsigned long time_for_A = 1000; unsigned long time for B = 2000; if(total_time == time_for_A) { Serial.println("It was item A"); if (total_time == time_for_B) { Serial.println("It was item B");