Resistance from Color Codes

Introduction

The program is designed so as to calculate the resistance of a resistor based on the color bands that the resistor has. Color bands are used as a simple way of representing the resistance values especially since indicating the figures may not be a feasible alternative, owing majorly to the small size of the resistors. The colors that are normally used in resistors (and their corresponding values) are: Black (1), Brown (2), Red (3), Orange (4), Yellow (5), Green (6), Blue (7), Violet (8), Gray (9), White (10). The color bands in a four band resistor each serves a special function, with the first two bands serving as an indicator for the numeric value of the resistor. The third band serves to denote the exponent to base 10 (power of ten multiplier) of the resistance value. The final band is used to denote the tolerance value, with the most common values for 4 band resistors being Gold (±5%), Silver (±10%) and ±20% when there is no 4^th color band.

Project Objective

            The objective of this project is to create a simple C-based program that will calculate the values of resistance for a 4-band resistor based on the colors of the bands. The program takes in the textual representation of the colors, performs calculations based on the values of these colors and returns the computed values to the code that called it. Or simply, the program will get the colors as input and give the resistance as output. The program serves to give an easy and efficient way to calculate resistance values with minimal computation and maximum accuracy

Methodology

The program first begins by prompting the user to state whether they want to calculate resistance for 4-band resistors based on their color values. If they accept (choose ‘yes’), the program prompts the user for inputs, specifically the color codes for the 4-bands on the resistor. The colors are represented internally as (an array of) characters. Along with the prompt, the program issues some directions to ensure that the values are entered correctly, and the colors chosen are from a subset of valid colors. Following the choice, the program gets the color codes and runs them through a search function that ensures the user input is of the right type, and that the colors are in the array that is used to store valid resistor band values. If valid, the program uses the color codes for calculation of resistance. Resistance is calculated as

Resistance = ((10*first color band + second color band )*(pow(10,third color band)))

The tolerance is calculated based on the fourth color band. Tolerance is calculated as a positive or negative factor on the resistance value, that is if the band denotes a tolerance value of 5%, then the resistance value will range from between 95-105% of the resistance.

Resistance Range = Resistance Value ± Tolerance

The program then outputs the relevant values for resistance and resistance range.

Algorithm Development

Pseudocode

DECLARE variables colorBand1 through to colorBand 4

WHILE user  wants to calculate the resistance for a resistor

PROMPT user for inputs colors

GET input colors

SEARCH for colors in array

IF colors are valid members of the array

            CALCULATE resistance

            DISPLAY resistance

            CALCULATE tolerance

            DISPLAY resistance range

END IF

END WHILE

EXIT

Flowchart

C-program Code

#include <stdio.h>

#include <stdlib.h>

#include <math.h>

#include <string.h>

int search (char codes[10][7], char target[], int size);

int main (void)

{

    char COLORCODES [10][7] = {“black”, “brown”, “red”, “orange”, “yellow”, “green”, “blue”, “violet”, “gray”, “white”};

    float resValue;

    char colorBand1[7];

    char colorBand2[7];

    char colorBand3[7];

    char colorBand4[7];

    int i,j,k;

    char question;

    printf(“\nDo you want to run the program now (Type ‘y’ for yes, or ‘n’ for no)?\n “);

    scanf(“%c”, &question);

    while (question == ‘y’)

    {

      printf(“\nEnter the colors of the resistor’s four bands. Type the colors in lowercase letters only. “);

      printf(“\nThe color for the three first bands are as follows\n\n”);

      printf(“\tblack\n\tbrown\n\tred\n\torange\n\tyellow\n\tgreen\n\tblue\n\tviolet\n\tgray\n\twhite\n”);

      printf(“\nThe colors for the tolerance band are as follows\n”);

      printf(“\n\tgold\n\tsilver\n\tnone\n\n”);

      printf (“Enter the first Band:”);

      scanf(“%s”,&colorBand1);

      printf (“Enter the second Band:”);

      scanf(“%s”,&colorBand2);

      printf (“Enter the third Band:”);

      scanf(“%s”,&colorBand3);

      printf (“Enter the fourth Band:”);

      scanf(“%s”,&colorBand4);

      i = search (COLORCODES, colorBand1, 10);

      j = search (COLORCODES, colorBand2, 10);

      k = search (COLORCODES, colorBand3, 10);

      resValue = ((10*i+j)*(pow(10,k)));

      if (i == -1 || i == 0)

      {

         printf (“Invalid colors : %s”, i);

      }

      if (j == -1 )

      {

         printf (“Invalid colors : %s”, j);

      }

      if (k == -1 )

      {

         printf (“Invalid colors : %s”, k);

      }

      else

      {

         printf (“Resistance value: %.0f ohms\n”, resValue);

      }

      double tolerance;

        if(colorBand4 == “gold”){

            tolerance = .05*resValue;

        }else if(colorBand4 == “silver”){

            tolerance = .1*resValue;

        }else{

            tolerance = .2*resValue;

        }

      printf(“\nResistance value range is between %.2f and %.2f Ohms”, resValue-tolerance, resValue+tolerance);

      printf(“\nDo you want run the program again (Type ‘y’ for yes, or ‘n’ for no)? \n”);

      scanf(” %c”, &question);

    }

system(“pause”);

return 0;

}

int search (char codes[][7], char target[], int size)

{

    int x = 0;

    int match = 0;

    int location = 0;

  while (!match && x < size)

       {

              if(strcmp(codes[x], target) == 0)

              {

                     match = 1;

              }

              else

              {

                     ++x;

              }

       }

              if(match == 1)

              {

                     location = x;

              }

              else

              {

                     location = -1;

              }

       return(location);

}

Testing and Results

Testing is done to ensure that the operation is as expected. The program is expected to give values that are consistent with what a manual calculation of the same will give. The tests that the program has to undergo include that it only accepts the right type of data, returns the right type of data and handles computation correctly. Final test involves ensuring that the loops in the program are coded correctly and logic does not fall through.

The test data for the program is as follows

#	COLOR BAND	VALUE
1	Brown	1
2	Black	0
3	Red	103
4	Silver	±10%

Table 1. Test data

From the test data, resistance should be calculated as ((1*10+0)*(10³) ±10%). The result that is expected is a resistance value of 1000 Ohms before factoring in tolerance. With tolerance, the value is expected to range between 900-1100Ohms.

The results of running the program on a similar set of data is as follows:

Fig 1. Computation results     

The program results are found to be consistent with the values that are expected for the test results.

Conclusion

The program is found to be an accurate way to get a quick assessment of the resistance value of a resistor based on the color bands that are printed on it. With this program, quick and accurate assessments of resistance values can be achieved. With a little extension, the program can be modified to carry out the same kind of computation on 3-band and 5-band resistors. Evidently, this is a handy tool for the said computation.