Mapping Values Without Using map() in Arduino

When working with Arduino, the map() function is a handy tool for scaling values from one range to another. However, there are times when you might want to achieve this without using the built-in function—either to understand the underlying logic or to handle custom cases. This post will guide you through the process of manually mapping values step by step.

Understanding the Basics

The goal of mapping is to scale an input range [input_start,input_end] to an output range [output_start,output_end].

For example:

• Input range: [0,1023](e.g., an analog sensor)
• Output range: [[0,255] (e.g., LED brightness for PWM)

To map an input value input from the input range to the output range, we need a formula.

The Formula for Mapping

The formula for mapping values manually is:

Here’s how it works:

1. Shift Input Range to Zero: Subtract input_start from the input value.
2. Calculate the Scaling Factor: Divide the difference of the output range by the difference of the input range.
3. Scale the Input: Multiply the shifted input by the scaling factor.
4. Shift to Output Range: Add output_start to the scaled value.

Simplifying with a Constant Slope

To optimize for repeated calculations:

• Precompute the slope:

The formula becomes:

output = output_start + slope × (input − input_start)

Writing the Code

Here’s a practical implementation in Arduino:

double calculateMappedValue(int input, int input_start, int input_end, int output_start, int output_end) {
  double slope = 1.0 * (output_end - output_start) / (input_end - input_start);
  return output_start + slope * (input - input_start);
}

void setup() {
  Serial.begin(9600);
}

void loop() {
  int sensorValue = analogRead(A0);  // Example sensor reading (0-1023)

  // Map the sensor value manually to 0-255
  double mappedValue = calculateMappedValue(sensorValue, 0, 1023, 0, 255);

  // Print the results
  Serial.print("Sensor Value: ");
  Serial.print(sensorValue);
  Serial.print(" -> Mapped Value: ");
  Serial.println(mappedValue);

  delay(500);  // Small delay for readability
}

Handling Edge Cases

• Floating-Point Precision:
  To ensure accurate results, use floating-point calculations:

double slope = 1.0 * (output_end - output_start) / (input_end - input_start);

• Rounding the Output:

If the output needs to be an integer, apply rounding:

#include <math.h>
double roundValue(double value) {
  return floor(value + 0.5);  // Round to nearest integer
}

Update the formula:

output = output_start + roundValue(slope * (input - input_start));

Example Use Case: LED Brightness

Suppose you want to control an LED’s brightness using a light sensor. The sensor outputs values from [0,1023], and the LED brightness needs to range from [0,255]. Here’s the implementation:

void loop() {
  int sensorValue = analogRead(A0);  // Read LDR value
  double brightness = calculateMappedValue(sensorValue, 0, 1023, 0, 255);
  
  analogWrite(9, (int)brightness);  // Set LED brightness
  delay(100);
}

Conclusion

Manually mapping values gives you more control and flexibility, especially for custom applications. By understanding the process, you can handle edge cases, optimize for precision, or adapt the logic for non-linear mapping.

For more Arduino tips and tutorials, visit diyusthad.com. Share your thoughts or questions in the comments below! 😊