Calibration

This example demonstrates one techinque for calibrating sensor input. The board takes sensor readings for five seconds during the startup, and tracks the highest and lowest values it gets. These sensor readings during the first five seconds of the sketch execution define the minimum and maximum of expected values for the readings taken during the loop.

Hardware Required

Arduino or Genuino board
LED
analog sensor (a photoresistor will do)
10k ohm resistor
220 ohm resistor
hook-up wires
breadboard

Circuit

Analog sensor (e.g. potentiometer, light sensor) on Analog input 2. LED on Digital pin 9.

click the image to enlarge

image developed using Fritzing. For more circuit examples, see the Fritzing project page

Connect an LED to digital pin 9 with a 220 ohm current limiting resistor in series. Connect a photoresistor to 5V and then to analog pin 0 with a 10K ohm resistor to ground.

Schematic

click the image to enlarge

Code

Before the setup, you set initial values for the minimum and maximum like so:

int sensorMin = 1023;        // minimum sensor value
int sensorMax = 0;           // maximum sensor value

These may seem backwards. Initially, you set the minimum high and read for anything lower than that, saving it as the new minimum. Likewise, you set the maximum low and read for anything higher as the new maximum, like so:

 // calibrate during the first five seconds 
 while (millis() < 5000) {
   sensorValue = analogRead(sensorPin);

   // record the maximum sensor value
   if (sensorValue > sensorMax) {
     sensorMax = sensorValue;
   }

   // record the minimum sensor value
   if (sensorValue < sensorMin) {
     sensorMin = sensorValue;
   }
 }

This way, any further readings you take can be mapped to the range between this minimum and maximum like so:

// apply the calibration to the sensor reading
sensorValue = map(sensorValue, sensorMin, sensorMax, 0, 255);

Here's the whole program:

/*
Calibration

Demonstrates one technique for calibrating sensor input. The sensor readings
during the first five seconds of the sketch execution define the minimum and
maximum of expected values attached to the sensor pin.

The sensor minimum and maximum initial values may seem backwards. Initially,
you set the minimum high and listen for anything lower, saving it as the new
minimum. Likewise, you set the maximum low and listen for anything higher as
the new maximum.

The circuit:
- analog sensor (potentiometer will do) attached to analog input 0
- LED attached from digital pin 9 to ground

created 29 Oct 2008
by David A Mellis
modified 30 Aug 2011
by Tom Igoe

This example code is in the public domain.

http://www.arduino.cc/en/Tutorial/Calibration
*/

// These constants won't change:
const int sensorPin = A0; // pin that the sensor is attached to
const int ledPin = 9; // pin that the LED is attached to

// variables:
int sensorValue = 0; // the sensor value
int sensorMin = 1023; // minimum sensor value
int sensorMax = 0; // maximum sensor value

void setup() {
// turn on LED to signal the start of the calibration period:
pinMode(13, OUTPUT);
digitalWrite(13, HIGH);

// calibrate during the first five seconds
while (millis() < 5000) {
sensorValue = analogRead(sensorPin);

// record the maximum sensor value
if (sensorValue > sensorMax) {
sensorMax = sensorValue;
}

// record the minimum sensor value
if (sensorValue < sensorMin) {
sensorMin = sensorValue;
}
}

// signal the end of the calibration period
digitalWrite(13, LOW);
}

void loop() {
// read the sensor:
sensorValue = analogRead(sensorPin);

// apply the calibration to the sensor reading
sensorValue = map(sensorValue, sensorMin, sensorMax, 0, 255);

// in case the sensor value is outside the range seen during calibration
sensorValue = constrain(sensorValue, 0, 255);

// fade the LED using the calibrated value:
analogWrite(ledPin, sensorValue);
}

[Get Code]

Calibration

Hardware Required

Circuit

Schematic

Code

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