Published February 15, 2019 © GPL3+

Is There Light?

Examining the use of photo resistors, also known as Light Dependent Resistors, as a vehicle to understanding timer interrupts in UNO.

BeginnerProtip15 minutes1,185

Things used in this project

Hardware components

Elegoo Arduino UNO Rev 3

Elegoo Photoresistor

Elegoo Resistor, 10 Kohms

Elegoo DuPont connecting wires

Elegoo Breadboard, any size

Arduino Uno Mega 2560, Raspberry Pi RAB 5 in 1 Breadboard Holder

Software apps and online services

Arduino IDE

Story

Introduction

This note examines the use of photo resistors, also known as Light Dependent Resistor, as an introduction to measuring light intensity. A photo resistor is often referred to as light dependent resistor owing to the fact that its resistance ranges from low to high corresponding to the intensity of the light exposed to it. When the light is high, the resistance is low. When the light is low, the resistance is high. There is a nominal lag in the response of the device from the exposure to light.

Photo (Light Dependent) Resistor

Applications

The primary applications for this sensor is in the following area but not limited to:

Photometry:
Light meters
Flash control
Street lighting control
Home automation:
Safety (smoke, flame and burglar) detectors
Room lighting
Surveillance cameras
Automobile applications:
Headlight dimmer
Rear view mirror
Obstacle detection
Industrial:
Position sensor
Control
Ambient light
Electronic toys
Annunciation

The resistance of the device changes inversely to light striking it. This change is proportional but non-linear. The device relies on the properties of cadmium sulfide for this phenomenon. There are regulatory statutes regarding the commercial use of this device owing to environmental concerns regarding the element cadmium. Restrictions on Hazardous Substances (RoHS) labels are applied on the packaging when applicable. The spectral response curve of cadmium sulfide is similar that of the human eye.

Specification

The following table summarizes the key characteristics of the device. There are many minor variations in the popular models. The reader should consult the corresponding data-sheets for more detailed information.

ParameterDescription

Peak wavelength 600 nm

Resistance at 10 lux min 1.8 K ohms, max 4.5 K ohms

Resistance at 100 lux 0.7 K ohms

Power dissipation 50 mW

Library

The use of the standard Arduino library call to read values from an analog pin that provides a value proportional to the resistance of the connected device.

AnalogRead

The standard function, analogRead, reads a value from an analog pin. The conversion of the analog signal to digital data is performed by the board through a 10-bit analog-to-digital converter. Owing to the latency of the operation, on the Arduino UNO board, this value cannot be read more than 10, 000 times per second since it requires approximately 100 µs to complete the operation.

The Arduino UNO board has six analog pins labeled from A0 to A5. The corresponding pin numbers for this function range from 0 to 5.

Usage

The assembly shown above will apply equally for the test cases in this elementary exercise:

Continuous polling of data
Interrupt driven data processing
Within interrupt handler
Inside the standard loop function

Continuous polling

This method is the simplest to understand since an explicit hold is placed by invoking the standard delay function directly as shown below:

void loop()
{
  Serial.println(analogRead(pinPHOTOsensor)); // read/write directly for serial plotter
  delay(1000);              // 1,000 ms = 1 sec
}

Interrupt handler routine

The use of the delay function may be expedient but there some handicaps for the duration that this function is active such as:

Other pin readings are suspended
Pin operations are blocked
Calculations cannot proceed

In practical terms, the activities crawl to a halt. For this reason, it may be preferable to use the intrinsic timing functions of the board to perform an operation at a regular frequency. Once an interrupt is setup, the board will invoke the corresponding service routine at the specified interval where custom operations may be performed. (The other type of interrupt will be examined in a subsequent note on other sensors where autonomous detection may be desirable). Please consult the references section for links to information on the use of interrupts and their associated handler routines.

There are two examples to illustrate the interrupt handler method:

Reading within handler
Reading outside handler

Reading within handler

This method does not require any operation directly within the standard loop function. The reading (and writing) of the data is performed within the interrupt service handler routine:

void loop()
{
}

Reading outside the handler

This method is primarily intended to avoid long running instructions within the interrupt handler routine. The interrupt service routine sets a flag which is examined in the loop function to perform the processing of the data from the photo-resistor.

void loop()
{
  if (interrupted)
  {
    interrupted = false;
    Serial.println(analogRead(pinPHOTOsensor)); // for use on serial line, stay tuned
  }
}

Results

References

Photosensitive resistors series

TO-18Ceramic Package Photocell

Simple test of the functionality of the photo resistor

Luminosity sensor Photoresistor GL5528 LDR THT for Arduino

Arduino Timer and Interrupt Tutorial

Arduino Timer Interrupt ISR example

Schematics

Code

/*
 * PhotoSensor-01.ino
 * A basic exercise to read the data from a photo-resistor continuously
 * 2018-10-12
 * armw
 * v0.2
 * © 2018 <reza@parkcircus.org> All Rights Reserved
 *  
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 * 
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 * 
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
 * MA 02110-1301, USA.  
 * 
 * Reference:
 * https://playground.arduino.cc/Learning/PhotoResistor
 */

/* Simple test of the functionality of the photo resistor

Connect the photoresistor one leg to analog pin 0, and pin to +5V
Connect a resistor (~10K should suffice) from analog pin 0 to GND

----------------------------------------------------

           PhotoR     10K
 +5     o---/\/\/--.--/\/\/---o GND
                   |
 Pin A0 o----------

----------------------------------------------------
*/

// 2019-02-16 v.2 armw pin mnemonic correction suggested by Peter Mansvelder
int pinPHOTOsensor = A0;     // analog pin 0 for the sensor - thx PeterM

void setup()
{
    Serial.begin(115200);  // use serial communcation for data display
    pinMode(pinPHOTOsensor, OUTPUT ); // redundant?
}

void loop()
{
  Serial.println(analogRead(pinPHOTOsensor)); // for use on serial line, stay tuned
  delay(1000);              // nominal expedient way, for now
}

/*
 * PhotoSensor-02.ino
 * Using compare and match timer interrupts to read the data from a photo-resistor efficiently
 * 2018-10-12
 * armw
 * v0.2
 * © 2018 <reza@parkcircus.org> All Rights Reserved
 *  
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 * 
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 * 
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
 * MA 02110-1301, USA.  
 * 
 * References:
 * https://playground.arduino.cc/Learning/PhotoResistor
 * https://oscarliang.com/arduino-timer-and-interrupt-tutorial/
 */

/* Simple test of the functionality of the photo resistor

Connect the photoresistor one leg to analog pin 0, and pin to +5V
Connect a resistor (~10K should suffice) from analog pin 0 to GND

----------------------------------------------------

           PhotoR     10K
 +5     o---/\/\/--.--/\/\/---o GND
                   |
 Pin A0 o----------

----------------------------------------------------
*/

// 2019-02-16 v0.2 armw pin mnemonic correction suggested by Peter Mansvelder
int pinPHOTOsensor = A0;     // analog pin 0 for the sensor - thx PeterM
bool interrupted = false;   // interrupt not detected

ISR(TIMER1_COMPA_vect)      // timer compare interrupt service routine
{
  interrupted = true;       // nominal flag
}

void setup()
{
    Serial.begin(115200);  // use serial communcation for data display
    pinMode(pinPHOTOsensor, OUTPUT ); // redundant?

    noInterrupts();         // disable interrupts while handlers are setup
    TCCR1A = 0;             // Timer/Counter Control Registers
    TCCR1B = 0;             // pre-scaler configured with these variables
    TCNT1  = 0;

    OCR1A   = 62500;        // 16000000 / 256 / 1; 16MHz/256/1Hz
    TCCR1B |= (1 << WGM12); // CTC mode
    TCCR1B |= (1 << CS12);  // 256 prescaler
    TIMSK1 |= (1 << OCIE1A);  // enable timer compare interrupt
    interrupts();           // re-enable interrupts for the assignments
}

void loop()
{
  if (interrupted)
  {
    interrupted = false;
    Serial.println(analogRead(pinPHOTOsensor)); // for use on serial line, stay tuned
  }
}

/*
 * PhotoSensor-02.ino
 * Using compare and match timer interrupts to read the data from a photo-resistor efficiently
 * 2018-10-12
 * armw
 * v0.2
 * © 2018 <reza@parkcircus.org> All Rights Reserved
 *  
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 * 
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 * 
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
 * MA 02110-1301, USA.  
 * 
 * References:
 * https://playground.arduino.cc/Learning/PhotoResistor
 * https://oscarliang.com/arduino-timer-and-interrupt-tutorial/
 */

/* Simple test of the functionality of the photo resistor

Connect the photoresistor one leg to analog pin 0, and pin to +5V
Connect a resistor (~10K should suffice) from analog pin 0 to GND

----------------------------------------------------

           PhotoR     10K
 +5     o---/\/\/--.--/\/\/---o GND
                   |
 Pin A0 o----------

----------------------------------------------------
*/

// 2019-02-16 v0.2 armw pin mnemonic correction suggested by Peter Mansvelder
int pinPHOTOsensor = A0;     // analog pin 0 for the sensor - thx PeterM
volatile bool interrupted = false; // interrupt not detected

ISR(TIMER1_COMPA_vect)      // timer compare interrupt service routine
{
  Serial.println(analogRead(pinPHOTOsensor)); // for use on serial line, stay tuned
}

void setup()
{
    Serial.begin(115200);  // use serial communcation for data display
    pinMode(pinPHOTOsensor, OUTPUT ); // redundant?

    noInterrupts();         // disable interrupts while handlers are setup
    TCCR1A = 0;             // Timer/Counter Control Registers
    TCCR1B = 0;             // pre-scaler configured with these variables
    TCNT1  = 0;

    OCR1A   = 62500;        // 16000000 / 256 / 1; 16MHz/256/1Hz
    TCCR1B |= (1 << WGM12); // CTC mode
    TCCR1B |= (1 << CS12);  // 256 prescaler
    TIMSK1 |= (1 << OCIE1A);  // enable timer compare interrupt
    interrupts();           // re-enable interrupts for the assignments
}

void loop()
{

}

Credits

Matha Goram

27 projects • 23 followers

Working with discrete electronic components for a very long time but still suffering from the occasional dry soldering results.

Contact

Thanks to Oscar Liang.

Comments

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Is There Light?

Things used in this project

Hardware components

Software apps and online services

Story

Introduction

Applications

Specification

Library

Usage

Continuous polling

Interrupt handler routine

Reading within handler

Reading outside the handler

Results

References

Custom parts and enclosures

Assembly

Schematics

Schematics

Code

Hello Photo (LDR) Resistor

Using a Photo Resistor with a timer interrupt

Using a Photo Resistor with a timer interrupt redux

Credits

Matha Goram

Comments

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Is There Light?

Is There Light?

Things used in this project

Hardware components

Software apps and online services

Story

Introduction

Applications

Specification

Library

Usage

Continuous polling

Interrupt handler routine

Reading within handler

Reading outside the handler

Results

References

Custom parts and enclosures

Assembly

Schematics

Schematics

Code

Hello Photo (LDR) Resistor

Using a Photo Resistor with a timer interrupt

Using a Photo Resistor with a timer interrupt redux

Credits

Matha Goram

Comments

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