Arduino UNO: LED Sensor, Part One

As seen in the previous blog postings, the LED (Light Emitting Diode) was used to DISPLAY the result of various sensors. It was also used to create a variety of Light patterns or sequences.

The LED is commonly used as an OUTPUT device, however, I have since found out: there is another option.

You can use the LED as an INPUT device !!

I have only tried this with a Yellow LED and a Red LED, however, this should work with any colour. Some sites recommend using a clear/transparent/colourless LED for best effect, but it depends on what you are trying to achieve.

The LED responds better to light of the same wavelength that it emits. So a yellow LED responds better to yellow light, and a red LED responds better to Red light.

The following experiment attempts to prove this theory.
A Red and Yellow LED alternate and fade in to maximum brightness using PWM (Analog Output). Meanwhile, a separate LED is used as an INPUT device to receive the light. The value obtained is plotted using a processing sketch.

The chart above used a Yellow LED to measure the light emitted from a Red and then a Yellow LED.

As the Yellow LED gets brighter, the INPUT LED takes less time to discharge, and thus produces a lower result. On the other hand, the Red LED has little effect on the INPUT LED, despite it's brightness.

Ambient light will produce different graph patterns.

Parts Required:

• Arduino UNO
• 2 x Yellow LEDs
• 1 x Red LED
• 3 x 330 ohm Resistors (choose resistors suitable for your LEDs)
• 5 wires to connect the circuit
• Breadboard
• Processing software

Here is the Sketch:

     Created with Fritzing : http://fritzing.org/

Here is the Arduino Code: which was adapted from this site.

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#define LED_Sensor_NEG 8 #define LED_Sensor_POS 7 #define Yellow_LED 10 #define Red_LED 9 int switcher=0; int lightLevelY=0; int lightLevelR=0; void setup(){ pinMode(LED_Sensor_POS,OUTPUT); digitalWrite(LED_Sensor_POS,LOW); pinMode(Yellow_LED,OUTPUT); pinMode(Red_LED,OUTPUT); //Turn on Serial Protocol Serial.begin(9600); } void loop() { //Alternate the flashing of the Yellow and Red LED if(switcher==0){ lightLevelR+=4; if(lightLevelR>255){ lightLevelR=0; switcher=1; } } else { lightLevelY+=3; if(lightLevelY>255){ lightLevelY=0; switcher=0; } } analogWrite(Red_LED,lightLevelR); analogWrite(Yellow_LED,lightLevelY); // Charge the LED by applying voltage in the opposite direction pinMode(LED_Sensor_NEG,OUTPUT); digitalWrite(LED_Sensor_NEG,HIGH); // Set pin 8 to read the input and Turn off the internal pull up resistor. // The greater the amount of light in the room, the smaller the number represented by variable "darkness". long darkness=0; int outputVal=0; pinMode(LED_Sensor_NEG,INPUT); digitalWrite(LED_Sensor_NEG,LOW); while((digitalRead(LED_Sensor_NEG)!=0) && darkness<80000){ darkness++; } outputVal=darkness/80; //Print the darkness level in the room. Serial.println(outputVal); }

Here is the Processing Code:     Code Source

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/* Processing code for this example */ // Graphing sketch // This program takes ASCII-encoded strings // from the serial port at 9600 baud and graphs them. It expects values in the // range 0 to 1023, followed by a newline, or newline and carriage return // Created 20 Apr 2005 // Updated 18 Jan 2008 // by Tom Igoe // This example code is in the public domain. import processing.serial.*; Serial myPort; // The serial port int xPos = 1; // horizontal position of the graph void setup () { // set the window size: size(400, 300); // List all the available serial ports println(Serial.list()); // I know that the second port in the serial list on my PC // is always my Arduino, so I open Serial.list()[1]. // Open whatever port is the one you're using. myPort = new Serial(this, Serial.list()[1], 9600); // don't generate a serialEvent() unless you get a newline character: myPort.bufferUntil('\n'); // set inital background: background(0); } void draw () { // everything happens in the serialEvent() } void serialEvent (Serial myPort) { // get the ASCII string: String inString = myPort.readStringUntil('\n'); if (inString != null) { // trim off any whitespace: inString = trim(inString); // convert to a float and map to the screen height: float inByte = float(inString); inByte = map(inByte, 0, 1023, 0, height); // draw the line: stroke(127,34,255); line(xPos, height, xPos, height - inByte); // at the edge of the screen, go back to the beginning: if (xPos >= width) { xPos = 0; background(0); } else { // increment the horizontal position: xPos++; } } }