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LED and 7-Segment Display

LED and 7-Segment Display
LED & 7-Segment Display

Let us now learn about Light Emitting Diode (LED) and 7-Segment display which is combination of eight LEDs. LED comes in many different colors now a day, like White, Red, Blue, Green and Yellow etc. LED are very commonly seen around us as indicators on different device to indicate power ON condition and used as decorative light around us. The most common place where LED is used but usually not recognized by us is the LED TV, where each pixel contains three micro LEDs with basic color i.e. Red, Blue and Green. If we see a large sized LED screens very closely, we may be able to identify 
how pixel is made
Pixel
this arrangement, specially the outdoor screens displayed in streets. So, if we have a HD screen it has resolution of approx. 1920x1080 pixels, that means there are 1920 pixels arranged horizontally and similar 1080 horizontal lines are there to fill the whole screen in the matrix form. Since each pixel contains three LED of basic colors, hence we can calculate total number of LEDs in such screen as 1920x1080x3 = 6220800 LEDs.


Colored LED
LED
LED is a PN junction semiconductor which emits light when current flows through it. Its symbol indicates a PN junction diode with two small arrows indicating emission of light. Being similar to PN junction diode it allows current to flow in only one direction from anode to cathode when it is under forward biased condition, not in reverse condition. Anode should be connected to positive supply and cathode should be connected to negative supply.

The light energy is generated when electrons combines with holes, it releases 
symbol of an LED
LED Symbol
energy in form of photons. Anode terminal is longer in size and cathode terminal is smaller in size. The color of light emitted depends on energy required by electrons to cross the band gap. White color is obtained by using phosphorus layer with semiconductor layer; similarly other colors are obtained by combination with different materials like Indium Gallium Nitrate (InGaN), Aluminum gallium indium phosphide (AlGaInP), Aluminum gallium arsenide (AlGaAs) and Gallium phosphide (GaP). Due to combination of different materials different wavelength of light is generated hence we see different colors.


construction on LED
LED Construction

Construction of LED shown here with a base called as substrate also. On
substrate there is a layer of ‘P’ type material with excess of holes and on top of that we have ‘N’ type material with excess of electrons. Under forward biased condition when holes and electrons combine with each other in middle (mentioned as ‘Active Region’) region, it releases light energy in form of photons.

high power LED
1 Watt LED
Broadly we have colored LEDs in visible light range, Infrared (IR) LEDs (used in remote controllers) and Ultra Violet (UV) LEDs used in machines for original currency verification and other industrial use. All these LEDs uses very low power to emit light, most of the small LEDs need 1.8V to 3.3V and current in range of 10mA to 20mA. So, if we calculate the power using P = V x I we get maximum power of 3.3Vx20mA = 66milli Watt. Since last many years we have got higher power LEDs in the range of 1W, 3W, 5W, 10W, 20W, 50W etc. which are now used in LED lamps, street lights, flood light in stadium etc. The more power they consume, comparable energy is also released in the form of heat hence for these high power LEDs we need to place big heat sink material mostly of aluminum to absorb the heat, else the LED will damage. These high power LEDs are much efficient as compared to conventional light bulbs, halogen bulbs or old mercury vapor and sodium vapor street lamps because LED releases less heat and more light compared to other lamps. Hence these LED lamps are now a day more popular and conventional old lamps are getting obsolete.


Note: We should not look directly from front of White LED and high power LEDs into it, as huge amount of light is generated in very small area inside LED so the intensity of light at this point is too high and looking directly into it may hurt your eyes.

simple LED circuit
LED Circuit
Let us learn how we can operate a LED with a battery and how we can decide 
the value of resistor to be used in circuit to control the flow of current through LED to avoid damaging the LED with over current. We will connect the LED with two pencil cells in series to get 3V as red LED need 1.7V hence single cell will not work as it gives 1.5V only. Different LEDs have different voltage drop across it during forward biased condition, as shown in table. Here anode of LED is connected to positive supply through current limiting resistor ‘R’ and cathode is connected directly to negative supply. Voltage across point ‘A’ and ‘B’ is 3V and we need voltage across LED to be at 1.7V i.e. across point ‘E’ and ‘F’, so we need to drop the remaining voltage i.e. 3-1.7 = 1.3V across 
forward voltage drop of different color LED
LED Voltage Drop
resistor ‘R’. Here we need to decide value of resistor ‘R’ to maintain current in circuit between 10mA to 20mA. To maintain 1.3V across point ‘C’ and ‘D’, considering average current 15mA (15/1000A) using Ohms law R = V/I = 1.3/15*1000 = 86.66Ohms, so we will take nearest available value which is 82Ω. So, the current through resistor or through circuit with 82Ω will be I = 1.3/82 = 0.0158 = 15.8mA which is within operating range of LEDs i.e. 10-20mA. Some other types of LED are combination of different color LEDs like RGB LED, dual LED, blinking LED, color changing LED etc.

That is how we can use different color LEDs with different resistor combination to operate them within given voltage and current conditions. Next let us learn about 7-Segment display which we see around us to display numbers on different devices like calculator, weighing machine and visitor counter at malls entry etc.

7-Segment Display

7-Segment Display 
7-Segment Display

7-segment display details
7-Segment Display
7-Segment display are used to display digits in different appliances like weighing machine, visitor counters etc. as the name suggest it consist of 7nos. LEDs arranged in format of number eight ‘8’ which can allow to display all number between ‘0’ to ‘9’. One additional led is also used to display decimal point to display any number higher than ‘9’ using two or more 7-segment displays. These are available in different colors like Red, Green, Blue and White etc.


These segments are denoted as letter ‘a’ to ‘f’ and ‘dp’ for decimal point, all these LEDs are connected to eight different pins of display with two common points on top and bottom as shown here. To display different digits we can refer table.

7-Segment Display Guide
Display Format Table
If we see the internal construction of these 7-segment displays we have two types of display, one is common anode and common cathode. In common anode type, anode terminal of all LEDs inside the display are connected together and is connected to common pin (com). Similarly for common cathode type, the cathode terminals of all LEDs inside display are connected together to common pin (com). Based on circuit requirement we can choose which type to use out of two types shown here. 


display format of digits on 7-segment display
Display Format of Digits

internal connection of 7-segment display
Internal Connection
Details inside diagram shows how the LEDs inside are connected and how each segment LED is connected to outer pin. We can see all anode or all cathodes are connected together to ‘com’ pin in these two types and rest terminals to other eight pins of display.

Watch out the video to see how digits are displayed on 7-Segment display. As of now you may ignore the circuit diagram as it is out of scope at this point of time.




That’s all about 7-segment display, for any doubt leave comment below, will respond ASAP. Keep watching for next topic.




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