Friday 29 March 2013
Seven Segment Display Everything you need to know about
Seven Segment display is the most common and fundamental display device which is used in many Electronic Gadgets and Appliances like Electric stove,Microwave Owen,Digital Clocks and also in many Digital Meters like Multimeter.
The seven segment display is called so since it contains 7 segments of LED which is assembled in a structure resembling like numeral ‘8’.This is because you can form all 10 digits (0-9) from the pattern ‘8’.
The 7 segments are named as A-G and the optional eighth segment is named as H.As you can see in the diagram the seven segment are arranged to form a shape of ‘8’.If all the segments are given the power then the number ‘8’ will be displayed and if you cutoff the power for segment G then it will result in number ‘0’.Similarly you can form the combinations to display numerals from 0 to 9.
Practically seven segment comes with two different configurations and both the type contains 10 pins.
The following are the two configurations of seven segment display
- Common Anode
- Common Cathode
In Common Anode configuration the pin 3 and 8 are shorted together with Anode of all 8 LED segments and this common pin is connected to positive voltage (VCC).Hence to light up any given segment the corresponding Cathode pin should be connected to ground or reference ,which completes the circuit and LED will be in forward bias.
Similarly In Common Cathode configuration the pin 3 and 8 are shorted together with Cathode of all 8 LED segments and this common pin is connected to Ground or reference point.Here if you give positive voltage to corresponding LED Anode pin the the circuit will be completed and the particular LED will be light up since it will be in forward bias.
Note that if you are using Ground as reference voltage then your VCC should be greater than 1.3 V,Which is minimum voltage to light up LED based on silicon.(Bandgap Energy).
Need for series Resistors
Practically we always use the power source(VCC) as +5V or Higher so it is necessary to connect series resistor to the Anode pins (Common Cathode) or to Cathode Pins (Common Anode configuration).This will ensure that the LED doesn’t burn.Here how the value of series resistor is determined
- Forward bias voltage : +2V
- Forward bias current : 20mA
Resistor value(ohm)=(Vcc – 2)/0.02 ohmsfor example if you are using +5 V then Resistor value=(5-2)/0.02=150 ohms.
Nowadays the usage seven segment displays are very limit due to the advancement in display technologies,Now Dot matrix display mostly displaced the place of seven segment displays but yet seven segment displays are good starting point to read about display technologies.
Bonus fact
Actually you can display all Hexadecimal using seven segment display,that is apart from digits 0-9 it is also possible to show A,b,C,d,E,F.Now I leave this part to you,Why the alphabet ‘b’ and ‘d’ is not represented in caps ???
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