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7 segment display

7-segment Display

An LED or Light Emitting Diode, is a solid state optical PN-junction diode which emits light energy in the form of “photons” when it is forward biased by a voltage allowing current to flow across its junction, and in Electronics we call this process electroluminescence.

The actual colour of the visible light emitted by an LED, ranging from blue to red to orange, is decided by the spectral wavelength of the emitted light which itself is dependent upon the mixture of the various impurities added to the semiconductor materials used to produce it.

7-segment display7-segment Display

Light emitting diodes have many advantages over traditional bulbs and lamps, with the main ones being their small size, long life, various colours, cheapness and are readily available, as well as being easy to interface with various other electronic components and digital circuits.

But the main advantage of light emitting diodes is that because of their small die size, several of them can be connected together within one small and compact package producing what is generally called a 7-segment Display.

The 7-segment display, also written as “seven segment display”, consists of seven LEDs (hence its name) arranged in a rectangular fashion as shown. Each of the seven LEDs is called a segment because when illuminated the segment forms part of a numerical digit (both Decimal and Hex) to be displayed. An additional 8th LED is sometimes used within the same package thus allowing the indication of a decimal point, (DP) when two or more 7-segment displays are connected together to display numbers greater than ten.

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Each one of the seven LEDs in the display is given a positional segment with one of its connection pins being brought straight out of the rectangular plastic package. These individually LED pins are labelled from a through to g representing each individual LED. The other LED pins are connected together and wired to form a common pin.

So by forward biasing the appropriate pins of the LED segments in a particular order, some segments will be light and others will be dark allowing the desired character pattern of the number to be generated on the display. This then allows us to display each of the ten decimal digits 0 through to 9 on the same 7-segment display.

The displays common pin is generally used to identify which type of 7-segment display it is. As each LED has two connecting pins, one called the “Anode” and the other called the “Cathode”, there are therefore two types of LED 7-segment display called: Common Cathode (CC) and Common Anode (CA).

The difference between the two displays, as their name suggests, is that the common cathode has all the cathodes of the 7-segments connected directly together and the common anode has all the anodes of the 7-segments connected together and is illuminated as follows.

1. The Common Cathode (CC) – In the common cathode display, all the cathode connections of the LED segments are joined together to logic “0” or ground. The individual segments are illuminated by application of a “HIGH”, or logic “1” signal via a current limiting resistor to forward bias the individual Anode terminals (a-g).

Common Cathode 7-segment Display

common cathode 7-segment display

2. The Common Anode (CA) – In the common anode display, all the anode connections of the LED segments are joined together to logic “1”. The individual segments are illuminated by applying a ground, logic “0” or “LOW” signal via a suitable current limiting resistor to the Cathode of the particular segment (a-g).

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Common Anode 7-segment Display

common anode 7-segment display

In general, common anode displays are more popular as many logic circuits can sink more current than they can source. Also note that a common cathode display is not a direct replacement in a circuit for a common anode display and vice versa, as it is the same as connecting the LEDs in reverse, and hence light emission will not take place.

Depending upon the decimal digit to be displayed, the particular set of LEDs is forward biased. For instance, to display the numerical digit 0, we will need to light up six of the LED segments corresponding to a, b, c, d, e and f. Then the various digits from 0 through 9 can be displayed using a 7-segment display as shown.

7-Segment Display Segments for all Numbers.

7-segment display numbers

Then for a 7-segment display, we can produce a truth table giving the individual segments that need to be illuminated in order to produce the required decimal digit from 0 through 9 as shown below.

7-segment Display Truth Table

Decimal
Digit
Individual Segments Illuminated
a b c d e f g
0 × × × × × ×
1 × ×
2 × × × × ×
3 × × × × ×
4 × × × ×
5 × × × × ×
6 × × × × × ×
7 × × ×
8 × × × × × × ×
9 × × × × ×

Driving a 7-segment Display

Although a 7-segment display can be thought of as a single display, it is still seven individual LEDs within a single package and as such these LEDs need protection from over current. LEDs produce light only when it is forward biased with the amount of light emitted being proportional to the forward current.

This means then that an LEDs light intensity increases in an approximately linear manner with an increasing current. So this forward current must be controlled and limited to a safe value by an external resistor to prevent damage to the LED segments.

The forward voltage drop across a red LED segment is very low at about 2-to-2.2 volts, (blue and white LEDs can be as high as 3.6 volts) so to illuminate correctly, the LED segments should be connected to a voltage source in excess of this forward voltage value with a series resistance used to limit the forward current to a desirable value.

Typically for a standard red coloured 7-segment display, each LED segment can draw about 15 mA to illuminated correctly, so on a 5 volt digital logic circuit, the value of the current limiting resistor would be about 200Ω (5v – 2v)/15mA, or 220Ω to the nearest higher preferred value.

So to understand how the segments of the display are connected to a 220Ω current limiting resistor consider the circuit below.

Driving a 7-segment Display

driving a 7-segment display

In this example, the segments of a common anode display are illuminated using the switches. If switch a is closed, current will flow through the “a” segment of the LED to the current limiting resistor connected to pin a and to 0 volts, making the circuit. Then only segment a will be illuminated. So a LOW condition (switch to ground) is required to activate the LED segments on this common anode display.

But suppose we want the decimal number “4” to illuminate on the display. Then switches b, c, f and g would be closed to light the corresponding LED segments. Likewise for a decimal number “7”, switches a, b, c would be closed. But illuminating 7-segment displays using individual switches is not very practical.

7-segment Displays are usually driven by a special type of integrated circuit (IC) commonly known as a 7-segment decoder/driver, such as the CMOS 4511. This 7-segment display driver which is known as a Binary Coded Decimal or BCD to 7-segment display decoder and driver, is able to illuminate both common anode or common cathode displays. But there are many other single and dual display drivers available such as the very popular TTL 7447.

This BCD-to-7 segment decoder/driver takes a four-bit BCD input labelled A, B, C and D for the digits of the binary weighting of 1, 2, 4 and 8 respectively, has seven outputs that will pass current through the appropriate segments to display the decimal digit of the numeric LED display.

The digital outputs of the CD4511 are different from the usual CMOS outputs because they can provide up to 25mA of current each to drive the LED segments directly allowing different coloured LED displays to be used and driven.

Driving a 7-segment Display using a 4511

driving a 7-segment display using a 4511

In this simple circuit, each LED segment of the common cathode display has its own anode terminal connected directly to the 4511 driver with its cathodes connected to ground. The current from each output passes through a 1kΩ resistor that limits it to a safe amount. The binary input to the 4511 is via the four switches. Then we can see that using a BCD to 7-segment display driver such as the CMOS 4511, we can control the LED display using just four switches (instead of the previous 8) or a 4-bit binary signal allowing up to 16 different combinations.

Most digital equipment use 7-segment Displays for converting digital signals into a form that can be displayed and understood by the user. This information is often numerical data in the form of numbers, characters and symbols. Common anode and common cathode seven-segment displays produce the required number by illuminating the individual segments in various combinations.

LED based 7-segment displays are very popular amongst Electronics hobbyists as they are easy to use and easy to understand. In most practical applications, 7-segment displays are driven by a suitable decoder/driver IC such as the CMOS 4511 or TTL 7447 from a 4-bit BCD input. Today, LED based 7-segment displays have been largely replaced by liquid crystal displays (LCDs) which consume less current.

45 Comments

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  • S
    Srinivas

    I saw a blunder in it.The LEDs are always reverse biased to glow. If Forward biased, they will act like normal diode.The article is written considering the LEDs to be used in Forward Bias.

    • Wayne Storr

      For LED’s to emit light, their PN-junction needs to be forward biased, that is their Anode is more positive than the Cathode.

  • f
    feyisayo

    very good

  • g
    govinda

    nice….

  • marcador electronico

    Very good article!! It explained everything perfectly.

    Greatings

  • T
    Tayyab

    I need help!! ASAP
    I want a block diagram of seven segment led display by using 7400 (7404,7408,7432 & 7493)
    Series IC’s.

  • D
    Double m

    How can I design 1,2,3,4,5,6,switch which can be used at hotel using 4611 Ic and it have only 4 input

  • t
    talha

    how do you show error when a number greater than 9 is to be converted in seven segment display , kindly tell urgent .

    • Wayne Storr

      4 data bits are used to display a number between 0 (0000) and 9 (1001) on a 7-segment display. Detect when the 4-bit code is 101X (10, 11) or 11XX (12, 13, 14, 15) where X is the dont-care condition and illuminate segment “G” (-) only.

  • f
    farhad hasan

    if i want to show minus sign then what to do

  • T
    TvLiquidator

    I like playing wtih & segment display to many things can be done with them 🙂

  • B
    Burak

    Hey, I have a 4 digit seven-segment display which is Common Anode but I am confused about something. To Illuminate each discrete seven segment, I am pulling the pin LOW since they are all kathodes. That’s OK. However, what confuses me is to enable the anodes and disable the others I had to force the desired anode pins to 0 in order to light its LEDs up. To give an example, here I got my 4 digits to enable the rightmost anode I have to write 1110 to Verilog Code(HDL is not important here, what matters is driving anode to 0). To enable the second I had to write 1101, each of which digit represents the anode signals of digits, respectively. I really don’t understand why anode must be driven to 0 to enable and to 1 to disable. Check out the BASYS2 FPGA board manual you will see what I meant. Thank you!

    • Wayne Storr

      For a Common Anode (CA) display, the Anode is held positive while the Cathodes are switched via a current limiting resistor either individually or in sets to negative or 0 volts to illuminate the relevant segments. In your case the switching of the Anodes is being controlled by a PNP transistor (see Basys2 board info) which requires the Base to be more negative than the Emitter to turn the transistor ON. Then a logic 0 (connecting the Base to 0v) will turn the transistor ON and therefore illuminate the display, while a logic 1 connects the Base to the same potential as the Emitter turning it OFF. Controlling the Anodes in this way allows each 7-segment display to be sequentially illuminated in turn to display the 4 digits.

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