7 Segment Display

      Seven segment display is widely use to display digit 0 to 9 and alphabets A to F. They find wide application in devices that display numeric information like digital clocks,Electronics meters etc.

     In this tutorial you'll learn how to drive single seven segment display.First off all we'll learn basic about seven segment display and how to drive seven segment display.

Basic about seven segment

        There are two types of seven segments Common Cathode (CC) and Common Anode(CA), common anode is widely used because many logic circuits can sink more current than they can source. 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.

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 applying a “HIGH”, or logic “1” signal via a current limiting resistor to the Anode of the particular segment (a-g).

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 current limiting resistor to the Cathode of the particular segment (a-g).

            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.

7-segment Display Truth Table

Truth table

Driving a 7-segment Display

        7-segment display has a 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 and amount of light emitted is 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.

       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 shown in below circuit.

Circuit diagram

7 Segment display driving circuit diagram

        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 commonly known as a 7-segment decoder/driver, such as the TTL7447.This 7-segment display driver which is known as a BCD(Binary Coded Decimal) to 7-segment display decoder and driver.