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Understanding the Basics of 7-Segment Display

Updated: Mar 28

Introduction


A 7-segment display is a type of electronic display that visually represents numerical digits using seven individually illuminated segments. These segments are arranged in a specific pattern to display numbers ranging from 0 to 9. The display consists of seven LED (Light Emitting Diode) or other light-emitting segments, each capable of being independently turned on or off.

The arrangement of these segments follows a standard pattern, and each segment is assigned a specific letter or designation. Usually, the 7-segments are identified by the letters a, b, c, d, e, f, and g. By selectively activating these segments, various numbers and some alphabetic characters can be displayed.

7-segment display configuration


Common Cathode Configuration:

The term "Common Cathode" (CC) refers to a display where all the LED segment cathode connections are collectively connected to ground or logic "0." A "HIGH" or logic "1" signal is applied to each individual segment to forward bias the individual anode terminals (a-g) and illuminate them through a current limiting resistor.



Common Anode Configuration:

The common anode (CA) display is made up of all the anode connections from the LED(Light Emitting Diode) segments connected to logic "1". Through the use of an appropriate current limiting resistor, a ground, logic "0," or "LOW" signal is applied to each segment's cathode (a–g) to illuminate it.


Since many logic circuits are able to sink more current than source, common anode displays are generally more widely used. Additionally, keep in mind that connecting common cathode displays to common anode displays and vice versa in a circuit is equivalent to connecting LEDs(Light Emitting Diode) in reverse, which prevents light output.


This specific set of LEDs(Light Emitting Diode) is forward biased, depending on the decimal digit to be displayed. For example, six LED(Light Emitting Diode) segments (a, b, c, d, e, and f) must be lit in order to display the number 0. As a result, a 7-segment display can be used to display the different digits from 0 through 9.

Then, as demonstrated below, using a 7-segment display, we can create a truth table that indicates each segment that must be illuminated separately in order to produce the necessary decimal digit from 0 to 9.

Driving 7 segment display

It is actually made up of seven separate LEDs(Light Emitting Diode), and as such, it requires protection from overcurrent. LEDs(Light Emitting Diode) can only emit light when they are forward biased, and the amount of light that is emitted is directly correlated with the forward current.


As a result, the light intensity of the LED(Light Emitting Diode) increases roughly linearly with increasing current. To avoid harming the LED(Light Emitting Diode) segments, this forward current needs to be managed and restricted to a safe value using an external resistor.


In order to properly illuminate, the LED(Light Emitting Diode) segments should be connected to a voltage source that is greater than this forward voltage value, with a series resistance used to limit the forward current to a desirable value. This is because the forward voltage drop across a red LED(Light Emitting Diode) segment is very low, at about 2-to-2.2 volts (blue and white LEDs(Light Emitting Diode) can be as high as 3.6 volts).


A typical red 7-segment display requires around 15 ma for each LED(Light Emitting Diode) segment to light up properly. Therefore, in a 5 volt digital logic circuit, the current limiting resistor should be 200Ω (5v – 2v)/15mA, or 220Ω to the closest higher preferable value.


Diodes are commonly used in the driver circuitry of 7-segment displays to control the flow of current, ensuring that each segment lights up when needed. They help prevent undesired backward flow of current.


In some applications, photodiodes may be used for light sensing or ambient light detection in conjunction with 7-segment displays. For instance, adjusting the display brightness based on ambient light conditions.


Rectifiers are typically used in the power supply circuitry that provides the necessary DC voltage to operate the 7-segment display. They ensure a unidirectional flow of current.


While diodes, photodiodes, and rectifiers are essential components in electronic circuits, their direct integration within a 7-segment display is limited. They are more commonly employed in the supporting circuitry, such as the driver circuits and power supplies, that enable the proper functioning of 7-segment displays.


Applications of 7 segment display


  • Electronic meters, digital calculators, clock radios, digital clocks, odometers, and other devices typically use 7 segment displays.

  • Currently, LCDs are used in the majority of seven-segment display applications because of their low power consumption.

  • Useful in situations when the display must function in dim or dark lighting, due to its bright luminance.

  • Measurement/sensor value is displayed using four characters when combined with four segments.


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