The circuit mainly consists of

AT89C51 microcontroller. AT89C51

belongs to the family of 8051

microcontroller. It is an 8-bit

microcontroller. This

microcontroller has 4KB of Flash

Programmable and Erasable Read

Only Memory and 128 bytes of RAM.

This can be programmed and erased

maximum 1000times. It has two 16

bit timers/counters. It supports

USART communication protocol. It

has 40 pins. There are four ports are

designated as P0, P1, P2, P3, P4.

Light Emitting Diodes are the semi

conductor light sources. Commonly used

LEDs will have a cut-off voltage of 1.7V

and current of 10mA. When an LED is

applied with its required voltage and

current it glows with full intensity. The

Light Emitting Diode is similar to the

normal PN diode but it emits energy in the

form of light.

The Anode is connected through a resistor to

GND & the Cathode is connected to the

Microcontroller pin. So when the Port Pin is

HIGH the LED is OFF & when the Port Pin is

LOW the LED is turned ON.

In this circuit, LEDs are connected to

the port P0. Port P0 will not have

internal pull- ups, while the other ports

have internal pull-ups. The controller is

connected with external crystal

oscillator to pin 18 and 19 pins. Crystal

pins are connected to the ground

through capacitors of 33pf.using an LED

you must use a resistor to limit the

current flow. If you just connect an LED

directly to a pin of the micro controller

you run the risk of damaging the micro

controller by allowing too much current

flow into or out of the pin.

Light Emitting Diodes are the semi

conductor light sources. Commonly used

LEDs will have a cut-off voltage of 1.7V and

current of 10mA. When an LED is applied

with its required voltage and current it

glows with full intensity. The Light Emitting

Diode is similar to the normal PN diode but

it emits energy in the form of light. The

colour of light depends on the band gap of

the semiconductor. The following figure

shows “how an LED glows?”

Thus, LED is directly connected to the AT89C51

microcontroller. The negative terminal of the LED is

connected to the ground through a resistor. Value of

this resistor is calculated using the following formula.

, where V is the input voltage.

Generally, microcontrollers output a maximum voltage

of 5V. Thus the value of resistor calculated for this is

330 Ohms. Thus this can be connected either to the

cathode or anode of the LED.

R= (V-1.7)/10mA

LED Interfacing with 8051 Circuit Applications:

LEDs are widely used in many applications like in seven segments.

They are used in dot matrix displays.

They can be used for street lights.

They are used as indicators.

They can be used in traffic lights.

They are used in emergency lights

They can used to make electronic designs.

Seven segment displays are used to indicate numerical information. Seven segments display can display digits from 0 to 9 and even we can display few characters like A, b, C, H, E, e, F, etc. These are very popular and have many more applications. Before going to start this concept, get an idea about how to interface LEDs with 8051 Microcontroller.

Interfacing 7 Segment Display to 8051 Circuit

Principle:

Seven segment displays internally consist of 8 LEDs. In these LEDs, 7 LEDs are used to indicate the digits 0 to 9 and single LED is used for indicating decimal point. Generally seven segments are two types, one is common cathode and the other is common anode.

In common cathode, all the cathodes of LEDs are tied together and labeled as com. and the anode are left alone. In common anode, seven segment display all the anodes are tied together and cathodes are left freely. Below figure shows the internal connections of seven segment Display.

Interfacing 7 Segment Display to 8051 Circuit Diagram:

Interfacing 7 Segment Display to 8051 Circuit Design:

Here, common cathode seven segment is used to display the digits. In this circuit, pins a to h of the 7 segment are connected to the PORT 2 of the microcontroller and com pin is connected to the ground through the 330 ohm resistor. This resistor is used to drop the voltage. Since we are using common cathode seven segment we need to send LOGIC 1 to the segments to glow.Figure shows structure of common cathode seven segments. Here dot is used for indicating the decimal point. Here all the cathodes of LED’s are connected to the Gnd pin. The operating voltage of this LED’s is 2 to 3V but from controller we will get 5V so to drop the remaining voltage we have to connect a to g pins to the controller through the resistor.

Digit Drive Pattern:To display the digits on 7 segment, we need to glow different logic combinations of segments. For example if you want to display the digit 3 on seven segment then you need to glow the segments a, b, c, d and g. The below table show you the Hex decimal values what we need to send from PORT2 to Display the digits from 0 to 9.

Digit Dp g f e d c b aHex

value

0 0 0 1 1 1 1 1 1 0x3f

1 0 0 0 0 0 1 1 0 0x06

2 0 1 0 1 1 0 1 1 0x5b

3 0 1 0 0 1 1 1 1 0x4f

4 0 1 1 0 0 1 1 0 0x66

5 0 1 1 0 1 1 0 1 0x6d

6 0 1 1 1 1 1 0 1 0x7d

7 0 0 0 0 0 1 1 1 0x07

8 0 1 1 1 1 1 1 1 0x7f

9 0 1 1 0 0 1 1 1 0x67

Interfacing 7 Segment Display to 8051 Circuit Applications:

Seven segments are widely used in digital clocks to display the time.

These are used in electronic meters for displaying the numerical information.

Used in Instrument panels

Used in digital readout displays.