Binary Coded Decimal: Seven-Segment Display

Theory

BCD to 7 Segment Decoder

In the Binary Coded Decimal (BCD) coding scheme, each of the decimal numbers (0 – 9) is represented by its equivalent binary pattern. A seven-segment display is an electronic device that consists of seven light-emitting diodes (LEDs) arranged in a specific pattern (common cathode or common anode) that is used to display the decimal numbers 0–9.

There are two types of seven-segment LED display:

Common cathode type: in this type of display, all the cathodes of the seven LEDs are connected together to ground, and the LED displays digits when a "HIGH" signal is applied to the individual anodes.

Common anode type: in this type of display, all the anodes of the seven LEDs are connected to +VCC, and the LED displays digits when a "LOW" signal is applied to the individual cathodes.

A seven-segment display does not work by directly supplying voltage to the various LED segments. First, the decimal number is converted to its equivalent BCD signal, then the BCD to seven-segment decoder converts the signals into a form that is fed to the seven-segment display.

The BCD to seven-segment decoder has four input lines (A, B, C and D) and 7 output lines (a, b, c, d, e, f and g), this output is fed to a seven-segment LED display that displays a decimal number depending at the inputs.

Truth Table – For Common Cathode type BCD to Seven Segment Decoder:

555 Timer IC

555 Timer is a highly stable integrated circuit that can produce accurate time delays and oscillations. It has three operating modes: bistable, monostable and astable mode. The internal schematics of 555 Timer consists of 25 transistors, 2 diodes and 15 resistors.

Represented with a block diagram it consists of 2 comparators, a flip-flop, a voltage divider, a discharge transistor and an output stage.

The voltage divider consists of three identical 5k resistors that create two reference voltages at 1/3 a