Understanding 74LS47 IC: A Comprehensive Guide to BCD-to-7-Segment Decoder Driver

The 74LS47 IC boasts a pair of active-low outputs tailored for driving common-anode LEDs or incandescent indicators. When fed a binary-coded decimal as input, it seamlessly transforms it into a corresponding pattern for illuminating a 7-segment display, showcasing the numerical range from zero to nine. This encoding process represents each digit's value through its respective 4-bit binary sequence (BCD).

Furthermore, a BCD to 7 Segment decoder employs specialized combinational logic circuitry to effectuate the conversion of BCD decimal digits into the 7-segment display format.

This comprehensive guide provides detailed insights into the 74LS47 decoder, including pin configurations, working, circuit, and more details, offering a clear and informative overview of the functions of the 74LS47 IC.

What is 74LS47 IC?

The 74LS47 is a BCD to 7-segment decoder/driver integrated circuit (IC). It takes a binary coded decimal as an input and transforms it into a corresponding pattern for operating a seven-segment display, effectively exhibiting the numerical range from 0 to 9. Binary coded decimal (BCD) is a coding system where every individual digit within a number is portrayed using its distinct four-bit binary sequence.

74LS47 Pinout

The IC 74LS47 pin diagram is outlined as follows:

IC 74LS47 Pin Diagram

Pinout Configuration

Pin No.	Pin Name	Description
1	B	BCD input of the IC
2	C	BCD input of the IC
3	Display test/Lamp test	To test the display LED
4	Blank Input	To turn off the LEDs of the display
5	Store	Store or strobe a BCD code
6	D	BCD input of the IC
7	A	BCD input of the IC
8	GND	Ground Pin
9	e	7 segment output 1
10	d	7 segment output 2
11	c	7 segment output 3
12	b	7 segment output 4
13	a	7 segment output 5
14	g	7 segment output 6
15	f	7 segment output 7
16	Vcc	Supply Voltage

How does 74LS47 Decoder Work?

The 74LS47 IC primarily processes the input data represented in Binary Coded Decimal (8421), using an internal mechanism to generate their complements. It further employs seven OR/AND gates for decoding and directly controlling the indicator LEDs associated with these seven segments.

Ultimately, the generated outputs align with the Common Anode (CA) configuration. In summary, the CA configuration is synonymous with the arrangement of the 7-segment display.

Working Principle of a 7-Segment Display

A 7-segment display operates in two main modes: common cathode (CC) and common anode (CA) configurations. In the CC mode, the negative terminals (cathodes) of the individual segments (a, b, c, d, e, f, g) are connected and grounded. The positive terminals (anodes) are linked to the corresponding outputs (a, b, c, d, e, f, g) of the 74LS48 decoder driver circuit. For instance, to display the digit '1', the decoder driver circuit sets segments b and c to a high logic level while the others are held low.

In the CA mode, the positive terminals (anodes) of the segments are connected and receive a 5V power supply. The negative terminals (cathodes) are connected to the corresponding outputs of a 74LS47 decoder driver IC. As in the CC mode, a current-limiting resistor is required in both configurations to prevent excessive current and protect the display. The choice of this current-limiting resistor is determined by subtracting the working voltage of the LED segments (usually between 1.8V and 2.2V) from the 5V power supply voltage and dividing by the desired current, typically between 10-20mA.

It's worth noting that for high-power 7-segment displays, resistor selection should consider the operating conditions and power rating.

Working Principle of 74ls47

The 74LS47 is a BCD-to-7 segment digital decoder and driver. Its primary function is to convert Binary-Coded Decimal (BCD) data into the corresponding numerical display on a 7-segment LED display. This decoding process simplifies displaying numbers on the 7-segment display, streamlining programming efforts.

The core logic function of a decoder is to translate binary input codes into specific high and low-level output signals. Common decoder circuits include binary decoders, binary-to-7-segment decoders, and display decoders. Decoding is the reverse encoding process, where the meaning assigned to the code is "translated" back. A logic circuit that performs this decoding process is known as a decoder. The 74LS47 is an active low 7-segment decoder, meaning it works when its outputs are low, and it is used in conjunction with 7-segment displays. The truth table for the 74LS47 is provided below, illustrating its relationship with 7-segment displays.

74ls47 Truth Table

(Note: In this context, "active low" means that when a particular output is active, it is in a low logic state, which turns on the corresponding segment in the 7-segment display.)

74LS47 Features and Specifications

The 74LS47, a member of the LS Technology family, serves as a decoder or demultiplexer and boasts the following attributes:

• A solitary channel designed for a standard 5V voltage.

• It necessitates a minimum VCC of 4.75V and can handle a maximum VCC of 5.25V.

• Adopts an open-collector configuration in a 4:7 ratio.

• Its maximum IOL is 3.2mA, while the maximum IOH temporarily registers at -0.05mA.

• Classified as Catalog, the 74LS47 operates within a temperature range spanning from 0 degrees Celsius to 70 degrees Celsius.

• Equipped with seven bits and a maximum digital input leakage of 5uA.

• Finally, it features an ESD CDM of 0.75 kV, and its ESD HBM reaches 2 kV.

All of these characteristics mentioned above collectively contribute to ensuring electrical safety while operating fundamental electronic systems.

74LS47 Application Circuits

The 74LS47 features built-in ripple-blanking capabilities, but it lacks data latches.

In Figure 1, you can observe the functional diagrams and pin assignments for these devices, each in a 16-pin package.

Figure 1

The 74LS47 is equipped with active-low outputs designed for driving a common-anode LED display using external current-limiting resistors (Rx), as illustrated in Figure 2.

Figure 2

Please note that Figure 1 reveals that each of these integrated circuits incorporates three input 'control' terminals, identified as LT (Lamp Test), BI/RBO, and RBI. The LT terminal activates all display outputs when it is connected to logic 0, with the RBO terminal left open or set to logic 1.

When the BI/RBO terminal is pulled low, it clears all the outputs, also serving as a ripple-blanking output terminal. Figure 3 guides connecting the ripple-blanking terminals for suppressing leading zeros on the first three digits of the four-digit display.

Figure 3

74LS47 Application

The 74LS47 is commonly used in devices that require seven-segment display functionality, such as:

• Calculators 

• Digital watches

• Various types of digital displays

Its application in these devices involves decoding and driving the seven-segment displays to present numerical or alphanumeric characters, providing clear and easy-to-read visual information to users. This integrated circuit plays a vital role in the presentation of numerical data, making it a fundamental component in designing electronic devices with digital displays.

74LS47 Equivalents

AM27S23AJCBMEMORY, AM27S23JCBMEMORY, N82LS135AMEMORY, N82S135AMEMORY, 63S281NLMEMORY, 63S281ANLMEMORY, 63S280NLXXXXMEMORY, 63S281NLXXXXMEMORY, DM74LS471VMEMORY, 53S281ANLMEMORY can equivalent to 74LS47.

74LS47 Datasheet

Download 74LS47 Datasheet PDF.

Conclusion

In brief, the 74LS47 takes Binary Coded Decimal as input and performs the conversion. Additionally, it aligns with the four BCD input data lines and generates their complements. Furthermore, the 74LS47 provides active-low outputs designed for powering a common anode LED display through external current-limiting resistors.

Read More

• Understanding IC 7447: A Comprehensive Guide to BCD to 7-Segment Converters

• Understanding Binary Counter in Digital Electronics

• 74ls90 BCD Decade Counter IC: Pinout, Application, and Datasheet

• Everything You Need to Know about 74LS74 Dual D Flip-Flop

• The Ultimate Guide to CD4017 Decade Counter IC

Previous: CD4017BE CMOS Counter: Circuit, Pinout and Datasheet

Next: The Ultimate Guide to 7 Segment Display: Pinout, Working, and Datasheet