Introduction
The 74LS47 is a popular CMOS BCD (Binary Coded Decimal) to 7-segment decoder integrated circuit (IC) manufactured by various companies like TI, NXP, STMicroelectronics etc. It is used for converting binary coded decimal digits from devices like counters, shift registers, microprocessors etc. to drive common cathode 7-segment LED displays.
Some key features of 74LS47 ICs are:
- Decodes 4-bit Binary Coded Decimal (BCD) inputs to 7-segment outputs
- Has active LOW outputs
- Outputs can sink up to 25mA current
- Operates from 4.5V to 5.5V supply
- High noise immunity
- Wide operating temperature range
- High speed CMOS design
In this article, we will learn about the internal circuitry, pin diagram, truth table and some applications of the 74LS47 BCD to 7-segment decoder.
Pin Diagram
The 74LS47 is available in dual in-line package (DIP) with 16 pins. The pinout diagram is shown below:
| Pin | Description |
|---|---|
| 1 | Lamp Test Input |
| 2 | Input B0 |
| 3 | Input B1 |
| 4 | Input B2 |
| 5 | Input B3 |
| 6 | Cathode g |
| 7 | Cathode f |
| 8 | Cathode e |
| 9 | Cathode d |
| 10 | Cathode c |
| 11 | Cathode b |
| 12 | Cathode a |
| 13 | Anode 7 |
| 14 | Supply Voltage |
| 15 | Ground |
| 16 | Lamp Test Output |
Internal Circuit
The internal block diagram of 74LS47 is shown below. It contains a 4-to-16 line decoder circuit and 7 AND gates.
The 4 BCD input bits B0 to B3 are decoded into 16 lines using a 4-to-16 line decoder. Only 7 outputs corresponding to valid BCD codes are used.
The 7 AND gates are used to drive the 7 segments a to g. The anode inputs of the display are connected to the active LOW outputs of the decoder.
Truth Table
The truth table of 74LS47 giving the outputs for different BCD digit inputs is shown below:
| B3 | B2 | B1 | B0 | a | b | c | d | e | f | g |
|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 0 | 0 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 0 |
| 0 | 0 | 0 | 1 | 0 | 1 | 1 | 0 | 0 | 0 | 0 |
| 0 | 0 | 1 | 0 | 1 | 1 | 0 | 1 | 1 | 0 | 1 |
| 0 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 0 | 1 |
| 0 | 1 | 0 | 0 | 0 | 1 | 1 | 0 | 0 | 1 | 1 |
| 0 | 1 | 0 | 1 | 1 | 0 | 1 | 1 | 0 | 1 | 1 |
| 0 | 1 | 1 | 0 | 1 | 0 | 1 | 1 | 1 | 1 | 0 |
| 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | 1 | 1 |
| 1 | 0 | 0 | 0 | 1 | 1 | 1 | 0 | 0 | 1 | 1 |
| 1 | 0 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
For all other input combinations, the outputs are OFF.
Applications
Some common applications of 74LS47 ICs are:
- Displaying output of Binary Coded Decimal (BCD) counters and registers
- Interfacing microcontrollers to 7-segment LED displays
- Decoding BCD output from shift registers to drive displays
- Embedded systems and logic circuits needing BCD to 7-segment conversion
Here are some typical application circuits using the 74LS47:
Interfacing 74LS47 with a BCD Counter
A 74LS90 counter can be used to generate a BCD count sequence from 0000 to 1001. The 74LS47 decodes the BCD value and displays it on a common cathode 7-segment display.
Interfacing 74LS47 with a Microcontroller
A microcontroller sends 4-bit BCD data through its I/O port to the 74LS47, which converts it into appropriate signals for the 7-segment display.
Cascading Multiple 74LS47 to Drive Large Displays
For driving displays with more than 7-segments like 14-segment or 16-segment displays, multiple 74LS47 ICs can be cascaded. The microcontroller sends the segment code to the 74LS47s which decode and drive the appropriate segments.
Frequently Asked Questions
1. What is the difference between 74LS47 and 74HC47?
The main difference is that 74LS47 is based on Low-Power Schottky TTL logic while 74HC47 is based on advanced CMOS logic. 74HC47 has higher speed, lower power consumption and better noise immunity than 74LS47.
2. How much current can the 74LS47 sink or source?
The output pins of 74LS47 can sink up to 25mA per pin. The inputs are CMOS type drawing very little current.
3. Can I connect a common anode 7-segment display directly to 74LS47?
No, you cannot connect a common anode display directly. The 74LS47 is designed for common cathode displays. For common anode, you need additional transistors at the output pins.
4. How do I generate BCD codes to test the 74LS47?
Simple ways to generate BCD test codes are to use a binary counter like 74LS90 or connect switches at the input pins. A microcontroller can also be programmed to generate test codes.
5. Can I cascade two 74LS47s to drive two 7-segment displays?
Yes, it is possible. One 74LS47 can drive one display and the other 74LS47 drives the second display. Additional logic may be needed to send appropriate codes to the two ICs.
Conclusion
The 74LS47 integrated circuit provides a convenient way to interface binary coded decimal output from counters, ICs or microcontrollers to common cathode 7-segment LED displays. With its simple BCD to 7-segment conversion logic, it eliminates the need of complex encoding circuits. It is useful for creating digital displays, logic systems, embedded designs and other applications which need to display numeric data.
