Introduction to the 74LS47

The 74LS47 is a member of the 7400 series of TTL (Transistor-Transistor Logic) ICs. It is designed to convert a 4-bit Binary Coded Decimal (BCD) input into a 7-bit output that can directly drive a 7-segment LED display. The IC simplifies the process of displaying decimal numbers by providing a simple interface between the BCD input and the LED display.

Key Features of the 74LS47

• BCD to 7-segment decoder/driver
• Low power consumption
• High noise immunity
• Wide operating voltage range (4.75V to 5.25V)
• Active-low outputs for directly driving common-anode LED displays
• Ripple-blanking input for leading/trailing zero suppression

74LS47 Pin Configuration

The 74LS47 comes in a 16-pin Dual Inline Package (DIP). The pin configuration is as follows:

Pin Number	Pin Name	Description
1	B	BCD input B
2	C	BCD input C
3	LT	Lamp Test input (active low)
4	BI/RBO	Blanking Input/Ripple Blanking Output
5	RBI	Ripple Blanking Input
6	D	BCD input D
7	A	BCD input A
8	GND	Ground
9	e	7-segment output e (active low)
10	d	7-segment output d (active low)
11	c	7-segment output c (active low)
12	b	7-segment output b (active low)
13	a	7-segment output a (active low)
14	g	7-segment output g (active low)
15	f	7-segment output f (active low)
16	VCC	Power supply (+5V)

BCD Inputs (A, B, C, D)

The BCD inputs A, B, C, and D represent the binary-coded decimal value to be displayed on the 7-segment LED. The combination of these four inputs determines the decimal digit (0-9) that will be displayed.

Lamp Test Input (LT)

The Lamp Test input is an active-low input that, when pulled low, causes all 7 segments of the LED to light up regardless of the BCD input. This feature is useful for testing the functionality of the LED display.

Blanking Input/Ripple Blanking Output (BI/RBO) and Ripple Blanking Input (RBI)

The Blanking Input (BI) is used to suppress leading/trailing zeros in multi-digit displays. When BI is high, the 74LS47 will decode the BCD input as usual. When BI is low, the 7-segment outputs will be forced to a non-display state (all segments off).

The Ripple Blanking Output (RBO) is used to cascade multiple 74LS47 ICs for displaying multi-digit numbers. The RBO of one 74LS47 is connected to the Ripple Blanking Input (RBI) of the next 74LS47. This allows the suppression of leading zeros to propagate from the most significant digit to the least significant digit.

7-Segment Outputs (a, b, c, d, e, f, g)

The 7-segment outputs (a, b, c, d, e, f, g) are active-low outputs that directly drive the individual segments of a common-anode 7-segment LED display. When an output is low, the corresponding segment will light up.

74LS47 Truth Table

The truth table for the 74LS47 shows the relationship between the BCD inputs (A, B, C, D) and the 7-segment outputs (a, b, c, d, e, f, g). The ‘X’ in the table represents a “don’t care” state, which means the input can be either high or low.

BCD Input		7-Segment Output
D	C	B	A	a	b	c	d	e
—	—	—	—	—	—	—	—	—
0	0	0	0	1	1	1	1	1
0	0	0	1	0	1	1	0	0
0	0	1	0	1	1	0	1	1
0	0	1	1	1	1	1	1	0
0	1	0	0	0	1	1	0	0
0	1	0	1	1	0	1	1	0
0	1	1	0	1	0	1	1	1
0	1	1	1	1	1	1	0	0
1	0	0	0	1	1	1	1	1
1	0	0	1	1	1	1	1	0
1	0	1	0	0	0	0	1	1
1	0	1	1	0	0	1	1	0
1	1	0	0	0	1	0	0	0
1	1	0	1	1	0	0	1	0
1	1	1	0	0	0	0	1	1
1	1	1	1	0	0	0	0	0

Applications of the 74LS47

The 74LS47 finds applications in various digital systems where displaying decimal numbers is required. Some common applications include:

1. Digital clocks and timers
2. Counters and score displays
3. Digital multimeters and panel meters
4. Instrumentation and control systems
5. Elevator floor indicators
6. Parking lot occupancy displays

Example Circuit: Single-Digit Counter with 7-Segment Display

Here’s an example circuit that demonstrates the use of a 74LS47 to display a single-digit decimal counter:

        +5V
         |
        -+-
         |
        | |
        | | R1 (330Ω)
        | |
        | |
        | |    7-Segment LED Display
        | |    ---------
        | |   |   a    |
        -+-   |  ----   |
         |    | |    |  |
 74LS47  |    | f|  | b|
-------  |    |  |  g|  |
|      |      |  ----   |
| A  9 |------| e|  | c|
| B 10 |------|  ----   |
| C 11 |------|   d     |
| D 12 |------|---------
|      |
|      |      Push Button
| BI 4 |------o/o----+
| RBI 5|              |
|      |             GND
|      |
| LT 3 |----+
|      |    |
 -------    |
            |
           GND

In this circuit, a push button is connected to the BI input of the 74LS47. When the button is pressed, the counter increments, and the corresponding decimal digit is displayed on the 7-segment LED. The current-limiting resistor R1 is used to limit the current through the LED segments.

Advantages and Disadvantages of Using the 74LS47

Advantages

1. Simplifies the interface between BCD input and 7-segment LED display
2. Directly drives common-anode LED displays without additional driver circuits
3. Supports leading/trailing zero suppression for multi-digit displays
4. Low power consumption and high noise immunity

Disadvantages

1. Limited to displaying decimal digits (0-9) and a few additional characters
2. Requires external current-limiting resistors for the LED segments
3. Not suitable for driving high-current or high-voltage displays

Frequently Asked Questions (FAQ)

1. What is the difference between a common-anode and common-cathode 7-segment LED display?

In a common-anode display, all the anodes of the LED segments are connected together and driven by a positive voltage, while the cathodes are individually controlled by the driver IC (e.g., 74LS47). In a common-cathode display, all the cathodes are connected together and grounded, while the anodes are individually controlled.

2. Can the 74LS47 drive a common-cathode 7-segment LED display?

No, the 74LS47 is designed to drive common-anode displays. To drive a common-cathode display, you would need to use a different decoder/driver IC, such as the 74LS48 or 4511.

3. What is the maximum current that the 74LS47 can sink per output?

The 74LS47 can sink up to 24mA per output. However, it is recommended to use current-limiting resistors to keep the current below this limit and protect the IC and LED segments.

4. Can I use the 74LS47 to display hexadecimal digits (A-F)?

No, the 74LS47 is designed to display decimal digits (0-9) only. To display hexadecimal digits, you would need to use a different decoder/driver IC, such as the 74LS49 or 4511.

5. How do I cascade multiple 74LS47 ICs to display multi-digit numbers?

To display multi-digit numbers, you can connect multiple 74LS47 ICs in a cascaded configuration. The RBO (Ripple Blanking Output) of the most significant digit’s 74LS47 is connected to the RBI (Ripple Blanking Input) of the next 74LS47, and so on. This allows the leading zero suppression to propagate from the most significant digit to the least significant digit.

Conclusion

The 74LS47 is a versatile and widely used BCD to 7-segment decoder/driver IC that simplifies the process of displaying decimal numbers on LED displays. By understanding its pin configuration, truth table, and functionality, you can effectively incorporate the 74LS47 into your digital projects that require decimal number displays. Its low power consumption, high noise immunity, and direct drive capability for common-anode displays make it an attractive choice for a wide range of applications.