The 7447 IC is a popular 7 segment display decoder chip used in many electronic projects and products. This article provides an overview of the 7447, how it works, and some example circuits using this versatile logic chip.
What is a 7 Segment Display Decoder?
A 7 segment display has 7 LEDs arranged in a rectangle to form each digit. To display a decimal number, the correct LEDs need to be illuminated. A decoder takes a 4-bit binary input and decodes it to turn on the proper LEDs to display the digit.
The 7447 contains the logic to take a 4-bit binary coded decimal (BCD) input and convert it to the 7 signals needed to drive a common cathode 7 segment display.
Pinout and Features
The 7447 comes in a 16 pin dual in line (DIP) package. The pinout is:
| Pin | Description |
|---|---|
| 1 | Lamp Test Input |
| 2-5 | 4-bit BCD Input |
| 6-12 | Cathode Outputs |
| 13-15 | Not Used |
| 16 | Ground |
Key features of the 7447:
- Built in BCD to 7 segment decoder
- Open collector outputs can sink up to 25mA current
- Has lamp test input to illuminate all segments
- Wide operating voltage range of 3-15V
How the 7447 Works
The 7447 contains combinational logic gates internally to translate the 4-bit BCD input into the 7 cathode control signals.
For example, if the BCD input is 0110, this represents the number 6. The logic gates inside the 7447 will turn this input into the correct output signals to illuminate segments A, B, F, G, C, and D to properly display a “6” on the 7 segment display.
The 7447 uses active low outputs – when an output is low, it will sink current to light the connected LED segment. Often current limiting resistors are used between the outputs and the LEDs.
Example Circuits
Here are some example circuits using the 7447 to drive 7 segment displays:
Simple BCD Counter Display
This circuit hooks up a 4-bit counter IC like the 7490 to the 7447 to create a decimal counting display:
Simple BCD Counter 7 Segment Display Circuit
The counter provides the BCD digits, and the 7447 decodes them and drives the common cathode 7 segment display.
Multiplexed Displays
Using ICs like the 74LS247 or 74HC4511, multiple 7 segment displays can be driven by the 7447 using time multiplexing. This reduces the number of I/O pins required to drive multiple digits.
In a multiplexed circuit, the 7447 cathode outputs would connect to the segments of each display. The common pins of the display would be driven by the digit selector ICs.
Driving Larger Displays
To drive larger multi-digit displays, shift registers like the 74HC595 can be used. The shift register outputs connect to the 7447, which controls the proper segments. Again, digit multiplexing can be used to control larger displays with just one 7447 chip.
Common Use Cases
Some common products and applications that use the 7447 7 segment decoder:
- Digital clocks
- Electronic counters and timers
- Measuring instruments like multimeters and oscillscopes
- Alphanumeric displays
- Basic calculators
- Appliances like microwave ovens
- Digital logic trainer boards
The 7447 provides an easy way to interface simple microcontroller or logic circuits to LED or LCD seven segment numeric displays. Its wide availability and easy interfacing makes it popular in many electronics hobbyist and commercial products.
FQA
What is the difference between common anode and common cathode displays?
Common cathode displays have all the cathodes or negative sides of the LEDs connected together. The 7447 is designed to work with common cathode displays. For common anode, a PNP transistor circuit is needed to invert the signals.
Can a 7447 drive an LCD display?
Yes, as long as it is a common cathode LCD display. The 7447 can provide the segment logic signals to properly drive the LCD segments.
How do I drive a decimal point with the 7447?
The 7447 does not directly drive a decimal point segment. An additional logic gate connected to one of the inputs can enable the decimal point on the display when needed.
Can I cascade two 7447 to control a 14 segment display?
Yes, the outputs of two 7447 chips can be paralleled to enable controlling two digit 14 segment displays, with one handling each digit. Extra logic may be needed to properly display all numerals.
How do I strobe or blink the display driven by a 7447?
A 555 timer or microcontroller output connected to the cathode pins can enable the strobing or blinking of the display by pulsing the cathode signals on and off.
