The 7447 IC is a popular 7-segment display decoder chip. This integrated circuit converts binary codes into signals that can drive an LED or LCD 7-segment display.
Key Features of the 7447 IC
The 7447 contains the following key features:
Decodes BCD to 7-Segment Display
- Accepts 4-bit binary coded decimal (BCD) as input
- Decodes the BCD into 7 signals that can drive a common cathode 7-segment LED or LCD display
TTL Compatible
- Uses standard TTL voltage levels
- Can connect directly to TTL logic chips or microcontrollers
Internal Pull-Up Resistors
- Contains built-in pull-up resistors for each output pin
- Eliminates need for external current limiting resistors
High Current Drive
- Each output can sink up to 25mA, allowing it to drive LED displays directly
- Useful for building large, high brightness 7-segment displays
7447 Pinout Diagram
The 7447 comes in a 16-pin dual in-line package (DIP) with the following pinout:
| Pin | Symbol | Function |
|---|---|---|
| 1 | BLANK | Display blanking input (active low) |
| 2-5 | A-D | BCD inputs |
| 6 | LT | Latch input (active low) |
| 7-14 | a-g | Cathode outputs for 7-segment display |
| 15 | Vcc | Positive supply voltage (+5V) |
| 16 | GND | Ground |
Sample 7447 Circuit
Here is a simple circuit using the 7447 to drive a common cathode 7-segment LED display:
Four switches allow inputting a BCD digit to display. The LT pin is connected to ground to disable the blanking function. Current limiting resistors are used for each LED segment.
Driving an LCD Display
The 7447 can also drive LCD glass 7-segment displays. An external resistor network is required to limit current through the LCD segments.
Additionally, the BLANK pin can be used to control the colon/decimal point on the display.
Conclusion
With its built-in BCD to 7-segment decoding functionality, the 7447 makes it easy to interface simple numeric displays to digital logic circuits. It remains a versatile and low cost choice for creating digital clocks, timers, counters and other numeric displays.
Frequently Asked Questions
What is the difference between common anode and common cathode displays?
Common anode displays have all the segment anodes connected together and cathodes separate. The 7447 is designed for common cathode displays where the cathodes are common and anodes separate.
Can a 7447 drive incandescent bulbs?
No, incandescent bulbs require higher current than an IC can provide. Use the 7447 with LED or LCD displays only.
How do I make the display blink?
Connect a pulse waveform from a timer IC like NE555 to the BLANK pin. It will blank/unblank the display at the pulse frequency.
Can I cascade two 7447 to create 14-segment displays?
Yes, the outputs can be paralleled to drive larger 14 or 16 segment displays. External current limiting may be required.
What is the maximum supply voltage for the 7447?
The chip can operate from 4.5V up to 7V maximum supply voltage. Avoid exceeding the 7V limit.
