What is the 7447 IC Decoder?
The 7447 IC is a BCD to 7-segment decoder integrated circuit (IC). It converts binary-coded decimal (BCD) input into a 7-segment display output.
Some key features of the 7447 IC:
- Decodes 4-bit BCD values into 7-segment outputs
- Has active-low outputs for driving LED or LCD displays
- Includes overrides on all outputs for blanking display
- Available in 16-pin dual in-line package
How Does the 7447 IC Decoder Work?
The 7447 contains internal logic gates that decode the 4-bit BCD input into 7 outputs that can drive an LED or LCD 7-segment display.
Here is the pinout diagram for the 7447:
| Pin | Description |
|---|---|
| 1-7 | Segment Outputs (a-g) |
| 8 | Lamp Test Input |
| 9 | Blank Input |
| 10-13 | BCD Inputs (D0-D3) |
| 14 | Ground |
| 15 | +5V Supply |
| 16 | Overflow Output |
The 4 BCD input pins (D0-D3) represent the binary-coded decimal value to be displayed. When a BCD value is applied to these inputs, the 7447 drives the corresponding segment outputs low.
For example, to display ‘5’, the inputs would be 0101. This would drive segments a, f, g, c and d low to form the correct character.
The blank and lamp test inputs can override the BCD inputs and blank or illuminate all segments as needed.
7447 IC Decoder Example Circuit
Here is a simple example circuit using the 7447 to drive a common cathode 7-segment LED display:
This circuit has 4 BCD inputs from toggle switches. The 7447 decodes these inputs and drives the 7-segment display over resistors. A current limiting resistor is used for each LED segment.
7447 IC Applications
The 7447 IC decoder is commonly used in:
- Digital clocks
- Electronic meters
- Calculator and counter displays
- Scoreboards and other numeric displays
It provides a simple way to decode BCD values from counters, processors or other logic circuits and drive visual numeric displays.
Some other similar decoder ICs are the 7448 (BCD to 7-segment with active high outputs) and 7446/7448 (BCD to 7-segment with open collector outputs).
Frequently Asked Questions
What is the difference between 7447 and 7448 ICs?
The main difference is that the 7447 has active-low outputs while the 7448 has active-high outputs. So the 7447 sinks current when a segment is illuminated while the 7448 sources current.
How much current can the 7447 outputs sink/source?
The standard 7447 and 7448 can sink/source up to 25mA per output pin. Special higher current versions are also available such as the 75447 (75mA drive).
Can a 7447 directly drive an incandescent 7-segment display?
No, an incandescent display requires higher drive current (350mA+) than a 7447 can handle. External transistors or driver ICs are needed to switch enough current for incandescent displays.
Are 7447 ICs still used today?
Yes, the 7447 continues to be a popular and low-cost way to interface simple BCD data sources to numeric LED, LCD and VFD displays. Modern variants are also available with higher drive capacity and other features.
Can a 7447 be cascaded to drive multi-digit displays?
Yes, multiple 7447 ICs can be cascaded to drive larger multi-digit displays. Microcontroller output ports can also digitally multiplex BCD data to multiple 7447s.
