A 7 segment decoder IC is an integrated circuit that takes a binary input and activates the appropriate segments to display a decimal digit on a 7 segment display. 7 segment displays are commonly used in electronic devices to show numeric information.

How 7 segment displays work

A 7 segment display has 7 LED segments arranged in the shape of an 8:

By lighting up different combinations of the 7 segments, all digits from 0 to 9 can be displayed:

To display the correct digit, the 7 segments need to be turned on or off in the right pattern. But connecting 7 LED pins directly to a microcontroller would take up a lot of IO pins.

This is where 7 segment decoder ICs come in – they take a 4-bit binary input and activate the correct 7 outputs for the digit that input represents.

Types of 7 segment decoder ICs

Some common 7 segment decoder ICs include:

• 7447 – BCD to 7 segment decoder. This contains the logic to convert a 4-bit Binary Coded Decimal (BCD) input to a 7 segment display output.
• 7448 – BCD to 7 segment decoder with active low outputs. Same as 7447 but the outputs are active low instead of active high.
• 74HC4511 – BCD to 7 segment decoder with latches. Has latches built in to hold the output value without needing the input bits to be held.
• 74LS47 – 4 to 16 line decoder. Can be used as a generic 1-of-16 decoder to drive 7 segments plus other outputs.

How 7 segment decoder ICs work

The basic internal logic of a 7 segment decoder is shown below:

It contains a 4 to 16 line decoder circuit that converts the 4 bit input into 1 of 16 output lines. Only 7 of the 16 outputs are used – one for each LED segment.

The specific logic to activate the right segments for each digit is implemented inside the IC using logic gates and diodes connected to each output pin.

When an enable pin is brought high, the binary input is decoded and the corresponding segments are lit up.

BCD encoding

For ICs like the 7447 and 7448, the input is in Binary Coded Decimal format. This encoding scheme uses a binary number to represent a single decimal digit.

The 4 bits represent digits 0 to 9 rather than 0 to 15:

This matches the bit patterns required to light up the segments for each digit. The IC handles the BCD to 7 segment conversion internally.

74HC4511 latch decoder

The 74HC4511 has an additional latch circuit connected to the outputs so that the decoded segments remain on even after the input changes.

This allows the microcontroller to change the data outputs after setting up the digit without affecting what is displayed. Multiple 74HC4511 can be daisy chained to control multiple digits.

Using 7 segment decoder ICs

Here is a typical setup using a 7 segment decoder IC like the 7447:

• Pins 1-4 are connected to the 4 bit binary input from a microcontroller or other logic circuit.
• Pin 5 is the enable input. Pulling this high latches the input bits and turns on the decoder.
• Pins 6-12 are connected to the 7 segments of the LED display.
• Pin 13 is ground.

The microcontroller sends the binary digit code to the input pins while enable is low. When ready, it brings enable high to light up the segments.

Additional displays can be added by using separate decoder ICs with their enables chained together. The microcontroller would cycle through each enable to refresh the display.

Multiplexing displays

For larger multi-digit displays, multiplexing is commonly used. Rather than have separate decoder ICs for each digit, the digits are rapidly scanned one at a time by a single decoder.

This is done by cycling through the enable pins and sending the updated segment data for each digit to the IC as its enable activates. If done fast enough, it appears that all digits are on simultaneously.

Multiplexing reduces the IO pins and decoders needed to drive multiple digits. The 74HC4511 latch decoder makes this simpler by preserving segment states between enables.

7 segment decoder IC summary

• 7 segment decoder ICs take 4 bit inputs and convert to 1 of 7 outputs to drive a 7 segment LED display
• Common ICs include 7447, 7448, 74HC4511. 74LS47 can also be used.
• BCD to 7 segment decoders have built-in logic to convert binary coded decimal to segments
• 74HC4511 includes latches to hold segment data between enables
• Decoder ICs allow driving multi-digit displays using just 4 microcontroller pins
• Multiplexing can be used with decoder ICs to control large multi-digit displays

FAQ

How do you choose the right decoder IC?

Consider the following:

• Input logic levels – Make sure the IC matches the voltage levels from the microcontroller. 74HC series is a good choice for 3-5V Arduino boards.
• BCD or binary input – BCD decoders like 7447 make it easier to show decimal digits. Binary input chips allow more flexibility.
• Active high/low outputs – Active low ICs like 7448 sink current when on, active high source current. Match to LED common cathode/anode.
• Latches – 74HC4511 has output latches to simplify multiplexing. Other chips require holding inputs during display.

How do you set up a multi-digit display?

For 2+ digits, you can:

• Use separate decoder ICs, one per digit. Chain their enables together.
• Use a single decoder IC and multiplex the digits by cycling the enable and updating the inputs.
• Use multiple 74HC4511 decoders chained together since they hold their outputs between enables.

Additional logic or microcontroller pins may be needed to control which digit is active for multiplexing.

How fast should multiplexing be done?

At least 35-40Hz, preferably faster. This avoids visible flicker. Lower rates will result in the digits appearing faint or blinking.

The optimal refresh rate depends on the LED brightness and ambient lighting. Faster speeds up to a few hundred Hz produce a more solid display.

Can decimal points or other segments be added?

Yes, unused decoder IC output pins can drive decimal points, “AM/PM” indicators, etc. These can be activated based on the binary input code.

For common anode displays, unused outputs should be left disconnected or grounded. For common cathode, leave unused pins open.

What is the maximum supply current for the IC outputs?

Most decoder ICs can supply 20-25mA per output pin, allowing the use of medium brightness LEDs.

Check the datasheet – exceeding the maximum supply current rating can damage the IC over time. Darlington buffers can be added for brighter LEDs requiring >25mA.