Introduction

The 74LS47 is a popular TTL logic IC chip used for decoding BCD (Binary Coded Decimal) numbers and driving 7-segment displays. It allows microprocessors with limited IO pins to display decimal digits on 7-segment LED or LCD displays. However, the 74LS47 has an Achilles heel – it can be easily cracked to enable displaying any arbitrary pattern on the 7-segment display instead of just 0-9 digits. This can be very useful for creating custom characters, animations, graphs etc. on 7-segment displays.

In this detailed guide, we will explore the internal workings of the 74LS47 and how to exploit its weaknesses to get full control over the 7-segment display connected to it.

How the 74LS47 Works

The 74LS47 contains the following key components:

BCD to 7-Segment Decoder

This component takes as input the 4-bit BCD code of the digit to be displayed. It decodes the binary pattern into a pattern of 7 control signals corresponding to each segment of the display. For example, BCD 0110 (decimal 6) is decoded to turn on segments A, B, F, G and C to display ‘6’.

Output Buffers/Drivers

The decoder outputs cannot directly drive the high-current requirements of an LED display. So output buffers are used to boost the current drive capability. These buffers are active-low, meaning a low level from the decoder will turn ON a segment, while a high level turns it OFF.

Lamp Test Input

This allows turning ON all segments of the display to test that all LEDs are functional. Overriding the decoder output.

Ripple Blanking Input

This input blanks the display output during the ripple carry propagation when driving multiple displays from sequential BCD counters.

Cracking the 74LS47

The key to gaining full control over the display is exploiting the ripple blanking input. When this input is driven HIGH, it forces all the output buffers into a HIGH impedance state essentially disconnecting the internal decoder from the display. This allows us to directly drive the 7-segment pins externally to show any pattern.

Circuit to override the IC

Only a simple transistor circuit is needed to pull the ripple blanking pin high and override the IC.

A logic HIGH on the ‘Display Override’ input turns ON Q1 pulling pin 15 high. This blanks the internal decoder and allows full control via the 7 segment pins.

A logic LOW on the input turns OFF Q1, allowing normal BCD digit decoding by the IC.

Driving arbitrary patterns

With the override in place, we can now connect microcontroller IO pins directly to the 7 segment pins of the 74LS47 to drive custom patterns.

Here are some examples of arbitrary dot patterns we can display:

Copy code

A F G _ _ | | |_| B C D E

We can display alphanumeric characters by referring to their 7-segment encoding patterns. For example:

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A F G A F G _ _ _ _ | | | | | | | | |_| |_| |_| |_| B C D E B C E Displays 'H' Displays 'b'

Applications

With the ability to override the IC and display any arbitrary pattern, many interesting applications are possible:

• Display custom characters and symbols unconstrained by just 0-9 digits. Useful for displaying unit symbols, arrows, animations etc.
• Graphing applications by lighting up bargraph patterns on the 7-segment display
• Scrolling text by continuing to override the IC and displaying alphanumeric patterns
• Displaying ASCII art patterns for graphics and games
• Educational coder/hacker kits to demonstrate how to override an IC by exploiting internal signals

Conclusion

While the 74LS47 was designed to just decode BCD digits, we have seen how it can be cracked open by manipulating the internal ripple blanking signal. This grants us full freedom to take over the 7-segment display and use it in creative ways beyond just decimal digit display. The simple override circuit makes this hack easy to implement using just one extra transistor. So experiment and see what new 7-segment tricks you can pull off!

FAQ

What is the 74LS47 IC used for?

The 74LS47 is an integrated circuit chip that converts 4-bit binary coded decimal (BCD) numbers into drive signals for a 7-segment LED display. It allows easy display of decimal digits 0-9 on 7-segment displays.

How does the ripple blanking input help crack the 74LS47?

The ripple blanking input when driven HIGH forces the output buffers of the 74LS47 into a HIGH impedance state. This effectively disconnects the internal BCD decoder and allows the 7-segment pins to be directly driven externally.

What components are needed to override the IC?

Just one NPN transistor and a couple of resistors allow the ripple blanking pin to be controlled via a microcontroller IO pin. When driven HIGH, the blanking overrides the IC.

What can you display by cracking the 74LS47?

With the BCD decoder overridden, the 7-segment display can show any arbitrary pattern – custom characters, shapes, graphs, animations, text etc. Anything can be displayed by programming the correct segments.

How do you determine the 7-segment encoding for a character?

Refer to 7-segment character encoding charts. These provide the segment patterns needed to display alphanumeric characters and symbols on a 7-segment display.