How to Modify 7447 IC Output to Improve 6 and 9 Display on a 7-Segment

I’m using the 7447 BCD to 7-segment decoder IC with a common anode 7-segment display, and I’ve noticed something odd with how it shows the numbers 6 and 9:

I’d like to tweak this so that:

Since the 7447 IC has fixed logic, I know I’ll probably need some external circuitry. I was thinking about using logic gates to detect when the BCD inputs correspond to 6 (0110) and 9 (1001) and then force the missing segments to light up. My questions are:

I’d really appreciate any advice, diagrams, or tips from people who’ve dealt with something like this before. Thanks!

the store I’m sourcing my components from doesn’t have the 74247 or any other ICs that support the "tails" for these digits. That’s why I’m trying to work with the 7447 and explore if there’s a way to improve the digit representation with the components I currently have

What about two resistors and two NPN transistors? Credit goes to @Sneftel for the initial idea.

To fix the "6": connect input bit 1 (2^1) to a resistor and a it to the base of an NPN BJT, with the collector connected to the output which turns-on the top segment. This bit will be 1 for the numbers 2, 3, 6 and 7.

To fix the "9": connect input bit 3 (2^3) to a resistor and a it to the base of an NPN BJT, with the collector connected to the output which turns-on the bottom segment. This bit will be 1 for the numbers 8 and 9.



simulate this circuit – Schematic created using CircuitLab

Since the outputs are special (open collector), you cannot simply add "logic gates".

Instead, use the pin-compatible 74LS247. It has this output (copied from the linked data sheet):



Instead of adding logic to the 74xx47, consider using chips with the output you actually want, eg the very similar CD74HCT4511E:



It's not quite pin-compatible, as it has a latch, and the blanking mechanism is slightly different.

Sure. All valid BCD inputs of the form x11x have the top segment illuminated and all valid BCD inputs of the form 1xxx have the bottom segment illuminated, assuming your desired "complete" numerals. Since this is an active-low open-collector output you can parallel it with another open-collector and they'll act as an "or". Easiest would be to use a 74x03 (quad 2-input NAND), with one of the NAND gates taking the second and third bits, and another one taking the fourth bit at both inputs. The first output gets connected to the top segment and the second output gets connected to the bottom segment. Don't forget to ground the unused gate inputs.

EDIT: thanks to the simplification of the logic devnull noticed, this can be done even more simply, with two inverting open-collector buffers in a 74x05: one with the second bit as input and the top segment as output, and one with the fourth bit as input and the bottom segment as output.