Pull-up or pull-down when building an LED ROM for 74HC logic levels?
This idea, though cute, will not work reliably as is.

The issue is LEDs have a typical forward voltage of 1.6V

With pull up that puts the low level output at 1.6V which is over the max Vil threshold for HC. So pull up is out.

With pull down the high level is 3.4V which will be above the max Vih, but is really close, especially when you subtract whatever you need to buffer the signals (See below). An additional transistor detector may be required here. See Spehro's Answer

Further, in order to get any decent light out of the LEDs the resistors will need to be small and the TTL will not be able to drive a full row of LEDs at once. You will likely need to add some sort of transistor to the decoder to push that much current.

All told, something like this may do it for you. LED present = 1 out.







simulate this circuit &ndash; Schematic created using CircuitLab