I have 10*22 cm perf boards, which are 78*34 holes, from which I estimate I can place 8 digits. Each digit would occupy a plot of 8*13 holes on board. Each segment will be done as 2 red LEDs in series, which will have forward drop of 3.7 volts, so, driving them from 5 volt supply might pose some challenge.

I envisioning this device as comprised from several parts:

1) 8 digits (obvious) made from 14 red LEDs each

2) enabler circuits, sourcing supply current to digits at the moments when they should light up

3) 3 bit counter with base frequency of at least 800 Hz with 3-to-8 decoder for addressing digits and their respective registers

4) 4-bit registers for each digit, connected to common bus through tri-state buffers

5) transcoder, the unit which should decode 4-bit binary number from common bus into the pattern of appropriate segments; also this unit will sink current from segment diodes

6) special registers: input data latch (4 bit) and address latch (3 bit), so one could input a digit in specified place

7) some other circuitry yet unknown.

Registers will be done as data latches made with dtl nand gates (all from discrete electronic components). Counter circuit will be made from dtl nand master-slave data flip-flops as ripple counter (the same circuit as was used in my "counter/clock" project.

For now, the biggest problem for me is the implementation of 3-state buffers with bjts. As of now, I still have no solution, and searching the internet turned out empty so far. Closest I found was open emitter circuit, but it can only source current ( pull up voltage) but can not sink (pull to ground).