Why this (IC7447 and 7 -Seg combo) not light up? I'm not sure as if what to do of the 3 lines of the IC7447 (Pin - 3,4,5) which I've depicted in GREEN below. Because at certain sites they have been shown ACTIVE LOW, but they are connected to +Vcc while many others even don't have connections for them(as seen on YouTube videos.they just leave them isolated). So I'm confused so as what combination of connection to have for them.
According to the datasheet for a TI family of BCD-to-seven-segment decoders (http://www.ti.com/lit/ds/symlink/sn5447a.pdf), these control lines are active-low, and should be either left floating or pulled high when not in use (footnotes on p. 3 and 4).
As for the LEDs not lighting up, what input combination have you tried on the decoder? You've left them disconnected on the picture and for all high inputs (=> 15) it outputs a blank display (p. 3)...
(Edit) I see you found that only a few segments light up when you pull pin 3 to ground. That's the Lamp Test Pin, and it should make all the segments light up. If they don't, something is broken. Have you tested all the segments by hand?
we have had the same problem as yours..The problem was caused since we had taken a common cathode display..apparently,the problem might be same with your circuit. The 7447 does not work for common cathode display. 7447 only works for a common anode display. So check and verify whether yours is a common anode display. Good Luck! :) From, (Agnel Institute of Technology and Design(AITD))