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.


This is the same IMAGE with adjustment so that wirings will be more clear to see. DECREASED CONTRAST TO SEE WIRING MORE CLEARLY This 7-SEGMENT is for my ELECTRONICS PROJECT of ALU(Arithmetic & LOGIC UNIT), for 4-bit adder.

  • \$\begingroup\$ Are none of the segments working or just some of them? Do they light when you attach the wire directly to the supply bypassing the chip? \$\endgroup\$ – John Meacham Nov 11 '16 at 8:54
  • \$\begingroup\$ None man. None of them are working! \$\endgroup\$ – Nitish Prajapati Nov 11 '16 at 8:56
  • \$\begingroup\$ Yes. i mean when one i directly connect -ve terminal to the other end of resistor(rather than allowing them through IC,)then the segment lights up. \$\endgroup\$ – Nitish Prajapati Nov 11 '16 at 8:58
  • \$\begingroup\$ UPDATE: when i connect the 3rd pin to ground rather than +Vcc(Without disturbing 4th and 5th pin),then the segment shows a Small "b". But The inputs are not showing up. Just a "b" that's it. \$\endgroup\$ – Nitish Prajapati Nov 11 '16 at 9:02
  • \$\begingroup\$ So pin 3 the lamp test pin should make all the segments turn on I believe. Not just b Are your breadboard connections flaky? \$\endgroup\$ – John Meacham Nov 11 '16 at 9:07

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?

  • \$\begingroup\$ It's only for a 4 bit display(BUT MAX = 7(i.e. 0111)...i guess it may show 15 as well, but in the format "F"). It's just in the picture that I've left them disconnected. Practically i tried various combinations of inputs, but of no use. Moreover, if i leave them blank, then it should show a 0, isn't it? \$\endgroup\$ – Nitish Prajapati Nov 11 '16 at 9:08
  • \$\begingroup\$ UPDATE: OK FINE, i see that the segment was broken since as i tested one by one, not all of them lit up. Now i changed it and all of them LIT UP. But now what,What's next? \$\endgroup\$ – Nitish Prajapati Nov 11 '16 at 9:11
  • \$\begingroup\$ It does not necessarily show '0' when you leave the inputs open, in fact it assumes the inputs are HIGH unless you actively pull them down. Now that you have a working 7-Segment display, try pulling pin 3 (the lamp test) to HIGH again and then apply different combinations of inputs. It should display numbers then (check the table on p. 3 of the datasheet for more details) \$\endgroup\$ – Akko Nov 11 '16 at 9:15
  • \$\begingroup\$ Ok.Cool. It's working now, as you said , It needed me to Actively pull the INPUTS DOWN OR push them UP rather than leaving them disconnected. Even if i had connected one,or two to HIGH and ignored others, even then it was showing blank. It meant that all Needed to be ACTIVE SOMETHING. Thanks. :) \$\endgroup\$ – Nitish Prajapati Nov 11 '16 at 9:47
  • \$\begingroup\$ No problem, glad I could help :D \$\endgroup\$ – Akko Nov 11 '16 at 9:51

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))


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