3-digit, 7-segment display (one unit) questions/help..

Hey gang–

I am looking for an Eagle symbol for a 3-digit, 7-segment led display (the all-in-one units, not 3 individual 7-segments displays) :slight_smile:

I am planning on use this with a a MAX7219/7221 chip, for reference during reading.

First… I have to ask about the differences between these two displays I found:

1 has 12 pins:

1 has 11 pins: (which might make more sense?..to me I guess? SegA-G + DP = 8, 3 digits = 3GNDS = 11 pins total?)

I searched and searched… and finally found a 3-digit, 7-segment display Eagle part/symbol…
but it has 12 pins…

C3 (which I’m assuming are the common grounds/cathodes?)

and then an “L” pin?

(possibly a LATCH pin perhaps?)

either way… I dont believe is the one/type of display I want to use in the schematic as it doesnt match any of the actual/physical displays I have found/will be using?

Anybody have one? a link to a lib with one? Used one in a project before?

Also some insight on these differences?

again… my current approach is to make a simple ‘backpack’ PCB (BOB) for this 3-digit display… with the MAX and vRegulator circuit(s) on the back…etc… (this is to be used as a counter)

the MAX needs 3 lines to the Arduino… and multiplexes so I don’t need to worry about high power requirements

thanks again!

3-digit_bob.zip (41.1 KB)

You need a 3-digit display with 3 common cathode pins, 1 per digit. I can't open those links - are either described as common cathode?

For example, browse the PDFs here, look for common cathode picture on first page. http://www.kingbrightusa.com/category.asp?catalog%5Fname=LED&category%5Fname=KC3%2DDigit+7%2DSegment+Thru%2DHole+Display&Page=4&orderby=

yes.. those are common cathodes.. (at least by link & auction description) :)

What is the difference between the 11 pin and the 12 pin?

(12 pin version Eagle symbol is was attached in .sch & .brd files above)

So in general.. I should just be looking for a normal (common cathode) 11 pin 3-digit display/unit.. right?

(I need to find an Eagle symbol now too!) LOL

morning bump… :slight_smile:

Sorry, forgot to look last night.

no worries..

just gave it the ol' bump in-case anyone else was around/had time.. :) (or you forgot!) ;)


Im still abit confused as to the ebay links posted above??..

1 has an 11 pin pinout.. and the other seems to have 12?

11 makes sense for 3 digits (8) + 3 gnds = 11 pins.. I have a 'used' one at home.. (not sure where it came from.. looks to have the 12 pin bent down.. not used?.. its part of PIC based board..that has no relevance) :)

so if I wanted to order some to play/work with... beside ensure they are common cathode.... does it matter?

The more important aspect is the Eagle footprint..

outside the one I posted above.. I found some in the

display-kingbright library...

under: B?56-21*WA sub-directory...

SEEMS to be the style have in my hand.. and on Ebay.. 12 pins..

however.. these are only 0.56inch in size?

I think the ones at my house I just measured were .75 in height (1.25 in width if I recall?)

anyone used a 3 digit numeric display before in there projects?


Im curious if anyone can answer this:

when they are describing the 'size' of the 7-segments displays.. (like in the ebay links above)..

the .56in are they referring to the actual NUMERIC height?.. or the overall height of the 'display/unit' itself?

the unit I have in hand.. measure .75in for the display in height.. but when I just measured the numeric portion.. it seems to be 0.56in?

(so I guess those 0.56in ones ARE the ones I need to order? i cant seem to find them at Digikey or Mouser though? tayada seems to only have 0.28in ones??)

12th pin if it exists it's an unused pin on all the displays. Quoted height is the actual segments, case adds to that dimension. You can create a symbol for any size you want, I know you have in the past. You can also check Newark.com for symbols, they have them for a lot of parts.


so I know that I am looking for the 0.56" ones.. common cathode (blue)

(I think the symbol is already in one of the default libraries.. display-kingbright?)

but I'll check Newark anyways.. to see about ordering some :)

Be careful with Newark orders, if they ship from UK it's expensive. Can download eagle symbols for free once you register with them.


heres the 99% completed pcb/schematic…

I wasnt 100% sure on the ‘size/window’ this needs to mount into… so I designed it all to be as big as the display itself… leaving freedom to make the outline/dimension layer as is needed with holes/cutouts…etc…

this is using 1206 components too… I normally use 0603 stuff… so this could could be smaller/cleaned up a bit I suppose…


feedback appreciated…

3-digit_bob.zip (42.1 KB)

  1. You don’t seem to have any layer 20 Dimension lines. Need those to determine board size.
    I made these a little too tight, need a little taller so traces at top/bottom don’t have manufacturing issues.

  2. Once added, you will likely need to add a via & name it Gnd to get both polygons to connect to the Gnd plane.
    You see can one at the left side.

1.) the pcb outline/shape (dimension layers) was left out on purpose..(as mentioned above) :) until I can measure tonight (physically) and determine the best size/shape... (I think I'll have some drill holes on either side.. the pcb will be bigger than the whole display by maybe 0.25" or so.. so it'll literally be just an OUTLINE needed..since the whole circuit was contained the size of the display/unit)

2.) hmm.. this is where I can learn from you then... I really only picked up the adding the polygon to both TOP/BOTTOM layers.. and then naming them GND form some articles/tutorials...etc...

and the only time I every use a VIA, is when I dropping a trace... and I click the mouse wheel to switch from top and bottom (auto adds a via) shortcut..

so adding one manually and how to get it to 'work' properly.. would be a nice lesson/tip! a general lesson/idea on how to drop GND vias? the correct & easy way? LOL

I tried to drop a via manually in the same location.. (and it added it to stage)

I went to NAME icon.. clicked on newly placed via.. named it GND... gave me pop-up saying.. do I want to merge N$1 to GND?...etc.. I say yes..

then I get a new 'AIR-LINE' (UN-ROUTED LINE) on the board..??

Click on via, click on the board, hit escape. Right click the via, Name it Gnd. Very easy. Connect the trace that appears, near the bottom left. No biggie.

ok.. I wasnt sure if that was supposed to happen..etc..

I'll make an update..

so this morning… i was attempting to drop the via (like you suggested)…

dropped on in the lower/left corner…

and the RATSNESt turns on… (showing the GND polys)… and the air/wire/line)

I connect the line to the closest ‘GND’ connection according to Eagle…

once done though… there are other air-wires… like I need to connect ALL GND points or something to that VIA?

In still not 100% clear on WHY I am doing this? I dont believe I have any isolated areas? and the planes run around/outside all the traces?

anyways… here is a quick image of what Im talking about…

if I connect that via to the closest GND point (which is GND pin on the MAX chip)… I still get the lines that I need to connect it to the other GND points too?

I have attached my FINAL (current) board files…

the only thing that I am leery about is this GND via?

check it out and let me know…(curious as to why dropping a GND via to connect GNDs on both sides… makes me draw 4-5 more traces? shouldnt it just connect to the HUGE GND plane? that those other components are connected to as well??

  • Im starting to overthink my other PCb project too now… (the one with MAX chip but 30 SMD leds instead of a numeric display)… I dont believe I manually dropped any GND vias??
    wouldnt the error checker alert the user to this is needed/missed?


3-digit_bob.zip (44.7 KB)

Let autoroute now - should make the Gnd connections to the plane for you. Thats all I did with the one I posted earlier.