I removed some led displays from some old digital clocks, but they don’t seem to follow the standard pin assignments for 7 segment led displays.
By trial and error I managed to get each segment to light, but I can’t figure out how to totally control them (eg. it will light up two segments when I only want one of them active).
My internet seach thus far has been unsucessful. Anybody know the schematic or have a good link?
Is this the most boring GQ ever?
I have designed and built such things but I am not sure what you are looking for. Normally you would have the 7 segments connecetd with either a commoncathode or anode. Then you would have a decoder from BCD to the 7 segments. Often you can have both integrated in one unit. And these days you have the entire clock and more integrated. You need to explain more so we can know what you need.
Is what you have got a display for one digit, or a display for serveral digits, or a display for several digits with associated decoding circuits/ICs? How many terminals?
Some of these displays scan across the digits so that only one or two segments are lit at one. The scanning is done quickly enough so the eyes don’t notice.
There’s a .pdf file linked on this page:
http://www.dalewheat.com/projects/4D7S/
The schematic in the linked pdf shows the decoder chip having basically 12 leads lighting 4 seven segment displays. Without a scanning feature, this number would be [7x4 (total segments) + 4 (common anode/cathode)] 32.
Thanks for the replies all, sorry I couldn’t get back sooner.
All I’m doing is building a seconds counter, 8 digits, so I can watch the seconds of my life tick away for the next couple years.
Sailor, yup, your standard 7 segment display is fairly simple. If say they are hooked up to a common cathode, you can make (with the appropriate decoding) the first digit with LED’s a1 to f1, the second digit with a2 to f2 and so on.
So I pulled apart an old digital clock, removed the LED display, and figured it would be pretty much the same for the 4 digits. It’s not. The display has a bunch of lines going in, I’ll denote the lines that seem to activate a segment pins 1 to 12, plus you have two other control pins say a and b.
When you put a control pin low and a segment pin high, you will get a segment lit. Thus you have 2(control)*12(segment)=24 possible segments. You actually only need 23 to make all the digits a 12 hour clock requires (minus center blinky and other various dots).
The problem is, each digit uses both control pins. For example the first digit has segmets a12,b12,a11,b11,a10,b10,a9 the second digit has a8,b8,a7,b7,a6,b6,b9 and so on. Therin lies the dillema, if you want to make an 8 on the first digit, you activate all the segments for digit one. Unfortunately, if you do this, you also end up activating b9 from the second digit.
There are another 7 leads in the don’t seem to do anything. So I was thinking that some connection with those would enable me to deactivate b9 above to make the 8 and nothing else. This was too tough for me to figure out and I was hoping somebody had data sheet that showed how this works.
Anyway, I’m starting to think that jnglmassiv has it correct, on the first pulse you activate the segments you want with control line a and the second pulse activates the segments with control line b, or just do them one at a time.
yes, most of those are multiplexed and you would need to build a scanning circuit. They do it like that because it is simpler and cheaper just like keyboards are also scanned by row/column for key closures.