Found a source of cheap 6 Digit 7 Seg LED displays locally - Page 1
Forum section: electronics / projects

author: Stonent
author_rank: Super Contributor
timestamp: November 10, 2013, 10:18:47 pm
content: I found this at a local electronics store for $1.49. They seemed to have quite a few of them.At first I thought it was part of a bench-top multimeter but after googling the name "Norand Corp" which was on it, it appears to be related to data gathering or barcodes. Norand was bought by Intermec several years ago and Intermec is sort of like Symbol and Telxon where they make barcode scanner equipment.I had a few ideas of what to do with this. I thought about getting several and doing some DeLorean time circuits but since they are just red, it wouldn't look as good and the first 3 digits on the real circuits are 14 or 16 segment displays.I thought well surely there's something interesting I can do from a microcontroller but was then trying to figure out how to drive it. One idea I had was an array of 75HC595 shift registers but was thinking maybe spreading it out that far might cause the display to be either dim or flicker badly. I looked at some 7 segment display driver chips too, but they didn't seem worth the cost to drive this. I was a tad bit disappointed that this is a common anode configuration, since I'll have to reverse the logic in my brain when it comes to wiring this thing up.So then I had the bright idea of taking a cheap multimeter and hacking the display somehow to drive this and enclose it in a box for a fake benchtop multimeter.I don't know I just love finding neat stuff and trying to figure out a project for it. Anyway, here's the details below, maybe there's a chance someone knows what the original purpose of this board was.Not that I've done a lot of LED or any 7 segment stuff but this is the first time I've seen 34R resistors used. I wonder what kind of voltage you'd expect to use with that.Edit: I just had a thought about that. Perhaps since all 6 displays are tied to that one resistor, they expect the current will be spread out therefore not needing to limit it as much.Connector 11. 8 Seg 1 Anode2. 8 Seg 2 Anode3. 8 Seg 3 Anode4. 8 Seg 4 Anode5. 8 Seg 5 Anode6. 8 Seg 6 AnodeConnector 21. Indicator LED Common Anode2. N/C3. Indicator LED 1 Cathode4. Indicator LED 2 Cathode5. Decimal point6. Mid -Connector 31. L Top |2. L Bot |3. Bot -4. R Bot |5. R Top |6. Top -Indicator LED 1 and 2 each have a 220R series resistor at the cathode.All segments have a 34R series resistor at the cathode that is shared on all 6 displays.There is no pin on the LED modules to drive the left decimal, so I suspect there is no physical LED under it.I also found out that the first 7/8 segment module has a burned out spot. The bottom left vertical bar is dead.

author: dannyf
author_rank: Super Contributor
timestamp: November 10, 2013, 11:23:07 pm
content: QuoteThat approach works, except that the segments for '1' are brighter than the segments for '8' -> the cure is to run the leds at higher current levels, which appears to be what the board is doing.

author: tsmith35
author_rank: Frequent Contributor
timestamp: November 11, 2013, 06:23:28 am
content: Quote from: Stonent on November 10, 2013, 10:18:47 pmSeems like it would make a nice digital clock display...

author: Stonent
author_rank: Super Contributor
timestamp: November 11, 2013, 08:32:57 am
content: Quote from: tsmith35 on November 11, 2013, 06:23:28 amYeah, it's got the perfect number of digits. If I get more of these, this one might end up being a clock if I can find a really cheap way of driving it, because of the burned out segment on the first one. I could still display a "1" on it

author: mamalala
author_rank: Unknown
timestamp: November 11, 2013, 08:58:01 am
content: Quote from: Stonent on November 10, 2013, 10:18:47 pmThe displays are multiplexed, that is why you have such low value resistors, and each resistor is "shared" among the (usually same) segment of all the digits. They probably run on 5V. Due to the multiplexing, on the 6 digit display you get 1/6th of the current on average on each digit. If you would put the "correct" value of resistor for 5V in there and then multiplex, the display would be rather dim.Greetings,Chris

author: tsmith35
author_rank: Frequent Contributor
timestamp: November 11, 2013, 11:45:21 am
content: Quote from: Stonent on November 11, 2013, 08:32:57 amYep, that should be all you need, assuming you don't go with military time. It's "1" or nothing for the left digit.