2 digit 7 segment LED questons and part recommendation? - Page 1
Forum section: electronics / projects

author: rcbuck
author_rank: Frequent Contributor
timestamp: May 03, 2021, 07:29:09 pm
content: I am looking at 3 different LED displays on Mouser's website with different luminous intensity. I have searched and found articles about luminous intensity. It appears to be a somewhat complex subject, no simple answer.LTD-6710P 950 ucdLDD-A514RI 3900 ucdLB-602VA2 16 mcdShould I expect the second display to be brighter than the first? And would the third display be a lot brighter than the second one?I actually have the LTD-6710 on the workbench with 20 mA flowing through each segment displaying the number 8. This is over the 15 mA data sheet rating but it has ran fine for an houlr. I have 470 ohm current limit resistors on each segment. The brightness is somewhat OK but I would like a brightter display.Any recommendations for a brighter 2 digit 0.56 inch (14.2mm) common anode red display?

author: Kleinstein
author_rank: Super Contributor
timestamp: May 03, 2021, 08:19:19 pm
content: The other 2 displays are allready brighter and should get a similar brightness with some 5 mA or 1.25 mA.With the about log response of the eye the perceived intensity is not that much higher as one may think.With only 2 digits one could get away without multiplexing, so drive the segments all the time.With red leds one could consider a red fitler foil / plexigals cover. This improves the contrast and readability as the off segments look darker.

author: rcbuck
author_rank: Frequent Contributor
timestamp: May 03, 2021, 10:41:49 pm
content: Kleinstein, Thanks for the reply.If the other 2 are already brighter at 5 mA or 1.25 mA then at 10 mA they should be much brighter. I guess I will just order a couple of each to run some tests.I am actually multiplexing 6 digits. These 2 are the last in line. I was running static brightness tests to get an idea of how bright the digits are. Obviously multiplexing speed will affect brightness and current draw so I will need to make adjustments in the resistors.

author: Peabody
author_rank: Super Contributor
timestamp: May 04, 2021, 02:05:30 pm
content: You might want to look at the Vishay TDSR1350 single-digit display for comparison. They come in various Luminance Intensity Classification groups, and group K produces 1800-3600 ucd at 1 mA. I have used the CC version (TDSR1360), and it's very bright on little current. Maximum current is 10mA. But I haven't found a 2 or 4-digit module using these.

author: bdunham7
author_rank: Super Contributor
timestamp: May 04, 2021, 02:38:24 pm
content: Look at the Broadcom HDSP-series. I'm using HDSP-H515A in a 6-digit multiplexed display and they look pretty good. You have less options in the 2-digit modules, but you'll still get pretty bright displays. I don't know how useful the mcd ratings are in comparing between brands when you are multiplexing--I think you have to just try them.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 04, 2021, 02:55:01 pm
content: AFAIK the red LEDs are usually still in a range where intensity is porportional to current. With multiplexing the average current is what counts. So 1:6 multiplexing would need 6 times the current to get the same intensity. There are usually 2 current limits: one for contineous and one for peak current.With 6 digits and high brightness one may need extra drivers for a higher current or split the mutiplexing in 3 groups of 2 digits.The µcd / mcd numbres should be comparable between manufacturers. However not directly between different sizes. Larger ones obviously need more light.

author: bdunham7
author_rank: Super Contributor
timestamp: May 04, 2021, 03:26:03 pm
content: Quote from: Kleinstein on May 04, 2021, 02:55:01 pmWhat I have found on the ones that I tried is that the peak currents are well beyond that proportional point and that is the problem. A typical case would be a module that specifies 30mA average current and 150mA peak current. A 'good' one might have generally increasing luminosity up to 50mA, a bad one will not increase appreciably after 15mA. The 'bad' one looks OK at 15mA, but if you x8 multiplex it as my application does, it's pretty mediocre at any current. The 'good' one actually looks OK at 5mA and when I x8 multiplex it at 40mA, it still looks OK.

author: rcbuck
author_rank: Frequent Contributor
timestamp: May 04, 2021, 05:18:27 pm
content: I ordered 2 each of the LDD-A514RI and LB-602VA2 for testing.What I am building is a 6 digit clock. The hours and minutes digits are 2.3 inches and the seconds digits will be 0.56 inch. I have 470 ohm resistors for the large digits which results in 20 mA current in static mode. I'm estimating they are receiving ~3 mA in 6 digit multiplex mode. They are plenty bright for a clock behind a smoked grey sheet of plastic.I have the seconds digits in the lower right corner and wanted them to be somewhat dimmer than the large digits. That would be less distracting but easy to see if you concentrate on them. I am using the same 470 resistors but the digits are slightly too dim. I tried lowering the 2 digit resistors to 82 ohms. They are brighter but not quite bright enough. Hopefully one of the two that I ordered will work. If they are too bright I can change the resistors to cut brightness.Schematic is attached showing how the multiplexing works.

author: floobydust
author_rank: Super Contributor
timestamp: May 04, 2021, 06:59:17 pm
content: TBH I'm having best luck with generic chinese 7-segment displays because they use the latest LED tech and there are no (inefficient) dim displays, no legacy stuff out there.I tried Lumex (YUCK) and Avago ($$$$) and findKingbrightare very good.Your clock circuit looks good, I would include a few usec dead-time between digit switching as it takes a while for the ULN2003 to turn off. It's just a check with a scope to ensure no overlap.I would add a phototransistor for a PWM dimming option, and maybe feed the LED's from an extra diode to 12VDC so mains brownouts can keep the 7805 input powered for a few 100msec. I've always found having good hold time or even a battery is needed. Maybe you're saving the time-zone in EEPROM.

author: rcbuck
author_rank: Frequent Contributor
timestamp: May 05, 2021, 12:55:18 am
content: I checked Aliexpress and Ebay but didn't find anyone who specified luminous intensity. Their prices appear to be fairly good unless the output is in the range of the LTD-6710P. If that is the case, they are more expensive than the Mouser price.Good point about the ULN2003 turn off time. The data sheet says minimum 0.25 us and maximum 1 us turn off time. I will measure it with the scope and try inserting some dead time to see what the effect is.I don't save the time zone in EEPROM but I do check it at the end of each multiplex cycle. The check is only 8 "C" instructions (haven't looked at the generated asm). The processor is a Bluepill module running at 72 MHz so I have plenty of time to perform the check. The Bluepill plugs into the PL4 and PL5 connectors.

author: james_s
author_rank: Super Contributor
timestamp: May 05, 2021, 01:09:15 am
content: The human eye is nowhere near linear, and there are a lot of other factors that can influence apparent brightness. With anything like this I always prefer to buy at least one sample of any display I'm considering and try it out at various currents under the sort of conditions I expect to use the finished device. If you have a few different displays you can compare the numbers with other parts you're considering to get a rough idea of how they will compare, although that still isn't as good as having the actual part you want to evaluate.

author: Peabody
author_rank: Super Contributor
timestamp: May 05, 2021, 01:46:41 am
content: An alternative to adjusting the resistors might be to make the ON time of the seconds digits longer than the other four. That would make the seconds brighter and the hours/minutes dimmer. You could even define the display as 8 digits, except the seconds digits would be allocated two digit times each. That would result in hours/minutes going from being on 1/6 of the time to being on 1/8 of the time. The seconds would go from being on 1/6 of the time to being on 1/4 of the time. Might have to speed up the refresh rate a bit if you get flicker. Well, anyway, I hope the new displays are brighter and that solves the problem.