How do I connect a 7 segment 2.5 digit module with only 5 legs? - Page 1
Forum section: electronics / beginners

author: Rocka
author_rank: Newbie
timestamp: June 26, 2023, 04:56:42 am
content: Found these 5 lead 2.5 digit 7 segment LED modules in my junk box.There is no part number on them.Underneath the potting the leads appear to be designated Hi, Co, Lo, -, +I connected a couple of them to around 3.5volts on - and + and they both lit up with '35' then I tried connecting other combinations of various pins and in one instance the display switched to '199', reverting back to 35 with just - & + connected.Any ideas how I could hook them up to use them?I cannot find anything similar in catalogs or image searches.UpdateI've tested resistance between Co and - and it measures 0 ohm,Shorting Hi to Co (or -) switches display to 199Shorting Lo to any pin does nothing.

author: ataradov
author_rank: Super Contributor
timestamp: June 26, 2023, 05:31:31 am
content: Those look like they may be dedicated indicators from the old PC cases. But numbers are really strange, I would have expected to see 33 , 66 or 166.Hi/Co/Lo is likely High/Common/Low.It may be worth trying to power using +/- and then just short Hi-Co and Lo-Co. And if the numbers change, then there would be no way to display arbitrary values.They usually had a bunch of jumpers to configure the digits, but I guess it is not impossible that fixed value models existed too.

author: beanflying
author_rank: Super Contributor
timestamp: June 26, 2023, 07:42:39 am
content: Old School Charlieplexing and with a little advanced site searching foo going my wayhttps://www.eevblog.com/forum/projects/amusing-5-pin-7-segment-led-display/

author: brucehoult
author_rank: Super Contributor
timestamp: June 26, 2023, 09:31:31 am
content: Quote from: beanflying on June 26, 2023, 07:42:39 amInteresting. Five tri-state pins to drive up to 20 segments.It would be awful to do in random logic, but in a microcontroller it's just a 40 byte table if you want the simplest code: 20x 1 byte of pin in/out direction plus 1 byte of pin output value / maybe pull-up on/off.. Or 15-20 bytes would do if you don't mind more complex code to interpret it e.g. 20x 3 bits each for HI pin, LO pin numbers = 120 bits 15 bytes if packed -- but that's both slower and probably about as much total data+code.

author: LinuxHata
author_rank: Frequent Contributor
timestamp: June 26, 2023, 11:54:27 am
content: Just to be noted, some MCU have protection diodes on their pins, tied to VDD/VSS, so current might always flow through these LEDS.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: June 26, 2023, 12:01:40 pm
content: "35" suggests to me that maybe these are voltmeter modules, not raw displays

author: MrAl
author_rank: Super Contributor
timestamp: June 26, 2023, 12:28:50 pm
content: Hello there,If this is really Charliplexed (easy to prove) then with 5 lines there should be 20 LEDs, if they decided to use the maximum number.I think the formula is:LEDs=n^2-nwhere n is the number of lines available, and LEDs is the maximum number of LEDs possible.That is the maximum number of LEDs though and doesn't mean they all have to be there.If there are 7+7+1 that's 15, but if there are 3 decimal points that would be 18 LEDs.The drawing shows the maximum number of LEDs using 5 signal lines. Note each unbroken thick red line represents 2 LEDs each.The above solves the problem of how many LEDs you can do with 'n' lines, but there is still one more problem to solve.That is, how many LEDs can you DRIVE at the same time, and can you drive more than one at a time and still get even distribution of energy to each LED that has to be lit. Since you can not turn all LEDs on at the same time, you have to multiplex them, but the multiplex pattern and duration would have to be varied so that if you display some blanks like _ _ 1 which would be just 2 LEDs, and you display the most 1 8 8 you still want all LEDs to have the same brightness, and both patterns _ _ 1 and 1 8 8 to also have the same brightness. It would be strange to see that '1' light up super bright, and that "1 8 8" be somewhat dim. This would take a little more thought. This would also face the peak current per LED constraint, which of course has to be taken into account.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: June 26, 2023, 12:44:45 pm
content: Why would there be a 10K resistor inside if it was a dumb display?

author: RoGeorge
author_rank: Super Contributor
timestamp: June 26, 2023, 01:07:00 pm
content: Looks like clock frequency displays from PC desktop cases. Used to be a thing in the past, some PC cases were having a mechanical button, too, for "TURBO" frequency, thus the display to tell the current CPU frequency.

author: themadhippy
author_rank: Super Contributor
timestamp: June 26, 2023, 02:12:19 pm
content: Quoteone at a time is all you need,just switch em fast enough so persistance of the eye does the magic,added bonus you can often shove more current up em for the brief time each led is on making things brighter.

author: RoGeorge
author_rank: Super Contributor
timestamp: June 26, 2023, 02:59:06 pm
content: If they are TURBO indicators for PC case desktops, then they are wired together such that they can display only 2 numbers (normal frequency and turbo frequency), no multiplexing, DC wired only. The +/- are for power, and Hi to switch the displayed number to whatever second frequency was corresponding to Turbo. Back then the processors were not self-tunning the clock, like nowadays.I still have in the scrap boxes a few such LED modules from former desktop towers I owned. They are wired to display only two numbers by switching a single pin from V+ to GND (usually powered with +5V). Though, mine are not potted, and the two possible numbers can be configured from jumpers. The 199 display means it was from some PC that was possible to clock at 35MHz normal and 200MHz TURBO.The "normal" (low frequency mode) was necessary because some programs and/or some games were using the CPU clock for timing, so to be able to play that MS-DOS game you would need to switch the CPU to 35MHz. I guess it should have been 33MHz, not 35, probably they were using 35 instead of 33 for marketing reasons. Some were displaying just HI and Lo instead of numbers.Found two of mine in the LEDs scrap box.

author: Rocka
author_rank: Newbie
timestamp: June 26, 2023, 03:23:04 pm
content: Thanks for all the replies!I'll agree with the suggestion that they are probably from a computer or some thing designed to display only two fixed values.If they were indeed Charlieplexed, I'd have expected to be able to light various other random segments by applying power to random pins however this isn't the case.Co appears to be internally joined to -veShorting Hi to Co or -ve switches display from 35 to 199Shorting Lo to any other pin does nothingCo=-ve and Lo does nothingSo only 3 pins seem to do anything, and that's display 35 or 199.For what its worth that resistor looks like 47ohm.I cant think of a use for these so back to the junk draw for now.

author: LinuxHata
author_rank: Frequent Contributor
timestamp: August 07, 2023, 06:26:37 am
content: 3 digit charplexed led modules are usually from power banks, they just display remaining capacity

author: iMo
author_rank: Super Contributor
timestamp: August 07, 2023, 07:34:28 am
content: Quote from: RoGeorge on June 26, 2023, 02:59:06 pmIt is only ~30 years since then.. Those magic 3 digits LED displays with the Turbo switch we operated manually..