Fast Optical Speed 7 Segment Display - Page 1
Forum section: electronics / projects

author: Stuper
author_rank: Newbie
timestamp: October 31, 2021, 02:42:21 pm
content: Hi, First time post, long time lurker.I'm trying to identify a seven segment display which has fast optical speed. I hope someone on the forum can give pointers on what to look for.My project uses seven segment displays which will be photographed using a high speed camera. Initially this will be hundreds of frames per second and eventually up to 1000 frames per second. The displays give a timing reference to the camera frames. I've not gone into my project further but I can do.I did some preparation work by checking a selection of LEDs I have handy. These were general purpose SMD LEDs. These optically switch fast enough for photographing at hundreds of frames per sec with no afterglow. My conclusion was that most general purpose LEDs are fine for my application.I've now got first proto PCBs made up with Kingbright SC56-21EWA 7 seg displays (GaAsP/GaP). The displays aren't multiplexed, and driven straight from decade counters - 4026N ICs.The problem I have is that the Kingbright displays aren't optically switching well at 100s of Hz, let along 1KHz. When monitoring the drive current this is switching fine with nice sharp transitions. If I drive a general purpose SMD LED next to one of the segments the difference can be seen with the camera and visually by scanning my eyes. The SMD LED produces discrete flashes with fast optical switching. The 7Seg display has a significant after glow which basically means the display is always on to some extent. I suspect this is intentional because most displays are multiplexed and strobing is undesirable.Datasheets for these displays don't give any info on optical speed. I need to order more display samples and would appreciate advice others can give me. Is there a particular LED chemistry which would be more suitable?Thank You, Stu.

author: TheMG
author_rank: Frequent Contributor
timestamp: October 31, 2021, 02:52:29 pm
content: That display according to the datasheet uses a red phosphor on top of an orange emitting LED.Any LED type that uses phosphors is going to have some decay time so will be much slower.Look for a 7 segment display that uses non-phosphor LEDs.

author: Stuper
author_rank: Newbie
timestamp: October 31, 2021, 03:16:39 pm
content: Thanks for the quick response. I did wonder if the 'Phosphide' in the chemistry was causing the afterglow. Most of the Red 7-seg displays include 'P' in the chemistry. However the displays I used particularly had an additional colour changing layer.I found some Red AlGaAs displays, such as Broadcom HDSP-515A.Is the solution simply looking for LEDs without the 'P' in the chemistry or did I misunderstand the phosphide/phosphor reference?Thank You, Stu.

author: Benta
author_rank: Super Contributor
timestamp: October 31, 2021, 03:49:11 pm
content: LEDs are generally good for several hundreds of kHz, if not MHz, both optically and electrically. The phosphor is the problem. Buy non-coated LED displays (which are actually the majority).

author: TimFox
author_rank: Super Contributor
timestamp: October 31, 2021, 05:22:27 pm
content: GaAsP is one of many semiconductors used to make LEDs: as a function of the material, both the emitted wavelength (color) and I-V curve change.A table of common materials can be found inhttps://www.electronics-tutorials.ws/diode/diode_8.htmlwhich reference does not address the speed.Since LEDs have a relatively narrow spectra, to get a more aesthetic or white color a short-wavelength LED is used to excite a phosphor (much like the phosphor coating inside a fluorescent light fixture, excited by UV from the mercury-vapor discharge inside the tube), which has much slower response than the diode itself.A simple discussion of LED speed can be found athttps://lednique.com/led-speed/Note that the LED itself is quite fast, but the overall switching speed is usually dominated by parasitic capacitance (and possibly inductance) in the total circuit.That reference has estimates of LED and LED-phosphor switching speeds.

author: floobydust
author_rank: Super Contributor
timestamp: October 31, 2021, 06:19:47 pm
content: I don't see anything suggesting phosphor in the KingbrightSC56-21EWA(unless it's the lens epoxy filler)"The High Efficiency Red source color devices are made with Gallium Arsenide Phosphide on Gallium Phosphide Orange Light Emitting Diode".Weird, is it red or orange LED's?CD4026is quite weak for output current, lucky to get 1mA at 5V and the datasheet shows using buffers to LED displays.Wondering if the output stage is lagging (overlap) due to the overload?

author: Stuper
author_rank: Newbie
timestamp: October 31, 2021, 09:08:58 pm
content: Many thanks for all the replies on a Sunday. I suspect I got very unlucky on the displays I used. I didn't choose them as such - they were the ones which were readily available to me. Because I didn't expect this issue I didn't have a particular requirement on the ones I used.I'm going to order samples of the Broadcom HDSP-515A. These use AlGaAs.I've attached some pictures of the problem.The scope screen shows the drop across the LED resistor for segment A, with the driver counting at 1MHz - it doesn't have a problem at this speed. Time/div is 1uS. There's a bit of an issue around the No.7 display which I'm addressing in next board iteration.The photograph is a single frame at a rate of 240 per sec. The digit on the left is 1/10s count, then 1/100s count etc. The second digit is seen transitioning from 2 to 3. Note: this wasn't just a lucky frame on the boundary of the change, the issue is consistently seen.Update: just to be clear the scope trace is on the A segment of the right hand display which is counting 0.000001 seconds. The scope trace on the second digit (not shown) is much squarer / cleaner.

author: BrianHG
author_rank: Super Contributor
timestamp: October 31, 2021, 09:19:36 pm
content: Go with super bright blue or white 7 segment displays.Drive them hard with an adjustable on-duty cycle (like powering them on for 10-20% of the time with a 100-200ma drive) which will allow you to correct for both LED after-glow and your camera's CCD speed.

author: StillTrying
author_rank: Super Contributor
timestamp: October 31, 2021, 09:27:11 pm
content: I can't tell whether you're working at 1kHz or 1MHz.At 1 MHz any resistance in series with the LEDs makes a difference to thelightedges.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 31, 2021, 10:24:35 pm
content: Slow LEDs are also electrical slow. At least for a first test one can measure Trr instead of the light outout. A few for the red and IR leds get a speed in the 1 µs range. Most are faster. With relatively slow LEDs a drive that can sink current can also speed up the LED to turn off. Stored minority carriers can be sucked out electrically before they produce light.The LED display picture looks like an superposition of the 2 and 3.

author: Benta
author_rank: Super Contributor
timestamp: October 31, 2021, 10:32:39 pm
content: Quote from: BrianHG on October 31, 2021, 09:19:36 pmRight! So your solution to luminous persistence (=phosphor afterglow) is to suggest white LEDs which are... Tadaa... blue LEDs with phosphor?Not the brightest idea (pun intended) IMO.

author: floobydust
author_rank: Super Contributor
timestamp: October 31, 2021, 11:41:45 pm
content: Quote from: Stuper on October 31, 2021, 09:08:58 pmLooks like around 0.2usec for the LED voltage to switch?It seems odd the B, C segments are bright. B-seg on for both the 2+3, but C segment only on for the 3. A,B,E on for both numerals. Unless it's the prior "1". You would think the segments on the longest excite the phosphor more and would appear brighter.I would take a blue LED and point it at the displays and see if they luminesce, to confirm that epoxy and maybe email Kingbright for details.You could also maybe slow the count down to measure the persistence time.

author: BrianHG
author_rank: Super Contributor
timestamp: October 31, 2021, 11:53:39 pm
content: Quote from: Benta on October 31, 2021, 10:32:39 pmBlue doesn't have phosphor, it's a fundamental 450nm light emitting diode. And also, in another thread here on EEVBlog, there was a 1MHz optically isolated probe created with 2 white LEDs and 2 high speed photodiodes. So we know that some white LEDs' blue core whose yellow phosphor which generates the white still turns off in the MHz range. Driving on the LED for a fraction of the time, say ~25-50% will only exaggerate an empty transition from 1 number to the next.

author: floobydust
author_rank: Super Contributor
timestamp: November 01, 2021, 12:28:44 am
content: I think it's this post, LED's short pulses and photodiode:https://www.eevblog.com/forum/chat/20w-halogen-bulb-viewed-by-a-photodiode/msg1751210/#msg1751210https://www.eevblog.com/forum/projects/ultra-short-ultra-fast-led-flash/msg2252829/#msg2252829

author: Benta
author_rank: Super Contributor
timestamp: November 01, 2021, 01:12:15 am
content: Quote from: BrianHG on October 31, 2021, 11:53:39 pm

author: BrianHG
author_rank: Super Contributor
timestamp: November 01, 2021, 01:25:44 am
content: Quote from: Benta on November 01, 2021, 01:12:15 amSee here:https://www.eevblog.com/forum/beginners/floating-probe!-for-$2-50/msg1834925/#msg1834925His scope timing measurements of a super bright white LED should give you and idea of how fast the phosphor decays.His white LED completely turns on and then turns off in ~150ns. Assuming that his photo-diode is infinite speed and doesn't add any additional turn-on, turn-off delay.

author: StillTrying
author_rank: Super Contributor
timestamp: November 01, 2021, 01:07:07 pm
content: Quote from: Stuper on October 31, 2021, 09:08:58 pmIn theory at 240 frames per sec. you should catch the 1/100s digit transitioning in around 1/3 of the photos, in practice the camera might be making that 1/240s frame up from more than one 1/240 frame because of the noise at low light conditions. so you catch the 1/100s transition in every photo?Have you got a not too bad still digital camera that can photo at 1/200 sec, probably at high ISO and wide lense because of the low light level.

author: StillTrying
author_rank: Super Contributor
timestamp: November 01, 2021, 04:08:45 pm
content: Quote from: floobydust on October 31, 2021, 11:41:45 pmIt looks to me like the red channel is saturatinginside the camera, making all the red segments a similar brightness so that we can't see from the photo which digit segments are on more or less then others.If I adjust the gamma of the red channel alot, it shows some of the variation in brightness between the segments.Unless it's a very warm white colour, I'm sure all these 7 segments LEDs will switch at 1MHz.If there were some that were slow enough to help hide the small on times and the multiplexing it would be on the data sheet.

author: Stuper
author_rank: Newbie
timestamp: November 01, 2021, 05:58:50 pm
content: Quote from: StillTrying on November 01, 2021, 04:08:45 pmThanks. This is very interesting because I'm also making progress on this front.I went back to the original tests I did with my assortment of LEDs and these performed very well on camera. The LEDs were green, and probably most important, in the setup I was modulating with a 50/50 square wave. The red Kingbright displays also performed better in this setup, but not as well as the green LEDs.The display board is driven by an FPGA which gives me good control and I was able to introduce blanking. The OE on the driver ICs are ganged together which isn't ideal. Anyway I targeted the blanking at the second LED. By blanking I mean turning off the output for a short time before the digit change and for a short time after. This made a big difference and there were no issues with frames showing blank digits. Possibly the camera is averaging / saturating as you say. This could be the issue all along. I was able to improve readability further by desaturating the colours.The aim is to be able to read the digits clearly - initially the first two, and later the first 3 with an advanced camera. I'm now where I need to be on two digits.When I re-spin the board I'm going to add per-digit blanking control. The fast camera will also be synced to the FPGA and I can tune the alignment of frames. So I'm feeling confident on 3 digits now.I think it is clear that the image I attached was transitioning from 1 -> 2 and not 2 -> 3. Your desaturated picture shows this very clearly.BTW: I tried illuminating the displays with a powerful ultraviolet lamp and there was no obvious fluorescence.In conclusion I don't think the Kingbright displays are significantly slower than other LEDs I've tried after all.Thanks everyone for the comments and contributed ideas. The ideas certainly got me thinking. I won't be shy about posting another time.

author: Zero999
author_rank: Super Contributor
timestamp: November 01, 2021, 06:18:03 pm
content: Quote from: TheMG on October 31, 2021, 02:52:29 pmWhat makes you think that?I think you're confusing the word phosphide, with phosphor. It's a completely different thing.Quote from: Stuper on November 01, 2021, 05:58:50 pmIt might be the parasitic capacitance of the LEDs keeping them on. Try bypassing the series resistor with a 100pF capacitor.

author: DavidAlfa
author_rank: Super Contributor
timestamp: November 01, 2021, 09:15:37 pm
content: The 4026 have pretty weak outputs (5mA?).Maybe a TTL equivalent would drive the leds harder.

author: StillTrying
author_rank: Super Contributor
timestamp: November 01, 2021, 10:09:55 pm
content: Youmightbe able to get the camera to use shorter shutter time by just videoing it in a lot more light, rather than having a very bright light directly on the LEDs just covering all the black chip areas with white card might be enough.I can't be bothered wiring a 7 segment up to see what the light from 1us switching times looks like, so here'sthe lightfrom the 8 digit red LED display on my PSU instead.At these speeds each digit seems to be cleanly switching on and off, after I average away the noise spikes.