LED visibility optimization at very low currents - Page 1
Forum section: electronics / projects

author: jmaja
author_rank: Frequent Contributor
timestamp: April 20, 2017, 08:48:22 pm
content: What is the best way to drive a LED to be visible in the dark? What kind of LED should be chosen for this purpose? The target is to be able to clearly see is the LED on or off from about 5 m distance. And only during night when you can't see a mechanical output. The device is targeted to consume less than 50 uA and it will have four LEDs than can possible be all on. The user can only see two at the time (red and green).Is it possible to reach this goal with max 10 uA/LED? Probably the LEDs could be blinked at ~1 Hz with 25-50% on time, which would help saving current at the same visibility.Is it better to use PWM or a constant current drive for better visibility at the same average current? PWM may be a problem, since the CPU needs to sleep.Does the size of the LED change the visibility? Is higher lm/W always better? lm/w is given at much higher current typically. Narrow viewing angle would help, but rather wide must be used.

author: Audioguru
author_rank: Super Contributor
timestamp: April 20, 2017, 09:03:04 pm
content: With LEDs you get what you pay for.My Philips Lumileds Luxeon Superflux LEDs are very wide angle and are very bright at low currents. The LEDs in my cheap Chinese solar garden lights have a very narrow angle and are not bright with the high current they use. I salvaged a few strings of outdoors LED Christmas lights and they have a wide angle and are bright at low current.

author: GeorgeOfTheJungle
author_rank: Super Contributor
timestamp: April 20, 2017, 09:15:09 pm
content: What hp did to save energy 40 years ago was to put a lens in front of very tiny leds: the bubble 7 segments displays google.com/search?q=bubble+7+segments+displays&tbm=ischWRT the max 10 uA/LED, that indeed sounds like a very little amount of energy... not sure about that.WRT "lm/w is given at much higher current typically" back then people used the LM3909 for that.

author: wraper
author_rank: Unknown
timestamp: April 20, 2017, 09:18:45 pm
content: Quote from: GeorgeOfTheJungle on April 20, 2017, 09:15:09 pmI don't think that lens had anything to do with energy saving. By using it, they could use one tiny LED die as a segment.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 20, 2017, 09:27:25 pm
content: For maximum visibility, (Cool) white, small size, narrow angle if your viewing angle . match angle to required viewing angle. If you want to stand out from other lights, maybe deep green, as this is at the peak or the eye's sensitivity.Where you want "noticeability" for emergency type applications, strobing can help as the eye is more sensitive to movement.In principle, for a steady brightness, PWMing doesn't give any advantage in terms of perceived brightness, but efficiency at low currents (< about 5% of nominal current) can vary a lot, particularly with white, so if only for consistent appearance, a higher current pulsed at a low duty cycle can be beneficial.

author: jmaja
author_rank: Frequent Contributor
timestamp: April 20, 2017, 10:19:13 pm
content: The LEDs need to be green and red. Not an emergency application, just sensor information to the user.The viewing angle must be wide, maybe 150 deg.Is efficiency better or worse at low current? Say 10 mA 0.1% PWM = 10 uA vs. constant 10 uA. Is there a big difference between these in lumens?

author: Zero999
author_rank: Super Contributor
timestamp: April 20, 2017, 10:22:16 pm
content: Quote from: mikeselectricstuff on April 20, 2017, 09:27:25 pmActually for maximum visibility in the dark, you need 498nm, the peak sensitivity of scotopic vision. Unfortunately LED efficiency (I'm talking about power in vs radiant power out here not luminous efficiency) drops, towards the middle of the visible spectrum, with a minimum around the greenish-yellow region, so it might be more efficient to use a slightly shorter wavelength or an indigo LED with cyan phosphor.https://en.wikipedia.org/wiki/Scotopic_visionQuote from: jmaja on April 20, 2017, 10:19:13 pmRed could be a challange, since the dark adapted eye is less sensitive to red, than any other wavelength.QuoteMuch worse with PWM. LED efficiency is generally poorer at higher currents.

author: GeorgeOfTheJungle
author_rank: Super Contributor
timestamp: April 20, 2017, 10:25:36 pm
content: Quote from: jmaja on April 20, 2017, 10:19:13 pmYes because @10µA constant current it won't even lit I think. That's why the LM3909 exists.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 20, 2017, 10:29:20 pm
content: Quote from: jmaja on April 20, 2017, 10:19:13 pmTest it and see. If this is something for production, do NOT run the LED at 10uA as you have no guarantee of performance at this current level. 1% at 1mA or 0.1% at 10mA will be inside the range that the manufacturer characterizes and tests at, so brightness will be more consistent from batch to batch. IFor indication it looks like you will have the opportunity to strobe it as well to save power - maybe 7-20Hz at perhaps 5-15% duty - again experiment to see what works best.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 20, 2017, 10:30:33 pm
content: Quote from: GeorgeOfTheJungle on April 20, 2017, 10:25:36 pmExisted. Long obsolete.

author: Zero999
author_rank: Super Contributor
timestamp: April 20, 2017, 10:32:27 pm
content: Quote from: GeorgeOfTheJungle on April 20, 2017, 10:25:36 pmThe LM3909 came out a long time ago, before efficient LEDs existed. It's no longer made but wouldn't surprise me if you can still get clones and new old stock on ebay.It's true that bright flashes will be more easily noticed, than a constant dim glow, so in that respect, pulsing can be useful. However, as the frequency increases to the point where the pulses merge together to form a dim glow, due to the persistence of vision, it's better to use use a tiny constant current, rather than short pulses.

author: GeorgeOfTheJungle
author_rank: Super Contributor
timestamp: April 20, 2017, 10:40:55 pm
content: Quote from: mikeselectricstuff on April 20, 2017, 10:30:33 pmOh dear, oh dear

author: james_s
author_rank: Super Contributor
timestamp: April 20, 2017, 10:48:12 pm
content: The LM3909 used an extremely low duty cycle in order to reduce power consumption and make the individual flashes relatively bright. Using PWM to get something resembling constant illumination is unlikely to have much benefit, you also have to factor in the power consumption of the PWM circuitry.It would be easy enough to do some experiments, get a bunch of LEDs, a multimeter and some pots and test them out at different currents in a dark room.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 20, 2017, 10:50:03 pm
content: Quote from: Hero999 on April 20, 2017, 10:32:27 pmBetter how ?Unless the LED datasheet specifies output at low currents, you have no guarantee of brightness.White LEDs in particular can show huge brightness variations below a few hundred uA.Some OSRAM lighting LED datasheets actually state that they should not be run below 10% of their rating for this reason.

author: Zero999
author_rank: Super Contributor
timestamp: April 20, 2017, 10:58:17 pm
content: Quote from: mikeselectricstuff on April 20, 2017, 10:50:03 pmThat's because the response of the phosphor in a white LED is often non-linear. It isn't just the brightness which can change but the spectral output and colour, hence manufatures are hesitant to specify their products beyond what they've themselves tested them to. In this case the user is not interested in white LEDs, so that's irrelevant. Whether or not the datasheet specifies it or not, luminous efficiency of the LED junction itself improves at lower currents.Quote from: james_s on April 20, 2017, 10:48:12 pmYes this is true.For short pulses, the eye will not be able to determine the length of the flash. A 20ms pulse will appear to be twice the brightness of a 10ms pulse, with everything else (intensity wavelength etc.) being equal. Thus, if it's flashes you need, you're better off using longer, lower current pulses, than short high current.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: April 20, 2017, 11:13:53 pm
content: Since your color choices are fixed and there is really no magic operating point for LEDs (other than not at the high end of their current capability) your only real flexibility is to combine persistence of vision/attention and human optical sensitivity with required data rate to reduce duty cycle.First, using an efficient LED find the lowest current/time pulse which can be detected by your operators in their environment. The eye is amazingly sensitive and if you can put your operators under a light hood long enough to dark adapt, and they have no other instruments or displays providing light you will be amazed how low it can go. Even moonlight and a complex scene can make a huge difference in this detection level. The message here is that your environment is critical to the question. You need to define the worst case from your design standpoint.Can your information be conveyed with one flash every second? Every 10 seconds? 50 seconds? If the information rate is low enough you might be able to get average current down where you desire. Persistence of vision is measured in tens of milliseconds so I am sure your average power desired cannot be achieved based on a duty cycle that makes the light appear constantly on.You will probably have to do some human factors testing in an area that I haven't seen much on. There will be interactions with things like human attention span, so something like three flashes at 0.5 second intervals with 10 second rest time might end up being your best operating point (those are made up numbers to illustrate the concept). Things like how well the operator has to localize the light will affect all of this. I am sure that the operators will not like the settings with the lowest usable powers. You will have a trade between operator happiness and power consumption.

author: jmaja
author_rank: Frequent Contributor
timestamp: April 20, 2017, 11:34:51 pm
content: I did some tests with a very old (90's?) red LED I happened to have by the bench. It is very clearly visible with 100 uA average current using 7.5 mA PWM. With 300 uA constant current it was much dimmer, maybe equal to 25 uA average PWM. Even at 5 uA PWM I could see the LED from 5 m, but not that clearly.I have to test with modern LEDs, but I was hoping to get some help selecting a good candidate. There are so many of them and I don't quite know what to look for in the specs.I'm hoping the new LEDs are much more efficient and thus may be as visible with 10 uA. Maybe even with constant current. I trying to find out is PWM an option or will the cortex-M0 take too much power while powering PWM during standby.The state of the LEDs should reflect the sensor output with around 1 s maximum delay. So flashing the LED needs to be around 1 Hz or faster. The processor will likely wake up 2-4 Hz so 25 or 50% slow PWM would be OK by just bit banging at wake up and no need to keep the fast PWM working.The user will be in the dark for a long time, so adapted to darkness. The LEDs need to be visible whenever light level is too low for seeing the sensor. So also in moonlight and in the evening. There are some displays and distant lights, but they are not bright in the dark.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 20, 2017, 11:48:38 pm
content: Quote from: jmaja on April 20, 2017, 11:34:51 pmHigh brightness and/or specced for low current ( few mA). but things like angle will make the biggest difference. Clear package rather than diffused.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 20, 2017, 11:54:34 pm
content: Quote from: jmaja on April 20, 2017, 11:34:51 pmThat would probably work fine. Assuming a 4Hz wake-up you may want a much shorter pulse than 250mS, but you could do this using a simple capacitor charge-pump arrangement - maybe arrange it so you get a bright flash for maybe 50mS, which then tails off to a lower constant-ish current, That way the flash attracts attention, but the tail-off preserves visibility for a quick glance.

author: LaserSteve
author_rank: Super Contributor
timestamp: April 21, 2017, 12:31:22 am
content: Green and yellow for the photopic and scotopic / mesopticPeaks. Blink with short flashes at 0.5 to 3 Hertz for maximum attention gathering factor. Using 100 uAFrom a current source at low duty cycle and flashing will be more noticble then 10 uA CW. You have other options such as double flashes and flash order to convey other meanings. Ie RG, GG, RR,. Angle does matter. Look at the flash patterns on light aircraft for examples, as they are designed for maximum recognition against a city or dark background.S.

author: GeorgeOfTheJungle
author_rank: Super Contributor
timestamp: April 21, 2017, 12:40:28 am
content: Quote from: jmaja on April 20, 2017, 11:34:51 pmThen you're done if there's a LED with 10x better lumen-per-watt efficacy than the old one you're using.https://en.wikipedia.org/wiki/Light-emitting_diode#Efficiency_and_operational_parameters

author: Cerebus
author_rank: Super Contributor
timestamp: April 21, 2017, 03:23:06 am
content: Back at Christmas I wired two very ordinary 3mm red LEDs (random picks from the parts bin) to two 3V lithium coin cells. One with a 330K dropper resistor and one with a 1M dropper resistor. The purpose of all this was to add a marker light to where the door handle is, as we'd hung a blackout blind on the window and now at night we get proper 'country' darkness rather than the perpetual demi-light us city dwellers have become accustomed to. Toes were stubbed. I wasn't sure what would be enough current so I had two tries and left them to run side by side.Both have been sitting next to my bedroom door running continuously since. In night-time darkness with the blind down both are clearly visible from 3 metres away and 40 degrees off axis. I'm not saying that's their limits of visibility, that's just where I can clearly see them from in bed. So that's one at around 5 uA, and one at around 2 uA; visually there's a difference in brightness but it's not instantly noticeable, you have to consciously look to see which is the brighter.

author: Dijital
author_rank: Contributor
timestamp: April 21, 2017, 05:00:09 am
content: I did an actual empirical test for yaUsing a Cree XQE (XQEROY-00-0000-000000L01) royal blue (465nm) I was able to see it in the dark from ~5m at 10uA directly driven (around 2.45V). It is very dim, but it stands out enough against the dark. However I feel that even in partial moonlight it may be too hard to see. I would shoot for green as it has the highest luminous flux of the bunch.

author: mjs
author_rank: Regular Contributor
timestamp: April 21, 2017, 07:30:54 am
content: If you can live with shorted flashes, use those with 2-4Hz repetition rate on a modern green LED. Note that modern bright InGaN LED have Vf higher (2.8-3.8V) than the older GaP ones.I did a daylight visible (in shadow, not direct sunlisght) blinker with 2N6027 running with 9V battery at about 5uA, about 0.5Hz rate. Forgot that completely and found it still flashing after 9 years!

author: Someone
author_rank: Super Contributor
timestamp: April 21, 2017, 11:48:41 am
content: Quote from: Hero999 on April 20, 2017, 10:58:17 pmI'm siding with mike on this, LEDs (even within the same batch) can vary wildly at low drive currents. And the luminous efficacy can drop as the current decreases..

author: jmaja
author_rank: Frequent Contributor
timestamp: April 21, 2017, 12:20:48 pm
content: I tested another LED. It's a new green one, but not a bright.http://datasheet.octopart.com/LTST-S110KGKT-Lite-On-datasheet-158286.pdfIt was clearly less visible than the old red one at the same current, but seems to have much wider viewing angle. It was extremely dim at 10 uA. There are LEDs that are much brigther at the same view angle. The one I have is said to have 130 deg and 35 mcd at 20 mA 2.0 V while this one is 130 deg, 995 mcd at 20 mA 3.2 V.http://www.sunledusa.com/products/spec/XZM2DG53W-6.pdfIt shows a straight intensity vs. current graph, thus it should have the same brightness at 1/30 current. At 300 uA the LED the green one was bright enough so 10 uA should be enough. The forward voltage is high for the system powered from a LiPo and using ~3 V regulator, but already at 5 mA Vf is 2.8 V and maybe 2.5 V at 2 mA.What would be a good way to pulse these with the cpu sleeping so that the average current would be ~10uA and 0 uA during daylight? Charge a capacitor and then let it pulse the LED while sleeping? Just a few uF ceramic and no resistors using two pins from the MCU. One for charging and the other for grounding the LED.So then there would be a few mA pulse + uA level tail every 500 or 1000 ms.

author: Zero999
author_rank: Super Contributor
timestamp: April 21, 2017, 12:23:05 pm
content: Quote from: Someone on April 21, 2017, 11:48:41 amThose graphs are meaningless, without real units, no mention of the type of LED and whether it's got a phosphor or not.Show me some real data for a bare LED (no phosphor), with the radiant power output (not luminous efficacy which varies depending on the lighting conditions!) vs power in and I'll take interest.

author: Zero999
author_rank: Super Contributor
timestamp: April 21, 2017, 12:34:39 pm
content: Quote from: jmaja on April 21, 2017, 12:20:48 pmThat LED is AlInGaP based which is a very old technology. Try a modern InGaN LED which will be much more efficient.QuoteThat makes sense. If the brightness varies linearly with current, then it's more efficient at lower currents, as Vf drops and power = V*I. Unfortunately, if you're limiting the current with a resistor or linear regulator, then there will be no efficiency gain, since the extra voltage is converted to heat anyway.QuoteFor ultimate efficiency, don't PWM the LED, use a buck converter, that way the output current can be lower than input current. It should be fairly trivial to implement with an MCU and since it's low power, no additional switching transistors are necessary.

author: Someone
author_rank: Super Contributor
timestamp: April 21, 2017, 11:10:09 pm
content: Quote from: Hero999 on April 21, 2017, 12:23:05 pmIf you wish to bring the data into question why can't you provide the data to show its wrong? Phosphors have well known saturation and temperature limitations at the high end, but I can't find any papers discussing non-linear effects towards zero. I'm not sure what you think luminous efficacy is, but its the well defined method of assessing visual brightness:https://en.wikipedia.org/wiki/Luminous_efficacyYes, it no longer applies in a scotopic vision environment but then you wouldn't be distinguishing colours clearly which is not what the OP is discussing. You raise a lot of questions without backing it up with data or references or even explanations to help people here. Sitting on your high horse and being pompous isn't of much value to the OP when you could be adding information to the discussion rather than trying to discredit it all without alternative facts to back it up.

author: Zero999
author_rank: Super Contributor
timestamp: April 21, 2017, 11:42:15 pm
content: Quote from: Someone on April 21, 2017, 11:10:09 pmI apologise if I came across as pompous.I agree with you about phosphors but that's not relevant here because the original poster is not using an LED with a phosphor. What I'm referring to isefficiency droop, i.e. there's no point in driving at high current with a low duty cycle. The LED should be run at the current, which gives the highest efficiency. Now I agree, if that's higher, than the OP's power budget then PWM may be a wise choice. In any case, a switch mode supply should be used for optimum efficiency.

author: Someone
author_rank: Super Contributor
timestamp: April 22, 2017, 12:18:44 am
content: Quote from: Hero999 on April 21, 2017, 11:42:15 pmAgain you're thinking upwards into the well known thermal/saturation limits of efficiency in LEDs, even the links from your lazy google search show that the peak efficiency is at a single forward current point with either too highortoo low drive dropping the efficiency (or efficacy):http://www.semiconductor-today.com/news_items/2016/feb/postech_030216.shtmlhttp://spie.org/newsroom/6402-demonstration-of-novel-high-efficiency-blue-leds-on-silicon-substratesWhich is the region the OP is considering driving the LEDs at. Mike had already summed it up neatly in his first post:Quote from: mikeselectricstuff on April 20, 2017, 09:27:25 pmThe OP was very clear on their operating point:Quote from: jmaja on April 20, 2017, 08:48:22 pmWhich I'll continue to suggest is usually well below the ideal operating current of an LED if you are trying to extract maximum efficiency from it, so the PWM concept at a peak current within the manufacturers specifications is a much better design.

author: Zero999
author_rank: Super Contributor
timestamp: April 22, 2017, 12:21:30 am
content: Quote from: Someone on April 22, 2017, 12:18:44 amI apologise, you're right. After doing some more reading, I've realised efficiency droop is more of a problem at higher currents, than at extremely low power, where efficiency also deteriorates. LED efficiency does improve at lower currents but this doesn't extend down to the 50µA and this is nothing to do with the phosphor I was so obsessed with. I think Mike was right all along! Sorry I was too pig headed to see that.Running around 10% of the LED's recommended forward current seems to be the best way to go (unless the datasheet suggests otherwise) and that's quite likely too high for the OP's power budget. For example, assuming a resistor is used to limit the current and the LED's recommended IFis 10mA, then run it with 1mA pulses at 5% duty cycle to give an average of 50µA. A buck converter, rather than a resistor, could be used to either reduce the current consumption or provide longer pulses, therefore brighter light for less power.

author: PointyOintment
author_rank: Frequent Contributor
timestamp: April 27, 2017, 11:16:05 pm
content: This project to automatedly characterize LEDs and laser diodesmight be interesting to everyone in this thread. His results show that the LEDs he tested could be more efficient at low currents.

author: Someone
author_rank: Super Contributor
timestamp: April 28, 2017, 07:46:17 am
content: Quote from: PointyOintment on April 27, 2017, 11:16:05 pmWhere? They plot the same singular peak efficacy point:https://hackaday.io/project/12874-automated-ledlaser-diode-analysis-and-modeling/log/44613-this-is-why-i-am-building-the-analyzerTheir curve matches very well to the data I have shared above showing that efficiency drops as the drive current is reduced. Its exceptionally close even though they have a much more limited linear range in their detector.

author: mark03
author_rank: Frequent Contributor
timestamp: April 28, 2017, 05:54:36 pm
content: [Sort of?] back to the original question. If I want to drive an LED with ~ 1 mA pulsed current, at perhaps 0.1% to 1% duty cycle, so 1-10 uA average, what is the most efficient circuit one could design for this task? Assume battery voltage > Vf.I am wondering if it is possible without resistors, or at least without resistors in the main current path of the LED. E.g. a simple buck converter with low-side NMOS. You would just need a nanopower source of low-duty-cycle ~ 1 us pulses. Is that feasible?

author: mark03
author_rank: Frequent Contributor
timestamp: April 28, 2017, 07:12:57 pm
content: Right, I figure this could be reduced to a constant-duty-cycle oscillator/pulser. Not much point in a feedback loop. So, basically, 10-kHz gate drive with 1-us on time in < 1 uA average current. If that is possible.

author: GeorgeOfTheJungle
author_rank: Super Contributor
timestamp: April 28, 2017, 07:39:18 pm
content: Quote from: evb149 on April 28, 2017, 06:09:37 pmhp did that in their calculators:"The image above shows the calculator with the back removed. The two sets of 4 passive components on the far left are coils used in the inductive circuit to drive the LEDs. This is a different approach where the current driving the LEDs is limited by the time it takes to build up in the inductor, rather than through a resistor in conventional LED displays. This strobed inductive drive circuit is more efficient and was invented and patented by HP for use in calculators. In the worst case, display power consumption is about 110 milliwatts (thirteen 8’s and two minus signs). The LED display is covered later in more detail."http://www.hpmuseum.org/cgi-sys/cgiwrap/hpmuseum/articles.cgi?read=1263google.com/search?q=hp+calculator+display+inductors

author: Someone
author_rank: Super Contributor
timestamp: April 29, 2017, 04:18:03 am
content: Quote from: mark03 on April 28, 2017, 05:54:36 pmYep, thats the way to do it and calculate the peak current from the inductor and pulse width. But be careful of the reverse voltage on the LEDs some are less robust than others. The dirty circuit is a boost/inverting design where you tie the diode back to the +input rather than ground, minimal leakage through just the FET switch, and at these low powers it can be switched directly from logic or a microcontroller pin.