Oscilloscope and probe to measure LED flicker? - Page 1
Forum section: electronics / testgear

author: erik_
author_rank: Contributor
timestamp: July 15, 2017, 06:23:05 pm
content: Hello fellow people of the internet!I get pretty bad eye strain, headaches and even severe migraine from LED light sources that flicker, even if it's fast enough that it can't be perceived. This happens with both computer monitors (and other monitors on electronic devices) and LED light sources, especially those for mains voltage that can be dimmed on the primary side. Flicker free (non PWM) displays are fine. No problems. DC driven LED's are fine too.Now, I know that it's very likely that these products use PWM for brightness regulation. I have had these problems for years, but the thing I don't know is which frequencies and duty cycles that are problematic for me. That's where you guys come in. I need equipment to measure this. I want to measure everything LED-based that I have access to, in order to try and figure out which light sources and displays that are problematic, and why.What I've managed to figure out so far is that I need an oscilloscope, and a photo sensitive probe of some kind, sensitive to the visible light spectrum. Thing is, my budget for this is pretty much non-existent.I want to be able to measure like whats shown in the picture on top of this page:https://www.notebookcheck.net/PWM-Ranking-Notebooks-Smartphones-and-Tablets-with-PWM.163979.0.htmlFrequency, how much difference there is between maximum and minimum brightness during a cycle, and how steep the rise and fall is, i.e. does it go down instantly, or does it slowly fade out. I want to be able to have somewhat good accuracy and resolution up to about 500 Hz, or 1000 Hz preferably, with decent accuracy and resolution to about 10 kHz, but I would also like to be able to detect frequency and duty cycle up to 100 kHz even though rise and fall isn't interesting at all at above 1-10 kHz really.The oscilloscope is only going to be used for this task for now, so I want the cheapest I can get away with. So with that in mind, will this one do:http://www.ebay.com/itm/Hantek-6022BE-PC-Based-USB-Digital-Storag-Oscilloscope-48MSa-s-20MHz-2-Channels-/272515225036?epid=1061057476&hash=item3f732c55cc:g:gYIAAOSwzhVWqIMy?And then the probe.. Where do you find something like what is used in this video:?I do also have a Fluke 117 that can measure frequency and duty cycle, which I guess would be a backup plan if I can find a probe, but it wont show me rise and fall curves.I hope you guys can help me out, because I want to get to the bottom of what causes my migraine attacks. They are no fun.

author: LaurentR
author_rank: Frequent Contributor
timestamp: July 15, 2017, 07:09:17 pm
content: A cheap solution I use occasionally is a photocell (photoresistor). These just need some voltage source (voltage not important) and a series resistor in the 5-10k range (exact resistance not important). A small cap (say 01.uF to 1uF) can help smooth out the signal.See one tutorial:https://learn.adafruit.com/photocellsTotal bill of material is < $1.If you stick the photocell on a screen that PWMs, you'll see the oscillations on the scope and you can measure the frequency.I have used this for measuring fan speed (shine an LED through it) or LED PWM frequencies.I spend 2min putting together a photoresistor I had on hand from a random electronics starter kit, a 4.7k resistor and 0.1uF cap, put the photoresistor in front of the screen of my bench DMM and got the pic attached.Having the scope measure frequency is a bit difficult as the results are not stable enough, but if you single shot, it's easy to measure. In this case, ~5 divisions @ 1ms per div = 5 ms / 200Hz.The photoresistor is a bit slow and the reason why the waveform is triangular, I believe, is because it is limited by the actual rise/fall time of the photoresistor, so you likely won't be able to see anything faster than a few 100s of Hz, but given the cost, it's a good start.

author: Jon.C
author_rank: Unknown
timestamp: July 15, 2017, 07:22:53 pm
content: I have some of thesehttps://www.aliexpress.com/item/Free-Shipping-10PCS-BPW34-DIP/1892868364.html?ws_ab_test=searchweb0_0,searchweb201602_1_10152_10065_10151_10068_10084_10083_10119_10080_10082_10081_10110_10137_10111_10060_10112_10113_10155_10114_10179_10154_438_10056_10055_10054_10182_10059_100031_10099_10078_10079_10103_10073_10102_10120_10189_10052_10053_10142_10107_10050_10051-10120,searchweb201603_1,ppcSwitch_5&btsid=517d1d76-e485-43ea-b86b-c080cb9f695c&algo_expid=01f1f463-7189-4b6c-a1e1-cefa731a28d3-3&algo_pvid=01f1f463-7189-4b6c-a1e1-cefa731a28d3Do you think they can serve?

author: erik_
author_rank: Contributor
timestamp: July 15, 2017, 07:30:42 pm
content: Quote from: LaurentR on July 15, 2017, 07:09:17 pmThanks for the suggestion, it may be worth a try at that price, at least to use with my multimeter. The slowness of the photoresistor may be a problem for accurate readings with a scope though. I really want to know if the fall is pretty much instant, or if it fades.Quote from: Jon.C on July 15, 2017, 07:22:53 pmI don't know? I have pretty much no knowledge of what is required from either a scope or a photoresistor. I don't even know what specs to look at. Hence this threadAny suggestions are welcome though. Solder free solutions preferred since I feel sick from the fumes too. Poor ventilation I guess.

author: SeanB
author_rank: Super Contributor
timestamp: July 15, 2017, 07:32:13 pm
content: PIN photodiode, it will work as a voltage source into a 1M scope input and give a noisy voltage, but will show flicker to a pretty high frequency. Better is to grab a small solar powered calculator ( get one with a real solar cell in it, there are fake ones) and use the small amorphous cell in it instead, larger area and higher output voltage as opposed to the sub 100mV the photodiodes will give without amplification. The solar panel ( even a small panel from any source that is around, just has to be around palm size and deliver some voltage in lowish indoor light) will do the job easier.

author: erik_
author_rank: Contributor
timestamp: July 15, 2017, 07:40:31 pm
content: Cool! So those react quickly enough? I do have a solar cell about palm size laying around actually, maybe I'll do some tests with my multimeter.Anyone have any opinions on whether the cheap scope in my original post will be good enough?

author: SeanB
author_rank: Super Contributor
timestamp: July 15, 2017, 07:48:13 pm
content: Yes on both counts, you are measuring up to 100kHz or so, and both will respond to at least that frequency.

author: Someone
author_rank: Super Contributor
timestamp: July 16, 2017, 05:45:52 am
content: Quote from: erik_ on July 15, 2017, 07:40:31 pmIf you're only interested in measuring visual flicker and backlight PWM rates, then a scope is probably unnecessary and distracting. No matter whats recording the signal you'll want a preamplifier for the small signals that these optical detectors produce. You can find many application notes for using opamps with photodiodes and instead of an expensive photodiode you can put an LED in the same place.Once its amplified, just send it to your computers sound card.

author: alm
author_rank: Super Contributor
timestamp: July 16, 2017, 05:55:20 am
content: Depending on what you want to know, a pulse stretcher (like in old logic probes) driving a LED might be sufficient (that would convert fast flickering into a slow blink).

author: erik_
author_rank: Contributor
timestamp: July 16, 2017, 10:47:11 am
content: Quote from: Someone on July 16, 2017, 05:45:52 amThe whole point is that I want to measure flicker that isn't visible to the eyes, as well as the difference between maximum intensity and minimum intensity during a cycle. Also the characteristics of the rise and fall is of interest, i.e. does it go to it's lower state instantly, or does it fade?Using a sound card could be interesting though, since I would then also possibly be able to use it with my phone for a portable setup that I can bring to stores to test light sources and displays before I spend my money on them. Would it give me enough information to see the difference in intensity between the high and low state of the cycle? I want to be able to calculate a percentage of flicker, like they do here:http://www.ledbenchmark.com/faq/LED-Flicker-Measurement.htmlQuote from: alm on July 16, 2017, 05:55:20 amI don't thing that would be sufficient unfortunately.---I did find out what kind of probe notebookcheck are using, and it's not cheap (https://www.thorlabs.com/thorproduct.cfm?partnumber=PDA100A-EC).I also did try a photovoltaic cell (about 100x70 mm size) connected to my multimeter, and it was able to measure the PWM frequency of my maglite mini at half power:It was however not able to detect anything on a laptop that I'm pretty sure is using PWM. I think the display was just too dim. 0.9 V at max brightness, 0.16 V at lowest. Nor did it manage to measure anything from a 12 V halogen light source powered by an AC transformer. I don't think the DMM can detect those small differences in voltage while measuring frequency. I was also wrong about the DMMs ability to measure duty cycle. It does not have that feature.

author: alm
author_rank: Super Contributor
timestamp: July 16, 2017, 11:32:29 am
content: If you also want level measurement, then you will also need a sensor with good linearity (or calibrate it yourself somehow). That is not something that is necessarily guaranteed for all sensors. You will also need to make sure that the light levels are within the dynamic range of the sensor. Too much light will saturate it, invalidating level measurements.With audio capture devices I would expect their AC coupling to distort your signal, since it will likely have some DC bias.

author: H.O
author_rank: Frequent Contributor
timestamp: July 16, 2017, 12:05:10 pm
content: QuoteUnlike LEDs the 12V halogen bulb (and incadescent bulbs in general) is incredibly "slow". Just look at it as you remove power from it, it takes a very long time (relative the cycle time of the AC driving it) for it to cool down so the flicker caused by it being driven from 50Hz AC must be miniscule.

author: ebastler
author_rank: Super Contributor
timestamp: July 16, 2017, 12:09:37 pm
content: Quote from: erik_ on July 16, 2017, 10:47:11 amAs sound cards are AC coupled, they will not be able to measure the average intensity, only the fluctuating part. Hence you cannot use a sound card to measure the percentage of flicker.But as you already own a nice multimeter, you should certainly be able to use that to measure average (DC) intensity and intensity fluctuation (AC), and then calculate a ratio. If you build a decent photodiode detector, and maybe an AC couple preamp, your multimeter should also be able to measure the frequency of the fluctuations.If you also want to observe the waveform, you could still use a soundcard for that. (And of course the soundcard provides an alternative way to measure the frequency.)

author: erik_
author_rank: Contributor
timestamp: July 16, 2017, 12:52:13 pm
content: Quote from: H.O on July 16, 2017, 12:05:10 pmOf course the flicker is miniscule, but it can still be measured with good enough equipment, which my DMM is obviously not for this task. An incandescent bulb driven at 50Hz can apparently be measured at 10 % flicker, according to ledbenchmark.com. Halogen is in my experience slower to decay.ledbechmark.com is apparently using an TSL257 IC to measure flicker, which seems like an affordable option. They are using one branded as Taos, which is quite expensive here, but there is also one branded AMS that is cheap. Don't know what the difference is.Quote from: ebastler on July 16, 2017, 12:09:37 pmSo, you think my multimeter would have a high enough sample rate to get an accurate average at these frequency ranges (50Hz to 500-1000Hz or so)? And then I can just use a sound card (or maybe even the microphone input on my phone) to see the waveform? That would be great as it would save me US $60. Although I would like to have an oscilloscopeCould you possibly get in to a bit more detail about how I can use DC and AC measurements to calculate a ratio? Not quite sure I follow.Wouldn't the mic preamp cause issues? I'm thinking that maybe it will affect the signal too much. I don't have a pure line input in any of my computers unfortunately. I know people are using the mic input on their phones so maybe not an issue?

author: ebastler
author_rank: Super Contributor
timestamp: July 16, 2017, 02:10:05 pm
content: Quote from: erik_ on July 16, 2017, 12:52:13 pmThe Fluke 117 is specified for AC voltge measurement up to 1kHz, so this should not be a problem.Your input voltage will have a fluctuating component (hopefully small) on top of a static component. When you measure the voltage in the meter's AC mode, it will "subtract" the stationary baseline, and only show you the amplitude of the fluctuating part. When you measure in DC mode, the meter will average out he fluctuations, hence show you the average total voltage. Hence, by dividing AC voltage / DC voltage, you get the relative percentage of flicker.Quote from: erik_ on July 16, 2017, 12:52:13 pmThe mic input will be AC coupled, hence -- similar to the multimeter in AC mode -- will subtract the static baseline and only let the fluctuations pass. There is a lower cutoff frequency in that coupling; e.g. 1 Hz fluctuations will not pass but will be subtracted. The exact cutoff frequency will vary between different brands, but I am confident that anything above 50Hz will pass undistorted. And I would not expect any lower frequency components to be relevant in LED flicker.

author: erik_
author_rank: Contributor
timestamp: July 16, 2017, 02:32:38 pm
content: Quote from: ebastler on July 16, 2017, 02:10:05 pmAh. This makes a lot of sense. Thank you!Quote from: ebastler on July 16, 2017, 02:10:05 pmOk, I think I'm going to find a suitable light to voltage component and build a probe to try with my sound card. It's going to be dirt cheap, which is good. If I feel the need I will then get a cheap oscilloscope.Thanks again!

author: JDubU
author_rank: Frequent Contributor
timestamp: July 16, 2017, 03:46:15 pm
content: A standard LED will also act as a light detector with ~microsecond response time.Here is the signal from an LED flashlight pointing at an IR LED with a 68K ohm load (LED output current --> voltage across resistor):

author: erik_
author_rank: Contributor
timestamp: July 16, 2017, 04:29:39 pm
content: I'll experiment with that too. Won't the voltage be awfully low from an LED though?

author: JDubU
author_rank: Frequent Contributor
timestamp: July 16, 2017, 04:38:22 pm
content: The peak to peak signal in the above test was 150mV.