I just fried my 2nd OPT101 by connecting the 9V battery. The sensor got pretty hot and then ceased to function. Pretty frustrating. Don't know what to do now. Maybe this approach is a dead end.
Homemade oscilloscope to detect PWM DIY guide
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If anyone still reads here, in a final attempt to measure anything useful I bought the handheld flicker meter Fauser LiFli ($300), and maybe the most important thing I found out so far is that flicker-free monitors do flicker, although very little, and even at 100% brightness. My two BenQ EW2740L flicker at 2% and the totally unusable Dell U2515H flickers at 4%. Still doesn't explain why one of the BenQs is not perfectly usable. Maybe very small differences in percentage make the difference. But 2% is the least you can measure with this device. My cheap Hantek 6022BE oscilloscope which I connected to the LiFli seems to suck too much to pick up such small differences. Currently I'm not willing to spend any more money for a better oscilloscope. I have an LED bulb that flickers at 4% when you turn it on and then it slowly continues down to 0% over some minutes. During the first seconds I get PWM symptoms. Later this LED is much better. So it seems the safe percentage for me might be somewhere between 0% and 2%. Which is insane since we're talking of frequencies between 20 kHz and 400 kHz. However, even this 0% LED is still buzzing slightly, made audible by the LiFli device's speaker. So there still is some ripple. It seems true flicker-free is not possible with wall-plugged devices. My small LED battery-driven flash light is at 0% without any buzzing.
I didn't measure flicker percentage (a-b)/(a+b) but ripple percentage (a-b)/a, which makes more sense to me.
Where a is max and b is min voltage.
https://www.fauser.biz/li/lifli_e.htm
Edit: With my new oscilloscope setup (a few posts below), it turned out that the low percentages (like 0-2 %) the device measured was probably the display's 60 Hz refresh rate flicker. I did not even know that LCD matrixes would pulsate at each refresh when displaying a constant image, but apparently they do.
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The 6022BE has much trouble with low voltages:
In the picture you can see a ripple of roughly 20%.
To me it looks like it is impossible to spot amplitude differences of 1% and even below, which seems necessary. Can any oscilloscope do this actually? If anyone knows how to amplify the signal before it enters the oscilloscope (it is a 3.5 inch audio to BNC cable), please let me know. Preferably with some purchasable device.
Just stumbled upon this post:
Like us he was interested in measuring PWM and compared different photodiode setups and their efficiency. Great work, must read!
http://budgetlightforum.com/node/61254
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@martin and anyone who is interested: With the help of the above link I managed to make my BPW34 very responsive for oscilloscope use. Older posts here helped me too, like TechSensitive mentioning a 100,000 Ohm resistor.
I also bought another oscilloscope, the PicoScope 2204A. It is their cheapest oscilloscope for ~140 € and does the job. They have excellent online support and replied to my questions in detail, recommending this model. In fact I do not understand why I had to search that long to find PicoTech when it seems to be the best USB oscilloscope manufacturer.
This is how I made the BPW34 responsive:
Note how the BPW34 has a dot on its detector surface, pointing to the end that gets connected to the resistor. It is a 100K resistor. The hook and grapple hold the resistor. The grapple also holds the black battery cable, negative pole (-).
The battery's positive pole (+) goes directly to the BPW34.
I do not know the maximum frequency that this setup can measure. The BPW34 PDF says the maximum responsivity is 100 ns with 10 V and 1K resistor. A 100K resistor amplifies the current voltage so it is easily detectable by the oscilloscope, however it may make the diode less responsive.
However, with this I measured 200 kHz very low amplitude ripple on my "flicker-free" monitor, so there is that. Anyone has an idea what the limits of this setup are?
sounds like a good project for my cheap hantek usb o-scope
KM Im happy to see this thread lives on and the oscopes we can create are improving. I lost track of it but Id like to get back to it. I have Hantek 6022BE and a batch of few of these LEDs with the small circuit. Your new setup seems a lot simpler to make. Can you recommend any good freeware software for the Hantek?
Ill get around getting these parts and see what I can make of it
martin Not sure what the best software is, but the OpenHantek software is pretty limited. I have a feeling the most developed software is HScope for Android, followed by some Windows 3rd party programs (which I did not try).
Today I tried to attach longer cables to my setup but it actually led to more noise and a worse signal. It is probably a good idea to keep everything as simple and short-cabled as possible.
valex13 I purchased the Radex Lupin. It works great for older laptops (like 5+ years old) as well as cars (even new cars). It also works great for detecting flicker in LED bulbs (I found that I was getting headaches, etc., from some LED bulbs that I had purchased). For me, LED bulbs below 5% flicker are just fine. Anything above, say, 30% flicker will cause noticeable problems. Anything in between is hard to say. I ended up walking around my house and replacing many of my LED bulbs with ones that flicker less.
KM I am very tempted to purchase the Lifli. It's totally worth the money to me if it works. I just don't want to waste $300+ on something that doesn't do what I want it to do. Would you say it does what's advertised? Specifically, will it detect PWM in newer laptops? I understand that it's not very precise because it only has 18 LEDs to indicate the amount of flicker.
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GregAtkinson I would not recommend the LiFli for displays. The reason is that it needs a pretty high brightness to operate. When the brightness is too low, the scale would just blink at 2%, indicating a lack of light. It would still output to an oscilloscope, but the output is very noisy, which is a problem since this hides low brightness fluctuations. The device works well for measuring room lighting and lamps, which I believe it is made for. But not for low brightness light sources like displays. On displays I need to show a white screen on a high brightness setting to get enough light.
For display measurements, you are much better off buying some BPW34 off eBay and build the oscilloscope circuit. It works even without soldering as you can see in the latest picture.
If you want to see even tiny fluctuations (1%) in low brightness modes, I can recommend the PicoScope 2204a. Its lowest range of +/-50 mV is a true 8 bit hardware mode. Plus it can do oversampling to simulate additional bits of vertical resolution, meaning it can filter out some noise. Up to 10 bits are useful. They told me the option which covers everything most accurate would be their true 12 bit oscilloscope for $800+, which no doubt would be great but seems overkill. Unless even fluctuations of far less than 1% are responsible for our eye strain, which at this point I have a hard time to believe.
The Hantek 6022BE has some accuracy problems in low voltage modes. Those modes are not real hardware modes but zoomed in from like the +/-100 or +/-200 mV mode. You will still see PWM, but maybe not tiny fluctuations (say below 5% voltage ripple) at low screen brightness. But thanks to the Android software HScope, the 6022BE might be the cheapest option to build a portable solution. We just need to make the photodiode probe circuit more robust so it can survive a trip in a bag easily. Then, with a powerbank and a compatible Android device, everything should be portable.
I just assembled a second BPW34 probe, this time without battery, just using a 100K Ohm resistor. And even then it is responsive enough, as seen in the following screenshots. Blue = battery + resistor, red = just the resistor.
The red line is waving a little, which is due to some neighbored 50 Hz noise. And differences in amplitudes are related to different viewing angles.
10,000,000 Hz - Now the LED gets noticably darker, probably because it can't keep up with the PWM speed anymore.
So it seems we don't even need a battery, just connecting/wiring/soldering a resistor to a BPW34 is enough. And then attaching the probes left and right of the resistor, right next to it.
KM Thank you, could you please again share a photo how the photodiode and the rezistor is attached together without the battery? Does the orientation of both matter? Does it also matter which clips of the osc go where?
I have downloaded picoscope, have hantek 6022BE and all the gear, I just need to put it together now.
martin PicoScope is a software only for PicoScope oscilloscopes. You need either:
Linux: OpenHantek (https://github.com/OpenHantek/openhantek)
Windows: The official software from Hantek for Windows (http://www.hantek.com/en/productdetail_2_31.html) or one of the alternative softwares for Windows. PCSCOPE was recommended by Wootever: http://www.eevblog.com/forum/testgear/hantek-6022be-20mhz-usb-dso/msg1099490/#msg1099490. There is also BasicScope, (http://pididu.com/wordpress/basicscope/) which looks a little strange: http://www.eevblog.com/forum/testgear/hantek-6022be-20mhz-usb-dso/?action=dlattach;attach=281643;image. There is also Open6022BE which I don't find the link for. It is buried somewhere in above eevblog thread. All those Windows versions might not run on Windows 10 as some users reported.
Android: HScope (http://hscope.martinloren.com/)
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Viktor Yes, I tried to do that. But the battery had no impact at all. Maybe my soldering was too bad. So I took my second OPT101 and used it without the attached board, effectively overheating it within seconds.
However, I think the OPT101 may be too slow due to the official bandwidth specification of 14 kHz.
Edit: Viktor I realized I soldered the OPT101 to its board the opposite way. Would this make any difference? I had to switch the probe tips to get a positive voltage.