@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🙂

    • KM replied to this.

      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.

      martin Good news: With the current setup I can detect 10 MHz PWM (and even more). It is a little out of shape, but clearly visible. And this was just the PWM of a small pin LED. A monitor is much brighter and should deliver even better results.

      • JTL likes this.
      17 days later

      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.

      • KM replied to this.

        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.

        • JTL replied to this.

          KM I would not recommend the LiFli for displays. The reason is that it needs a pretty high brightness to operate

          I suspected the same as well.

          13 days later

          KM Could you please share the name of the rezistor?

          • KM replied to this.
            18 days later

            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.

            100 Hz

            100,000 Hz

            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.

              • KM replied to this.

                martin Orientation does not matter without the battery. If the signal is negative, below zero, just switch the probes. I will upload a photo in some minutes.



                  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/)

                  KM Have you tried connecting the OPT101 like shown here?

                    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.

                      My recent LED measurements (see above) which showed no perfect square waves bothered me a little and I began to wonder whether the LED or the photodiode was too slow for the PWM signal. So I wanted to know how much impact the 9V battery really has, and as you can see in this screenshot, it speeds up the rise time a little.

                      Blue=9V battery, Red=no battery; Frequency: 5000 Hz, 100K resistors

                      However, reducing the resistance from 100K down to 10K speeds up the diode almost proportionally by the factor 10:

                      Blue=10K resistor, Red=100K resistor; No batteries

                      The downside is that 10K is not only 10 times faster but also delivers a 10 times smaller voltage, so I believe for now 100K are a good trade-off. If anyone has ideas how to make this faster or can measure his OPT101 rise times, let me know. With the Raspberry Pi How-To you can create your own PWM-controlled LED circuit and measure the speed of your oscilloscope setup: https://ledstrain.org/d/375-howto-create-pwm-led-flicker-with-a-raspberry-pi

                      OPT101 without battery, 5000 Hz PWM:

                      So it seems the OPT101 gives up much earlier.

                      KM There is a small notch on the photodiode to help with the orientation, this should be the correct layout:

                      Edit:
                      Test with a Nokia 6 at <17% brightness:

                      result from notebookcheck.com:

                      Edit2:
                      I tried to create a pwm signal with the arduino status led:

                      Code:
                      void setup()
                      {
                      pinMode(13, OUTPUT);
                      }

                      void loop()
                      {
                      digitalWrite(13, HIGH);
                      delayMicroseconds(100);
                      digitalWrite(13, LOW);
                      delayMicroseconds(100);
                      }

                      The opt101 can also be connected without soldering:

                        It looks like both the working OPT101 + 9V battery solution and the BPW34 + 9V battery + 100K resistor share the same responsiveness, with the OPT101 delivering a higher voltage.
                        Both suffer from their slow rise and fall times.

                        I am thinking about a new approach the whole day now: connecting multiple BPW34 in parallel to maximize the photocurrent, and using a low impedance resistor - 10K or lower - to maximize the voltage. High responsiveness and still decent voltage output.

                        dev