Homemade oscilloscope to detect PWM DIY guide

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.

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

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:

Hanzebenger I think you should even see 10 MHz PWM - as long as it's full on/off PWM. As you can see in my pictures in which I compared battery vs non-battery waves, a few posts above: https://ledstrain.org/d/312-homemade-oscilloscope-to-detect-pwm-diy-guide/52
The wave will not look like rectangular PWM anymore, and it will float above zero, but you can see spikes that have the correct frequency.

When the flicker is just a small ripple riding on a DC signal though, as seen in "flicker-free" displays, it might not be displayed. But such small flicker may matter: FRC/temporal dithering, unresolved software eye strain - all those have just a very tiny flicker. So I imagine if a backlight has a similar small flicker, it might trigger eye strain, too.
You can see such small waves a little easier when you switch your PicoScope oscilloscope to "AC" mode. The Hantek 6022BE doesn't have such a mode.
In fact I think there is a higher frequency riding on top of your waves (where there is no thin line but distortion) which you could make visible by adjusting the time axis.

I believe the largest part of your rise times is related to the slow CCFL response. You should easily detect 1000 Hz PWM.

Here you can see notebookcheck's PWM setup in action as they test the iPhone Xs:

Oscilloscope: Rigol DS1052E, ~300 €
Photosensor: Thorlabs PDA100A-EC, 327,29 € (PDA100A2) excluding VAT, https://www.thorlabs.com/thorproduct.cfm?partnumber=PDA100A-EC
The latter mentioned here on their German website: https://www.notebookcheck.com/Wie-wir-testen.1948.0.html

Fauser LiFli (red) vs. BPW34 @ 100K 9V (blue), 50 kHz PWM:

martin I think their implementation of their frequency software counter only recognizes waves that have a high peak-to-peak difference. OpenHantek suffers from the same problem. But what you could do is try and move the rulers to manually mark a single wave. Then the frequency might appear somewhere. As seen here in the OpenHantek screenshot: https://ledstrain.org/d/312-homemade-oscilloscope-to-detect-pwm-diy-guide/39 the rulers are labeled "1" and "2" and can be moved manually. Hopefully Hantek's software has such rulers, too.

martin When its set on 10x it doesnt show any readings, so I leave it at 1x.

That is normal behavior, we need to keep it at "1x".

By the way, even PicoScope's frequency counter is not very accurate, so I always use the bars there, too. The counter is useful only for quick evaluation but I rarely use it. Stopping and then manually marking the wave is always more accurate.

martin I am confused as then I do not know how well to trust the measurements I make with this. When I measure with the soundcard oscope, it shows PWM at all brightness levels of 15Khz.

I would trust the oscilloscope, as it is the more precise tool. If you want to, you can create pretty precise PWM with a Raspberry Pi and low budget parts (the breadboard isn't even needed) and verify if the frequencies are correct: https://ledstrain.org/d/375-howto-create-pwm-led-flicker-with-a-raspberry-pi