Monitor flicker detection sensor

I recently wanted to buy a monitor, and I found that people may be affected by the mobile phone industry and are very concerned about the stroboscopic problem of monitors. I was wondering why LCDs have stroboscopic effects. However, it turns out that it does exist and it is very common. I guess high-end monitors also have stroboscopic problems. PWM dimming is mainly to ensure that the backlight color temperature is consistent, which is basically the case for displays with color accuracy requirements. I made such a sensor to test the display.

The sensor is an older IT model opt101, which is a photodiode integrated with a transimpedance amplifier. It costs 5 yuan a piece on Taobao. The entire board costs less than 10 yuan. If you have an oscilloscope, you can make one for fun. The nominal bandwidth is 14khz, so if you want to restore the waveform better, you can probably only measure the waveform below 5khz. You can also see the square wave at 20khz, but it is seriously distorted. If you just want to determine whether the screen is flickering, this sensor is enough. , I want to see the high-frequency waveform and I will make a high-end version of the sensor later.

The CR927 lithium battery is mainly used to control the size of the board. I chose the smallest 3v button battery. The power supply current is a bit choked. Be careful not to short-circuit. The short-circuited battery will directly run out of power. The rise and fall time measured using a 3v button battery is better than that of a 3.7 lithium battery.

Test status of each display

After receiving the chip, I first tested the monitor I was using. The effect was shocking at first glance. It was a very standard square wave. The result was shocking. It was a 240hz PWM and the monitor was not an OLED screen. The picture below was measured about 40cm away from the screen. The amplitude is larger when it is close to the screen. DC dimming needs to be turned on to 40% brightness. After checking the promotion page, the promotion is that there is no flicker, and it is a major international manufacturer, not to mention who it is.

Then test a mobile phone with DC-like dimming for comparison. In the normal mode of Xiaomi Mi 11, 482hz PWM adjusts the brightness by adjusting the duty cycle.

Then there is the PWM that maintains a duty cycle of about 95% of 60hz under medium and low brightness in anti-flicker mode.

The waveform measured by the mobile phone screen is not a standard square wave. The reason is that the OLED does not light up and off the whole screen at the same time, but lights up and off in intervals. Therefore, it is almost a straight line when measured about 3cm away from the mobile phone screen.

Finally, let’s test a display that exceeds the bandwidth range of this sensor, the m1 version of MacBook Air and MacBook Pro. The two models have the same measurement results. There is a 20khz strobe at medium and low brightness. The depth and specific waveform of the strobe are not sure, etc. I'll test it again later with the high-end version.