We all experienced it: light flicker! When concentrating on a piece of work or dead tree text, the light suddenly goes berserk and flash like a stroboscope on the dance-floor.

After fixing a TV with LED backlight, let’s dive in different performance of light fixture/light bulbs.

This effect is especially present in LED light fixtures where there is no inertia: incandescent or fluorescent light have a thermal or phosphor inertia and continue to glow a flick of second after the power is cut. But not the LED (even though there is some phosphor in them), this leads to have a close relation of the light intensity and the supply voltage.

Most of LED light bulb are supplied from main, the quality of the AC/DC converter takes a main role here. Because good rectification and filtering cost, manufacturer are cutting them, and you get this effect.

You can find two type of power supply on LED bulbs: simple rectification with LED string to match the AC rectified voltage. Or AC/DC converter with inductor and constant current control (this is what you want).

I wanted to “quantify” it, more that just a feeling, but didn’t want to drop hard cash (k€$ range) for dedicated equipment.

So I used one of the simplest and cheaper photo sensor on the market: a NPN phototransistor, (not a photoresistance as the bandwith is to small), in addition with a 10k resistor, connected to the GND (ring 2) and MIC (sleeve).

Then use a simple Android software, called Oscilloscope

The result is then very straightforward, especially in spectrum analyzer:

I now always have this small 3.5mm 4 ring photo-transistor adapter in my pocket when shopping light bulb/appliances in home improvement, so I can point it and discard the worst devices.

If someone comes up with a better idea, to measure more accurately amplitude and ideal the spectrum (would need a dedicated color sensor though), with an easy to use interface, that would be awesome.