LEDs will work, depending on how bright or how large an area needs illuminating. But I've use printed strobe disks to check the speed of my mill against multiples of the 50Hz mains.

Bill

And this graph from Wikipedia gives an explanation:

Basically at ~100Hz you need only a very low depth of modulation between bright and dark(er) for the effect to be visible.

More at: en.wikipedia.org/wiki/Stroboscopic_effect

Edited By Neil Wyatt on 24/09/2020 21:32:16

I built a light beam communicator some 55 years ago. The DC of a torch bulb was modulated with an audio signal. I used a microphone and small power amp for this. The torch beam was demodulated by using a cadmium sulphide cell and amplifier. With good torch optics, ranges of several hundred yards were possible and speech was very intelligible. I have no difficulty in believing that light output from a GLS lamp is modulated by mains frequency and that this can be detected, without much difficulty.

Andrew.

You can get apps for smartphones that act as a variable frequency strobe light, so you can do away with the strobe discs and separate light source completely if you wish.

I've been using this one on an iphone that gives an RPM equivalent

https://apps.apple.com/us/app/strobe-light-tachometer/id978182913

I sometimes notice 50Hz flicker on old bulbs out of the corner of my eye. The effect is more obvious on 40W bulbs than 100W bulbs.

I've read somewhere than machine lamps are run at low voltage partly for safety reasons and partly because thick high current filaments in low voltage lamps cool down more slowly than the thin filaments in high voltage bulbs of the same wattage.

Day full of appointments but whilst in my Sunday best waiting for the next one I programmed an Arduino to flash a yellow LED. (I believe these don't have any phosphor, just light off the junction.) I can just see a trace of flicker with 2mS pulses (500Hz). Distinct flicker at 150Hz and no doubt at all at 50Hz. Anything below 25Hz is sick making annoying until individual flashes appear at about 5Hz. If it has to flash, 1Hz is bearable!

Whoops, time to hit the road again!

Dave