Why do 7 segment LED displays break apart when I brush my teeth?

Question

This question might sound silly, but it is very serious (although geeky, I must admit). When I use my electric toothbrush in front of my alarm clock (one of those with a big red LED display), the numbers seem to break apart. Why?

With my current alarm clock the first 3 segments (a,b,c) make one group and the other 4 make another one. The striking thing is that both groups seem to slowly oscillate, in anti-phase. I cannot affirm that the grouping is the same for every display of the kind -- and I cannot check now because I only have this one LED display at home -- but I've seen this illusion on many different displays in the past.

I think this illusion is fascinating (told you I'm a geek) and I would like to understand why it is...

I believe that it has something to do with 1) the vibrations of the toothbrush making my eyes oscillate in their orbits so the image seems to move (a bit like when you touch your eye on the side with your finger and the whole image seems to move), 2) with the periodic refresh of the segments. I suspect that the two groups I'm seeing actually correspond to two groups of bars blinking in sync, so there really are two groups, (but why do they dislocate like that?) 3) with our periodic perceptual "refresh rate". Something similar to what makes us see car wheels as stationary when they rotate at a certain speed.

I must say my competence in electronics is close to zero, so my questions might be trivial to you guys. How does the refresh of the line segments in a 7 segments display happen? Is it cycling (a,b,c,d,e,f,g,a,b,c...)? How come I see two groups, then? At what frequency are the LEDs blinking? I understand some of these questions depend on the specifications of my alarm clock, but since I've seen this illusion on basically every type of LED display I passed by with my toothbrush (microwave, VHS video recorder (yes the story started long ago. I bought the toothbrush I'm now using just for understanding this silly illusion, but it was actually as a teenager that I noticed it)...), I guess there is something constant there...

Answer 1

Yes, it sounds like you are seeing artifacts of toothbrush frequency vibrating your head and therefore your eyes, and that beating against the LED refresh frequency.

This is a similar effect to eating potato chips (actually anything crunchy) while watching the LED display. In that case the head vibrations are more random, so parts of the LED display will appear to jump around randomly. Some segments will be displayed during a head-high part of a vibration, and others during head-low. These will appear in different locations.

LEDs are refreshed all kinds of ways. A lot has to do with how clever or competent the engineer was that wrote the firmware. I've seen naive refresh algorithms that simply do each digit in order. Those have the most apparent flicker for any one refresh rate. Better means interleave digits, sortof like interlacing of old TV scans lines. The whole display is still refreshed at the same rate, but the apparent flicker is at least in part related to the interlace rate. There are fancy schemes which interleave both digits and segments, but these are often not possible with common displays where whole digits are already partially wired together.

Answer 2

This can be experienced with any vibration of the head in front of certain LCD displays - usually microwave ovens for some reason - most amusingly by vibrating your lips the way you would as a kid when imitating a car or motorbike sound - no oral hygiene technology necessary.

If you purse your lips and modulate your trumpeting frequency from high through low, you can see different effects at various frequencies - at higher frequencies the letters merely vibrate in a boring green manner, but at lower frequencies they seem to oscillate like a loch-ness beastie swimming across your display - which has been known to reduce a roomful of trumpeting students to hysterics, as if the Great and Holy Defy they had gathered to worship with all manner of purséd lip, had just told them a very dirty joke.

So yes, it's obviously as a result of beats created between the refresh rate of the display and the frequency of your vibrating skull (and thus the eyeballs too - assuming you've been blessed with the standard human eyeball-socket configuration). Because the digital oven display right next to my microwave shows no effect whatsoever (either a much higher frequency or simply a DC on off arrangement . . ?

I'm afraid my knowledge of electronic displays is sorely lacking). So if at first you experience nothing, do not be disheartened - simply move onto the next household appliance (pausing briefly to give the victimised appliance a quick wipe-down).

However, if you try this at home, might I recommend starting out alone. Your kids will love it, but it might be just the excuse your spouse has been waiting for to invite those nice men in white jackets to afternoon tea.

Answer 3

As other answers have stated, because the LEDs are being scanned at a rate that interferes with the vibrations in your head/eyes. However, you can actually measure the resonant frequency of your eyeballs by looking at a slow trace on a scope and hitting yourself on the head! You will see the trace "jump" and from the calibrations get a measure of impulse response of your eyes. Not sure if it works on digital scopes, but it certainly did on the old analog ones.

Answer 4

The power line noise could be beating against the LCD display refresh rate (assuming an underfiltered or undershielded power supply and a scanned matrix display output). The motor in the toothbrush can create an oscillating EM field which can couple to power lines. This is just like playing 2 slightly differently out-of-tune notes and hearing a "beat", except visually.

Answer 5

So this doesn't happen when the brush is merely running but not in your mouth; you have to be actually brushing your teeth? If that's true, then I think you nailed it, that it's a stroboscopic effect between the brush interacting with your eyes, and the refresh rate of the clock's display.

I see something related when I'm driving on the highway at night and approaching a car with LED tail lights, if I sweep my eyes across the road quickly while scanning traffic. Those tail lights look like a dotted red line, while the incandescent ones look like a single red streak.