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

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.

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.