# [ELI5] Why do digital clocks go absolutely nuts when you put your phone camera on them?

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Feeling super stupid sorry for dumb question…

In: Technology

I think what you are seeing is what’s called multiplexing: the screens aren’t all lit up at once, the are lit up in sections one after the other in very fast succession, this makes it so you can make the control circuit smaller by reducing the number of segments it needs to drive

Your phone camera has a frame rate that likely won’t match the multiplexing frequency, this means that in every frame a different segment will be lit, so it looks all flashy

to save on wiring, for a big array of LEDs, rather than linking each segment to a seperate pin on the controller, they link the positive pin of all the same segments together across all the digits, and link the negative pin of every segment in the same digit together. in that way, by controlling which segment electricity enters into, and which digit it is allowed to leave from, they can greatly reduce the number of pins. for example, for a 4 digit 7 segment display, they need 7 anode and 4 cathode pins, for a total of 11 pins. this is better than the 7×4=28 pins which you’d need if you didnt do this technique, called multiplexing.

But since you want different digits to light up different numbers, you have to show one number on one digit at a time eg: for 1493, you first setup the anodes to light up the segments and show 1, then you connect the cathode of digit 1. you then disconnect the cathode of digit 1, set the anodes to display 4, connect cathode 2, set anodes for 9, connect cathode 3, etc…

This means only 1 digit is on at a given time, and they flash past in sequence much faster than the human eye can perceive.

when you point a camera at them, the camera captures 30 frames per second, so sometimes it catches digit 1 on, sometimes digit 3, sometimes none of them, its very chaotic. for that reason, the image comes out looking super choppy and flashy.

hope this helps!