Naked Science Forum

General Science => General Science => Topic started by: AllenG on 09/01/2009 09:19:18

Title: Why does my alarm clock display strobe?
Post by: AllenG on 09/01/2009 09:19:18

I'm not sure where to put this so I picked General Science.

Why is it that the LED display on my alarm clock has a strobe effect when I look away from it, but is steady when I look directly at it?
It also happens if I pick up the clock and wave it back and fort.

Many thanks,

Allen

Title: Why does my alarm clock display strobe?
Post by: Chemistry4me on 09/01/2009 09:43:00

Hey, that has happened a lot with my alarm clock too! I've got absolutely no idea why...

Title: Why does my alarm clock display strobe?
Post by: lyner on 09/01/2009 18:17:27

Yes. You'd assume that the whole of the clock circuit works on DC and the LEDs would just be switched to be on or off.

Title: Why does my alarm clock display strobe?
Post by: RD on 10/01/2009 09:15:09

Multiplexed display: each of the 7-segment LED numerals are individually driven (lit) in rapid succession, not all lit at once ...

Quote

Multiplexed displays are electronic displays where the entire display is not driven at one time. Instead, sub-units of the display (typically, individual pixels but often individual characters or numbers) are multiplexed, that is, driven one at a time, but the electronics and the persistence of vision combine to make the viewer believe the entire display is continuously active.

A multiplexed display has several advantages compared to a non-multiplexed display:

Fewer wires (sometimes, far fewer wires) are needed
Simpler driving electronics can be used
And both lead to reduced cost
Reduced power consumption

Because most multiplexed displays do not present the entire display simultaneously, they are subject to "break up" if the observer's point of regard is in motion. For example, if the observer were to rapidly swing their vision across a multiplexed display, they might see a jumble of individual digits rather than a coherent display. This effect can also sometimes be provoked by chewing hard candy*; this causes vibration of the user's eyes, leading to the break-up of the display.

http://en.wikipedia.org/wiki/Multiplexed_display

*or using an electric toothbrush.

Title: Why does my alarm clock display strobe?
Post by: AllenG on 10/01/2009 15:01:32

Thank you RD.
That has been bugging me for quite a while.

Title: Why does my alarm clock display strobe?
Post by: AllenG on 10/01/2009 19:53:05

Could the fact that the periphery of one's vision is more sensitive to low light (as how averted vision (http://en.wikipedia.org/wiki/Averted_vision) in astronomy becomes obvious) have any effect?

Title: Why does my alarm clock display strobe?
Post by: Pumblechook on 10/01/2009 20:30:38

The multiplexing might show up if you were to look at with a video camera.