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Even on a current new 2012 car, when the green LED clock inside the car is quite bright, but when the car's headlight is turned on, the clock dims down to only about 1/4 of its brightness, which makes the time hard to see.

I thought the clock requires merely a watt or even less for its brightness, and the car stereo which output at least 30 watt of music (if comparing a 30 watt speaker for the PC to the car stereo), won't be less loud when the headlight is turned on.

Why does the clock dim down so much? Is it explained by Ohm's law? Also, can't it be "parallel" instead of "in series" with the headlight, so that the brightness is not affected?

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This is likely a deliberate feature rather than a side effect of drawing current. If the dashboard lighting is too bright it will interfere with the drivers vision in the dark. The other instruments don't dim because they are passive while the headlights are off (when it can be assumed to be bright out). –  dmckee Jul 4 '12 at 19:05

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up vote 4 down vote accepted

The answer is in physics, but not ohm's law.

The lights are dimmed when you turn on the headlights because the car assumes that if the headlights are on, then the cabin is dark. If the cabin is dark, then control lights need to be dimmed so they don't distract the driver.

This usually works just fine, unless you turn on the lights when it's not all that dark.

The physics of this is called "dynamic range" - the lights will look brighter when you don't have the sun shining around them.

In most cars, you can adjust the brightness. Check your manual.

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