by u.. I meant you people.
How were we to know, 'u' in an electronic forum is translated 'micro'

the dead esy way:

While I vehemently object to the use of one-shots (one shot in the foot), whatever you do, there are exceptions to every rule. Since you will see an occaional flicker in the intensity (shiny car passing in daytime, headligthts at night and such) one more flicker a day will not kill you

so here is the simple solution:

use a l/f such as http://www.selectronic.fr/includes_...TSL220.pdf and connect it to a retriggerable multivibrator which control power to the LEDs. The cap on the one-shot set the intensity.

If you already have a uC involved, you can connet the l/f to an interrupt and use a timer as the 'one shot'.

I have since abandoned the above method to improve hysteresis and react less to passing cars, but that solution is proprietary.

Erik