LED's with Auto dimming display

i'm building a digitally based, analog display clock using LED's for the display. also, a microcontroller is used ( ATmaga168). i need the LED's to be Auto dimming (reduces the intensity
of the LED display when the environment is darkened and restores
brightness in normal light). actually im kinda lost at designing such circuit. should i use a photo resister? is it possible to use shift registor?? any idea will be useful. thanks in advance..

If you would like all the LEDs to dim equally, a photo-resistor would probably be the easiest thing to use. Since the resistance of the photo-resistor changes as the intensity of light changes, you could determine the relative intensity of the ambient light by measuring the voltage across the resistor with an ADC. In order to dim the LEDs, you could simply send a PWM signal at the appropriate frequency to the LED. Combining these two, you can use the output of the ADC to create the appropriate PWM signal to dim the LEDs to the required brightness setting. I have included links to a few tutorials that should give you a very good start...

Using a photo-resistor with the ATmega168
http://jumptuck.com/2011/11/24/photoresistor-atmega168-adc/
http://www.avrfreaks.net/index.php?name=PNphpBB2&file=viewtopic&t=56429

Creating PWM signals on the ATmega168
http://www.protostack.com/blog/2011/06/atmega168a-pulse-width-modulation-pwm/

Regards,
Willis