Light Level Indicator Using a Window Comparator

The second example below uses a LDR (light dependent resistor) 

to indicate some desired light level. The LDR has a large dynimic 

range and varies in resistance from less than 100 ohms on 

a cloudy day to over a megohm in total darkness. A 2K pot was 

used to adjust the window range for usual room light
conditions. This setup might also be used to indicate 

sunrise/sunset conditions.