William Dudley @ dudley.nu has designed a motorcycle headlight modulator based on 555 timer IC and photoresistor. A headlight modulator will make the headlight to pulse during the day and be steady at night. He writes:
Unhappy with a headlight modulator I purchased, I decided to make my own. Even though it would be a trivial programming project to use an Arduino Teensy or similar to do this, I decided to do it the “old fashioned” way, using a 555 timer. The 555 is a clever chip; not only will it supply the oscillator for the flashing effect, it has a reset pin that can be used to force the output to a known state (low) when (other circuitry tells it that) it’s dark outside.
Headlight Modulator for Motorcycle – [Link]