This is a proof of concept/how to for a drawbridge that opens at dawn and shuts at night automatically. Its not meant to look nice, but it does easily show all of its workings. It is triggered by a "redstone link" from a separate circuit. This circuit is a rising edge/falling edge detector that sends a single pulse when the daylight detector turns on or off (changes state). You could also trigger this manually with the same "redstone link" channel.