Having automatic room light controller can save some energy. Deekshith build such controller which senses if human entered room. It has couple optical sensors fixed on a doorway that detects if someone enters. Seven segment LED display indicates current number of people inside and it keeps counting in bidirectional way. If someone enters it counts up if leaves, counts down.
An if people counter is larger than zero, then light is turned on. Still there is a problem with counting when two or more persons enter at the same time. So it may not work correctly in this situation. And probably a light sensor could be added to detect if there is a need to turn light at all. Anyway great project that helps turn on light when someone enters in the dark and tries to find light switch. And also it makes sure no one leaves light on when last leaving the room.