This is the simplest of the projects described in this site. It uses a photo light resistor to control some LED lights, lights are permanently turned on during all night long, and turned off during all day. When the light sensor reaches a determinate level automatically turns the lights on, and when it goes below that level it turns the lights off.

These LED lights are strategically placed in the second floor near the stairs, so during the night people can walk clearly along all stairs and annexes corridors. Also, this clearness is good for security reasons.

It is implemented a system with three-way switches that allows to control the lights from two separate points, one of them is the normal switch on the wall and the other is the Arduino circuit.

It is inspired in this link: Using photoresistors with an Arduino

List of materials and budget:

• Arduino Uno, $ 10.99
  
• Breadboard, $ 5.51
• Relay module, $ 7.99
• Power adapter, $ 7.49
• 1 x Photoresistor, $ 0.24
• 1 x 10k Ohm resistor, $ 0.05
• Some jumper wires, $ 0.65
  
• Homemade wooden case, $ 4.00 (Estimated)
  

The approximate total budget is: $ 36.92

The wooden case is mounted on the suspended ceiling and it is not visible at a glance. Also, there is space (in the breadboard and relay module) for futures circuits to control other devices.