Boktai: The Sun is in Your Hand/Controls

Light sensor
The solar sensor is located within the GBA cartridge. If it is broken, the game will not operate.

If you want to fool the sensor, you can obtain a UV light and shine it on the sensor. Because of inherent danger of UV radiation, this should only be performed in short bursts. Holding bulbs too closely to the sensor may cause damage to the GBA, and exposing yourself to UV radiation can have the same effect as sunburns or other serious health issues.