Playing "Boktai" requires sunlight. The "solar sensor" responds to the player's environment and reflects the amount of "solar energy" (sunlight) in the game at real time.

When there is strong sunlight, solar energy charges up quickly. When weak, it charges up slowly.
Sunlight is required mainly to charge energy to the solar gun which is the player's only weapon, and to fight the boss at the Pile Driver.

And during moments other than these, the sensor will detect solar energy, causing the game content to change.

There are two gauges at the bottom right of the screen.
The one on the top is the "solar gun gauge" that shows how much is left in the battery of the solar gun. The one on the bottom is the "solar gauge" that shows how strong the sunlight detected by the solar sensor at real time is. In order to charge solar energy to the solar gun,
press the A Button when the solar gauge is responding.

On the side of the "Boktai" cartridge WITHOUT the label is the "solar sensor."
You can charge solar energy by making sunlight hit this sensor.

The direction and tilt of the solar sensor does make a difference in the intensity of the sunlight detected. When you can't charge up sunlight well, try pointing the sensor in different directions or tilting it at different angles while keeping an eye on the solar gauge.
When playing with a Game Boy Advance SP, make sure the solar sensor is not in the shadow because of the cartridge's position.
However, in given environments, the solar sensor will respond even when not under direct sunlight.

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(C)2003 Konami Computer Entertainment Japan