The solar sensor is an essential part of Boktai gameplay, and is essentially the reason why Boktai is so famous. The solar sensor effects gameplay as it absorbs rays of sunlight and channels through to the game. The strength of the sun determines how well the sun can be used in the game, and the strength of the rays of sunlight are shown on the solar gauge.
 What the sun does to gameplay
Contrary to beliefs, the game does not power the Game Boy Advance, SP or Nintendo DS through the use of sunlight. The sun affects the surroundings in general and also charges battery (in Boktai: The Sun is in Your Hand and energy (in Boktai 2: Solar Boy Django and Boktai 3: Sabata's Counterattack). In all Boktai games, when the player is in a dungeon, when there is sunlight striking the sensor, beams of sunlight may come to the ground where the player can charge their energy. These openings are called skylights.
In Boktai: The Sun is in Your Hand, it was not possible for any bars to be on the solar gauge at night, even if a Sunny Clog or Mr. Rainnot was used. This feature was removed in Boktai 2: Solar Boy Django, and as such a player could do sunlight based activities during night time (e.g. Solar Forging). In Boktai 3, at night you can use the solar gauge just as you do during the day. Charging, Solarsmith, and any other activity is available at night, once the solar gauge is full. .
Aswell as charging energy and changing surroundings, the sun also has the potential to kill some forms of Undead. Since most undead enemies are of the dark element, they are harmed by the sun and as such, if they were brought into skylights or open areas of sunlight, they would be hurt and eventually defeated.
Collecting sunlight also contributes to the growth of the Solar Tree.
 Compatibility with Lunar Knights
In Lunar Knights, the gameplay is still affected by the sun. However since it is a Nintendo DS game, a solar sensor could not be fitted to the game cartridge. As such the state of the solar gauge on Lunar Knights is random, depending on the weather and time of day/month/year.
However, the geniuses at Konami managed to make the game compatible with all 3 Boktai games with the use of the GBA slot on the Nintendo DS. Using this, the player can go outside to play the game with the use of sunlight by using the sensor on any of the 3 Boktai games. The games also give a special item to the player depending on which cartridge is inserted.
To use this feature, the game must be turned off before inserting the Boktai game cartridge. The player should then start the game and resume the main game. From there on, the solar sensor can be used as long as the Boktai game cartridge is still inserted. However, the player should not remove the game cartridge to attempt to cheat the game; this will cause many technical problems and difficulties.
 How it works
Before I begin the walkthrough, the Solar Sensor deserves some explanation. The Solar Sensor is a tiny mechanical device in your Boktai cartridge that picks up UV radiation (such as from the sun) and reports what it's sensing to your game. This is measured on your screen on the Sun gauge, out of a possible 8 bars (in Boktai: The Sun is in Your Hand) or 10 bars (in Boktai 2: Solar Boy Django and Boktai 3: Sabata's Counterattack). When the sun is shining on your cartridge, the sun is shining in the game, which is important since, as the name implies, our Solar Boy protagonist needs sunlight to power his arsenal of magic.
The player has the standard HP and MP (called ENE in Boktai 2 and 3, but battery in Boktai: The Sun is in Your Hand) bars. The most immediate and obvious effect of the Solar Sensor is that, as long as the player is standing in sunlight, the ENE/battery bar will slowly refill. By holding down the A button, the player can perform a Solar Charge that accelerates this process at the expense of not being able to move. Although there are items that restore ENE/battery, for the most part, sunlight is the only reliable way to keep slinging those spells.
Energy also accumulates in the Solar Stations as long as you're getting sunlight. This acts as reserve energy that can be absorbed from Stations placed throughout the dungeons in a pinch, but perhaps more importantly, this energy can be transferred from the Station to the Solar Bank, where it becomes Soll, the currency of the Boktai world. Your Solar Bank account gains interest daily, even when you're not playing thanks to a battery-powered real-time clock also built into the cartridge. In Boktai 2 and 3, as the game goes on, shops will open where you can buy items, Protectors, and weapons using your hard-earned Soll, and of course sell unwanted items to earn more, and if need be, you can transfer your Soll back into the Solar Station, turning it back into energy.
Excessive sunlight is not, however, always a good thing. For one thing, most items in Boktai 2 and 3 are perishable, and more sunlight makes them perish faster. These will make them rotten. Rotten Items can still be eaten, but not without ill effect.
Although there are a few sources of artificial sunlight in the series, for the most part it is NOT possible to beat the game without it, and it's certainly not possible to get everything. However, there are a couple ways to fool the Solar Sensor. Most blacklights give off a fair bit of UV at close range, for example. I do NOT recommend playing the game this way, as it detracts from the feel quite a bit, not to mention holding the game too close may melt parts of your cartridge, GBA, or hands. But it may be the only option for people who work for most of the day or have similar problems keeping them from soaking up those rays. Black light room bulbs are another way of fooling the sensor, although this is MUCH more dangerous as it can severely damage your skin. Excessive UV radiation striking the skin can even cause forms of skin cancer (so the scientist). With room bulbs, it's not the game at risk; it's you. If you MUST use a room bulb, use it in VERY small doses.
 In-game Artificial Sun
Throughout the series there are three sources of in-game artificial sun.
- One is Sunny Clogs. When you use one of these, you will randomly summon either clouds or sun. If you summon clouds, your sunlight will be limited for the remainder of the day. Not good. However, summoning sun adds two bars to your sun meter for the entire day! Just save before you use one. The best source is an INVISIBLE brown chest in the Ruins. They are also randomly found in Azure Sky Tower.
- Second is Mr Rainnot. These aren't so good because you can't get any until the Solar Tree is already fully grown. What you have to do is get a Tonniar RM (the best source is a brown chest in Dark City (Boktai 2), or after you defeat The Count in Bloodrust Mansion (Boktai) and then stand under the Solar Tree at sunrise to purify it. This is just like a Sunny Clog, only not random, so in terms of USING them, they are better.
- Lastly, there's the Rising Sun spell/grenade. This adds FOUR bars of Sun to the meter, but only lasts 30 seconds and uses ALL of Django's energy.
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