The Big Bell is a collectible Item in Yomawari: Night Alone. It is only accessible in a random event with the Monster Cat.
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It is only accessible during the random event with the Monster Cat. When this random event is active, the Protagonist will find a lone white cat with brown and black spots guarding the Big Bell in the northwest Vacant Lot with the stereo. If the Protagonist gets too close with the Big Bell, the cat will transform into a Monster Cat and attempt to kill the girl before she can get the Item.
It is fast, so if you use normal means, it's normally too difficult to get the Big Bell with the Monster Cat trying to kill you. The only way to get the Big Bell is by distracting the Spirit with an Animal Feed while its still a cat or with a Doll while its in the Monster Cat form. However the time it is distracted with the Animal Feed is much shorter than with the Doll and it's probably not enough to get out alive.
Once you obtain the Big Bell, the Monster Cat will disappear.