Walker Brains are used for crafting a couple of different components in No Man’s Sky. Most players will need it for the conflict scanner for their ship(s), but they can also be used to make explosive drones for freighter missions.
It’s not a bad idea to stock up on a few of those in case that you need them in the future (another conflict scanner for another ship).
To get them, simply go to any planet with sentinels and start attacking them. Make sure you’ve got some decent weapon like maybe plasma launcher, and just keep killing them until sentinel walkers show up. Once you destroy those, simply loot their combat supplies and you’ll get the walker brain.
You can also sometimes get walker brains from the scrap dealer NPC on space stations when buying stuff they sell.
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