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Adafruit 3.5 / 1.3mm or 3.8 / 1.1mm to 5.5 / 2.1mm DC Jack Adapter Cable [ADA2788]
2.1mm inner diameter (5.5mm outer diameter) is the most common type of DC plug, but you can use this adapter for anything else that has a 1.1 or 1.3mm ID (3.5-3.8mm OD) DC jack! The adapter has little springy parts inside so it can be used with 3.5mm to 3.8mm OD plugs.