The D3100 unfortunately does not have the infrared remote option that was present on earlier models and restored on the D3200, which allowed a simple remote release to make long time exposures. But the timer/intervalometer shown in the above article looks relatively inexpensive, and should work, and adds some conveniences including the interval function, which is not present at all in the D3xxx family.
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I found this article on the net in which a D3100 was used with pretty interesting results:
(see here)
The D3100 unfortunately does not have the infrared remote option that was present on earlier models and restored on the D3200, which allowed a simple remote release to make long time exposures. But the timer/intervalometer shown in the above article looks relatively inexpensive, and should work, and adds some conveniences including the interval function, which is not present at all in the D3xxx family.