Mike has the best solution; use a single power FET and control it with a PWM output from Arduino. And it (could) handle a lot more current than an LM317T.
Nothing against the LM317T. I once bought a barrel of 14,000 of them for $55 from a junkyard near IBM I’ll send 5 to anyone who’ll send a SASE… I still have about 500 left.
UPDATE: Sorry! I’m always criticizing Acronyms…
Self-Addressed-Stamped-Envelope (sent to me, will return 5 LM317t’s…)
152 Colby Road
West Topsham, VT 05086
But: 3 weeks till I get there from the Middle East…