Hello Tony,
A few comments:
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If this is among your first projects, use a transistor. As a rule, they are less fussy. It’s been my experience that transistors offer a shorter path to success.
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Protect your Arduino. The Arduino has a maximum current per single I/O pin. It also has a maximum current for a group of pins. Since you have 6 drivers, you run the risk of damaging the common I/O such as the internal bond wire for the ground pin.
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Putting ideas 1 and 2 together, a Darlington pair may be appropriate. As a starting point, consider the MPSA29. You correctly mentioned “wiggle room.” I agree, but like to have a bit more headroom especially for parts that may be reused in other projects. With that said, also consider the BD679AS and the TIP122G. You could also use surface mount devices such as the MJD122. After you have the prototype working, you can select a low-cost components optimized to the task at hand.
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My understanding of your circuit is presented below. Please let me know if this is correct.
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Your comment about power is a classic mistake. But it’s a good mistake — a rite of passage — because it shows that you are thinking about the topic. This is a thing to celebrate! Know that the power is given up to the LEDs, not the transistor. In this example, the transistor power is very low. It is calculated as P = IE where I is approximately 0.25 A and E is in the 0.2 range for a transistor in forced beta. At full power the LEDs consume 6 W while the transistor is at 50 mW. Here “full power” assumes a 24 VDC source and the absence of R2.
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Know that I’m making some assumptions that may be incorrect with regards to R2. I don’t know if your LEDs have and an integral current limiting resistor. As presented, R2 is the high-power device and should have a rating of at least 1/2 W.
Let me know if this is helpful or if you have any additional questions.
Best Wishes.
APDahlen
P.S. Be sure to take a look at this article regarding forced Beta: