The R1 receives 5 volts from the arduino, but with a low current, so the BJT is used to amplify the current, but every example I have seen of this, does not have the R2 in that way, what is the R2 doing here?
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It's most likely a pull-down resistor to guarantee that the transistor remains off during the MCU's power-up. The value was chosen so as not to steal too much of the base current. I don't know that it's necessary, but having it won't hurt anything.