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I have a slight issue with the circuit, in that an arduino can only source 40mA max. I have measured several laser diodes, and the lowest current required for lasing by any of them was 60mA. Might I suggest a single transistor switch be used for each laser. I haven't completed my unit yet (still waiting for the 3d printer to arrive). My own idea would simply be to connect a 10k resistor to the arduino output, from there to the base of a 2N3904, collector to 5V (separate 7805 supply), emitter to the positive of the laser, and negative grounded. Once I have it up and running I'll post my findings.