so circuit 2 is the way my mind intuitively wants to do it.
My calculated series resistors on the LEDs are 18ohm if I remember right.
can that be 1 resistor or do they each need their own?
As a rule of thumb if a resistor for an LED turns out to be lower than 33R (ohms) then you need some better way of limiting the current like a constant current supply. At these sort of loads you also need to consider the gain of the transistor and calculate the value of the base resistor.
I am having a hard time thinking how you can put one resistor in series with 7 outputs and 7 transistor bases. So you must not be understanding the concept of a series base resistor.
sending an IR signal, so pretty low duty cycle. Probably <1%
I am not sure why you think you will get a low duty cycle with data. IR has to be modulated normally at 38KHz but can be between 32 and 48KHz depending on the receiver. During the time of the pulses you will have a 50% duty cycle. You can't average it over a long period.
I see what you are saying about the transistors, yes you need a separate base resistor for each one. You calculate the base resistor by taking the current you want to switch and dividing it by the minimum gain of the transistor from the data sheet. This gives you the base current you need. When feeding the base from an arduino output you will get 5V out (for the purposes of this calculation) and the base voltage will be 0.7V when the transistor is on. Therefore you calculate the resistor to have this current flowing down it with 5 - 0.7 = 4.3V across it. You might want to make the base current a bit larger to be on the safe side. No harm will come by having this up to 10 times larger than it needs to be.If driving lots of transistors from the one output make sure the base currents they require does not exceed about 30mA, if it does use more than one output and split your transistors over these outputs.
(...) (my test setup used 4, with no base resistors, and it worked fine, but I only ran it a little while).
It'll probably actually be a pair of 1/2w 10R in parallel.
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