Log FuelFlow -Reading 4 frequency inputs (digital on/off) and logging to SD card

Projects General Guidance
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And a question for dlloyd:

How do you use an Arduino Due for this when the flow meters work with 4.5-24 V and the max input on the Due is 3.3 V?

I purchased a Due which I'm trying to get working, but I have this voltage problem, you see...