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Old 05-24-2011, 10:59 PM   #2 (permalink)
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With 12 to 14 volts on the MPGuino input pin you would see about 0.5 mA current draw. About 0.3mA will go through the 50K pull down resistor and most of the rest travels through the Zener diode to ground with the current limited by 50K input resistor. The current into the actual processor pin is only about 1 ľA.

Assuming you have the current pre built MPGuino that includes that pull down resistor the wiring can be very simple. You could simply connect the input pin to 12 volts through the read switch. I would add a 1K resistor or small value fuse where you connect to 12 volts to protect incase of shorts.

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