3V-5V Switchable I2C Real Time Clock Shield proven with Arduino Due

Other Hardware Hardware Development
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Using the Maxim/Dallas 1307 RTC that runs at 5V with the Arduino Due that runs at 3V. FETs are used for bidirectional level shifter per a Philips app note. This shield has been tested on the Arduino Due (3V), Arduino Duemilanove (5V), and Arduino Leonardo (5V).

More details including schematics at http://techwithdave.blogspot.com/2013/09/3v-5v-switchable-i2c-real-time-clock.html

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For the next version, you could get a rev3 type protosheild, you could remove the voltage selection jumper. The rev3 boards have an IOREF pin on the header specifically for this purpose.