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I have a sensor which gives an output of 3.9V when high and 0V when low. Perfect for the RPi, except is it?

What happens if I put the 3.9V high signal directly into the GPIO and if bad things will happen what can I do to bring it down to the right range?

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You will at the very least destroy the pin, and most likely the entire Pi. The Pi is not 5V tolerant. All of the GPIO pins are 3.3V tolerant only.

To bring the voltage down to the safe level you can use a level shifter or a voltage divider circuit.

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    Potential dividor it is then! – GPPK Apr 6 '16 at 11:33
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There is a big chance that you may loose your pi, As pi GPIO pins are 3.3V tolerant.I lost my raspberry Pi, happened with me. Well in your case you can also use 3.3V zener diode which is a voltage regulator.

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    Sorry to hear that. – PNDA Apr 6 '16 at 14:53
  • A Zener diode might not be the best bet and should not be used without a series resistor to limit the current (see here) – Ghanima Apr 6 '16 at 15:31
  • Thanks for the info :) Yes we must use resistor to limit the current. – shekhar Kumar Apr 7 '16 at 13:17

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