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  • That MOSFET is 4.5v logic level. The Espruino is a 3.3v device. You can't fully turn it on with 3.3v

    That's an N-channel MOSFET. Yet you say you're using it to switch the Vcc side. In order to turn it on, you would need to apply a voltage 4.5v above the source voltage to the gate - meaning you'd somehow need to get and control 7.8v+ just to control the mosfet.

    You need a P-channel logic level MOSFET if you want to switch the positive side, and you need it to be rated for Vgs < 3.3, otherwise you can't turn it on.
    Alternately, you can use an N-channel logic level MOSFET, and switch ground side, but you still want one with RDSon spec'ed for Vgs under 3.3v

    Actually, I think you may have to use N-channel fet and switch low side if it's I2C, because if you switched the high side, it would draw current through the pullups - probably not enough to work, but enough to waste power.


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