I bought the below FTDI programmer for use with my 3v3 arduino pro mini. But even when the switch is set to 3v3 it is powering the board with 5v, (hence the IO voltage is also 5v). Note that when the pro mini is powered by batteries on the RAW input the IO voltage is 3v3 as expected. The problem is that when programming at 5v, any 3v3 peripherals (such as the BMP180) will be surely damaged.


Now on the eBay description it says:

This board ship default to 5V, but you can cut the default trace and add a solder jumper if you need to switch to 3.3V

Which trace needs to be cut, and where does the jumper need to connect to/from?

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1 Answer 1


The 5V/3.3V switch is fed into the "VCCIO" pin which sets the voltage of communication and nothing else. The 5V pin of the header is connected straight through to the USB port's 5V pin.

You can either take a separate 3.3V feed from the 3V3OUT pin of the FT232RL (limited to 50mA) which is easiest to take from the 3.3V side of the switch's pins (on the underside, either or both of the two pins with no visible trace connecting to them - the lowest two in your top picture), or you can re-wire the header so it takes its voltage from the output of that switch (the middle two pins). It looks like it would be a case of cutting the left hand portion of the long fat trace at the top edge of the back (on your top picture) then soldering a wire from the 5V pin on the header to the center pins of the switch. That should make the output power switchable between 5V and 3.3V.

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