"LiquidCrystal_I2C lcd(0x3F, 2, 1, 0, 4, 5, 6, 7, 3, POSITIVE)"
interpreted as [( 7bit_addr, EN, RW, RS, D4, D5, D6, D7, Backlight, POLARITY) ]
the PCF8574 datasheet indicates the 7bit addr range is 0x20--0x27
0x3f is not valid afaik.
default with a0:2 open = 7bit_addr of 0x27 , pbp addr would be $4e
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