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PCF8575 IO Expander module schematic/manual?


These modules are quite common (see attached photo), but I can't find out two things - why there's 3.3V LDO on board, while IC itself can deal with 5V supply and what are these two transistors for?


Transistors are for conversions between levels - when your i2c using other voltage, vdo may be bypassed (2nd side on the board) or it can provide 3.3V to IC.when u power it up with higher voltage and want to drive 3.3V outputs.

Speaking simply, I want it to use for driving LEDs from 5V power supply, my MCU will also run from 5V. Do I need to re-solder anything?

There's better ways to be driving LEDs, like actual led driver chips.
With that port expander IC, you still need 1 resistor for each led to limit the current, (unless you plan to rely on the port expander's pin current limit). With a led driver chip, you can set the limit for all leds with one resistor.

There's led drivers that behave like shift registers, you basically just have to send 8 bits into the shift register and enable the driver to change the state of all leds at the same time to the new values.  You can also chain multiple such led driver chips or you can use drivers with more channels.

Here's an example of such basic "shift register" like LED driver IC  : https://www.digikey.com/en/products/detail/stmicroelectronics/STP08DP05MTR/1653915

Here's one with 16 channels, still "shift register" like driver, very easy to use (max 60mA per led) : https://www.digikey.com/en/products/detail/lumissil-microsystems/IS31FL3726A-ZLS4-TR/13281511

or a more fancy one which also allows you to adjust brightness of individual leds : https://www.digikey.com/en/products/detail/stmicroelectronics/LED1642GWPTR/4441170

I found one here: https://www.utmel.com/components/pcf8575-io-expander-pinout-schematic-and-datasheet?id=411


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