Use an arduino as i2c expander

Do you think the PCF8574 on the back of an lcd screen could be repurposed as a general i/o expander. After all that's what it is right? I have quite a few 16X4 LCD screens and they are all 4wire PCF8574

Of course! If you can get it loose. :face_with_raised_eyebrow:

They are (or used to be) dirt cheap, much cheaper than the module illustrated in #18.

Only one limitation is that while 7 pins connect directly to the chip, the eighth is buffered by a transistor (open-collector) to switch the backlight and depending on the board it may be PNP or NPN. This may be particularly useful or a problem, depending on what you need to connect it to. :grin:

Oh yes - some of the very oldest ones did not have the address select pads IIRC. :roll_eyes:

It is hard tp get cheaper yhan a dollar.

As for the LCD daughter cards no need to remove

Perhaps I have not looked for a couple of years but in the past the general-purpose expander was about twice as expensive as the "backpack".

OK, a bit of research suggests that the difference may be less, mostly due to the substantial increase in "shipping costs" which mean that **neither** can actually be obtained for anything approaching a dollar. :face_with_raised_eyebrow:

To my knowledge, they will only give 7 IO pins directly on the header, not 8. Just to be aware of :wink:

Ebay as the board for $1.50 free shipping.

Interesting. I am wondering what search terms you used as "PCF8574" finds nothing under US$2.10 including shipping.

As explained in #22. :sunglasses:

$1.50 USD

$1.37 USD

it may be country ?

for the backpack;

why bother with all the pins if it only has 8 useful outputs ?

You are correct. Neither of these items ships to Australia, so they cannot appear in my listings.

Which is of course, somewhat curious as they are all coming direct from China. :face_with_raised_eyebrow:

Is this another subtle aspect of the the trade embargo they have placed on us because we (our politicians; Scomo) publicly mentioned the Uyghurs?

I havnt got a pinout of the piggyback pcf from the lcd but I do get 8 controlable outputs but they don't all relate to the pins labeled D1-8 on the LCD


Indeed. In some combination (depending on the backpack version) they correspond to pins 4,5,6,11 to 14 and a transistor controls either 15 or 16.