Have the problem to use the I2C-Adpter without Display don't work. I think the ports are open collector and can drive a 10mA LED. I can readout the adress with I2C-Scanner (27) but I cannot drive the port. Someone know if there is a special Adapter? I thought that is a normal PCF8574. :o
The PCF874 has no output drive capability. Well, it can drive 300 ua but that isn't very much current.
It can sink at least 10ma and typical is 25 ma.
What are you trying/wanting to do?
If you want to use a PCF8574 LCD backpack with a hd44780 display, I would recommend installing the hd44780 library and use the hd44780_I2Cexp i/o class.
First read the documentation, then run the included I2CexpDiag sketch which will report if everything is working.
--- bill
Yes I know this Kathode LED to Port0 at PIN7 and Anode LED over 330R to +5V. This works when I take a "normal" PCF8574 but not with this Modul. I think I must buy the portexpander Modul at same price but I would interest the reason why that don't work because I have four HD44780serialmodules and don't need it. 
I looked at the photo.
You are not using an HD44780 LCD but rather are attempting to use the backpack as an i/o expander.
The pcf8574 works as it should.
It isn't working because you wired up the LEDs to header pins that are not connected to anything.
The pcf8574 only has 8 i/o pins so it must run the hd44780 lcd in 4 bit mode.
hd44780 pins db0 to db3 are not used in 4 bit mode.
You have used db2 and db3 which are not connected to anything.
The pcf8574 on hd4480 backpacks is wired up and controls
rs,rw,en,backlight transistor, db4, db5, db6, db7
If you look closely you should be able to tell which pcf8574 Pn pin is wired to which header pin.
If not, use your ohm meter to track them down.
--- bill
Ooops.
Sorry it looks like the leds are attached to the header pins going to hd44780 pins DB0 and DB1 and not DB2 and DB3.
Still, those header pins are not connected the PCF8574.
--- bill
Oh yes I now I have tested directly at SMD-IC and then works, the pins are not connected that's right. Many thanks to bill(bperrybap). :o
thmoe:
Oh yes I now I have tested directly at SMD-IC and then works, the pins are not connected that's right.
There is/was no need to test them at the IC.
All eight i/o pins on the pcf8574 can control pins on the 16 pin header.
7 of them go to header pins directly and one of them goes through a transistor for higher drive capability.
--- bill