i need an alternative to my pcf8574. it seems the pcf8574 cant supply enough current to fully turn on my mosfet "irf540"
aside from using transistor to turn on mosfet i would like to know is there any gpio expander that can supply more current than the pcf8574?
Hi.
Haven't got a direct answer which expander meets your requirements, but have a look at the following link below. It list all kinds of I/O expanders, buffers and switches. You can use that to find datasheets of expanders
What you need to turn on the IRF540 completely, is voltage instead of current.
The IRF540 gate requires a voltage of 10V or higher.
Consider changing to a logic level FET instead of changing the I/O expander.
As long as your project is running at 5V (or 3.3V?), you never will not be able to turn on the IRF540 completely, no matter what I/O expander you have with its powerful drive current capabilities.
EDIT:
If you use switching frequencies above about 10kHz, the gate drive current is also important.
However, gate voltage is important regardless of switching speed.
i tried to use this mosfet board,
looks like it using lr7843 as mosfet and pc817 as opto isolator. There appears to be a 221 ohm resistor between the input signal and opto isolator, there is a 1k ohm between mosfet and opto isolator with a looks like a 10k ohm between two legs of the mosfet.
When i signal the pcf8574 to turn on the mosfet with a load on the mosfet it apears to turn on according to the led on output side of mosfet. when i connect a 1 amp 12v load to the mosfet the output led flash dimly 1 time and remains off as if the mosfet tries to turn on but don't
On my benchtop psu. it appears to require atleast 3.5-3.9v @ 0.009 amps to turn on fully if i try to run is less than 3.3v with a 12v load the mosfet explodes
If you are using that board, why did you write "IRF540" in first post? 
The driver board with an optocoupler, you need enough current to operate the optocoupler.
But, the PCF8574 can be able to output enough current to light the LED built into the optocoupler.
In other words, there is no problem with the choose of components, and the cause of malfunction is considered to be elsewhere.
What voltage is the VCC of PCF8574?
So first you have an IRF540 mosfet you're trying to control, then all of a sudden its a mosfet board with entirely different components - and it's not a mosfet in the first place you're trying to control, but an optocoupler.
Let's get this out of the way: please show a full (and accurate!!) schematic of your project.
In principle a PCF8574 should be capable of supplying the current the optocoupler would draw if it does indeed operate at 3.3V.. Your '221' resistor is 220 Ohms (22 * 10^1...not 221 Ohms...), at 3.3V this gives (3.3V - 1.2V forward voltage of led = 1.2V) / 220 = ca. 5mA. No problem for the PCF8574, and it still wouldn't be a problem if you run it at 5V.
However, like I said, show a full schematic as there's likely another problem somewhere. Possibly/probably it has to do with how you supply power to the mosfet board. LR7843 is not a logic level mosfet so there's a concern there.
One more illustration of why I'm not a huge fan of these mosfet boards btw. A single logic-level mosfet would have worked just fine in this case, with much less possibilities for confusion and error...
sorry i have both baords and the both operate the same way it seems.
i plainly connect benchtop psu 12v output to mosfet board Vin, gnd to gnd. second channel of benchtop psu set at 4v connect to optocoupler input. all channels grounds connected. then output from mosfet into programmable digital load set to 1.25 amps.
that configuration works requiring .009 amps @4v to operate the load.
or,
benchtop psu 12v output to mosfet board Vin, gnd to gnd. arduino and pcf8574 gnd connected to gnd from psu "all on same ground bus" pcf8574 output to optocoupler. mosfet output to 12dc 1 amp solenoid. in this configuration the mosfet output led does not stay illuminated it flashes and goes out quickly. if i remove the solenoid it stay illuminated.
not sure why im get this behavior
and sorry for confusion about irf540. when you said it require much more voltage than pcf8574 can supply i tried a different board. but neither of these are working. it seems the same effect on either
(3.3V - 1.2V) / 220Ω = 2.1V / 220Ω ≒ 10mA
if i bypass the optocoupler the mosfet will turn on. even the solenoid will operate if i just use the benchtop psu to drive input signal at 4v 0.009 amps
Sorry, Monday morning & insufficiently caffeinated 
Argument still stands though.
Please provide the complete schematic, even handwritten is well.
Also, what is the voltage of the signal input pin of the board when outputting HIGH from PCF8574?
And again, what voltage is the VCC of PCF8574?
Of course.
Like I said, and @chrisknightley as well: post schematic.
what kind of schematic should i post. its just Vin to Vin gnd to gnd and input signal. all gnds from everything shar same bus. input signal directly from pcf8574. i didn't build the mosfet board its just a prefab board.
Voltages, im showing 5.7v floating from pcf8574 without connecting to optocoupler input. when connected to optocoupler input im showing 2.7v from pcf8574.
@anon35827816
I noticed the problem.
This board doesn't have a VCC terminal on the input signal side, and the optcoupler Anode as the direct use to input, probably requesting source current from the PCF8574.
The PCF8574 can only drive powerfully with sink current, but bit poor source current.
Perhaps there is a problem with the circuit.
so i need to supply power to the optocoupler from Vcc 12v source?
Yeah, looks like it'll source 4mA max. Recommended only 1mA.
Again, this is why I'm not a fan of these mosfet boards. Had OP used a single, bare, logic level mosfet, it would have worked already...
You could drive a small logic level mosfet (although even a 2n7000 or so would do) from the PCF8574 and then have the small mosfet drive the larger mosfet board.
Or like I said: just get an appropriate mosfet and forget about that stupid mosfet board. It's a silly contraption.
First, check whether there is continuity between the terminals of GND1 to GND4 on the board with the DMM.
If there is no continuity and the terminals from GND1 to GND4 are independent, try the following procedure.
Connect all PWMn terminals on the board to 5V (that is VCC of PCF8574), and connect GND1 to 4 to the output pins of PCF8574.
Then, the logic of the output signal is inverted.
@anon35827816
Please review the above process. 
Uhm, to be honest, I'm done with trying to get this board to work 
If this were my project I'd be soldering the mosfets from the board right now (also the screw terminals, they might come in handy at some point) and chuck the pcb into the bin.
i dont feel like soldering together a 16 output mosfet board. which is why i bought these. figured they would be nice considering they are optically isolated.