First, thanks for your patience as this is very new to me - and probably a very simple question but I did not know where else to turn. I'm trying to use an I2C I/O expander (PCF8574) to run some relays, but currently just testing w/ LEDs. I have the I2C working fine, and the chip seems to be working correctly, except that the LED is extremely dim, and not lighting up much at all. I had an old RadioShack (I know, it's cheap and lame) meter I was using, but it has stopped functioning so I cant immediately test my volts/amps - but I'm assuming something along the lines of an amps problem? This is confusing me because the documentation clearly says it should be able to drive LEDs directly:http://www.nxp.com/pip/PCF8574.html
I simply have the chip setup to the I2C on the arduino, and the output pins on the I/O chip directly tied to some LEDs (actually just one right now), and then to ground. This is how I know the chip seems to be working fine - as I can barely see the LED lighting up when it is supposed to. I have tried tying multiple I/O lines into the LED - and it does glow a little brighter - but nowhere near full brightness, and nothing compared to my debug LED on arduino pin 13 and ground.
So, what am I missing? I'm betting it is some basic electrical circuitry knowledge, that I'm hoping someone here can help me fill.