Is there a wiring schematic for the pull-up resistors to trigger a relay from a esp8266. the pcf8574 wont put out voltage to trigger?

What kind of relay? Need to know voltage and current for the relay coil. Best bet is to use an fet to drive the relay. Still need to know your relay specs to answer completely.

Are these only digital I/O or also analog?

These are discrete only, no analog. Best to use them as active = low. I follow them with a 74LVC14A when I need active = high. Note the chips come in 2 address ranges. I have been receiving the low range with consistency.

Leds connected with vcc = 3.3v to board make them too dim. is this usual?

There are a couple of reasons this could happen. The pins on the PCF8574 can only sink 25 mA of current and, if I'm reading the datasheet correctly, can source much less. So you'll want to put the LED from Vcc to the output pin (so that low = on) and use at least a couple hundred ohms of series resistance.

Where should the pullup resistor be placed to trigger a relay from an mcu?

You can use any of the I/O pins of the Device. Do not exceed rated current!

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ASIN	B01ICN5JB6
Best Sellers Rank	#1,550 in Single Board Computers (Computers & Accessories)
Brand	HiLetgo
Compatible Devices	Arduino, Raspberry Pi, and other single-board computers with I2C interfaces
Connectivity Technology	I2C
Customer Reviews	4.0 4.0 out of 5 stars (60)
Manufacturer	HiLetgo
Mfr Part Number	3-01-1150
Model Name	3-01-1150
Model Number	3-01-1150
Operating System	Linux
Processor Brand	Broadcom
Processor Count	1

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HiLetgo 2pcs PCF8574 PCF8574T IO Expansion Board I/O Expander I2C Evaluation Develop Module

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