The AB Electronics UK Knowledge Base provides support solutions, tutorials and troubleshooting guides.
The output from the IO expansion boards can drive up to 25mA per channel but to drive higher loads such as relays or some opto-isolators you will need to boost the output using a transistor or mosfets.
The circuit below shows how you can operate a 5V coil relay using either a GPIO output from the Raspberry Pi or using one of the output pins on the IO Pi and IO Pi Zero boards.
If you would like to connect an opto-isolator to the IO Pi, you can remove the relay and protection diode and connect a series resistor in line with the opto-isolator in place of the relay.
You can use our LED Calculator https://www.abelectronics.co.uk/tools/resistor-led-calc.aspx to find the correct series resistor to use with your opto-isolator.
32 channel I/O expander for the Raspberry Pi
16 channel I/O expander for the Raspberry Pi Zero