I want an I/O pin to return current in a circuit, similarly to the function of ground.
Which of these will achieve what I require?
pinMode(pinnumber, OUTPUT) digitalWrite(pinnumber, LOW)
Your first snippet is the correct solution.
Be careful though, you can only sink up to 6mA or 9mA per pin (as per the documentation ["Input and Output" section], I never tried more). If you need more current, use a transistor (this looks like a good example, you just need to invert the output pin to
When the pin is configured as input it is supposed to be high impedance, not what you want.