My washing machine is equipped with a capacitive start button. A "button" is marked with paint on the plastic front and on the backside of the panel there is a spring extruding from the control PCB, pressing against the inside of the plastic panel. I would like to trigger this using an Arduino. While searching for answers I've been trying to use a metal foil, grounded through a relay to trigger the button, but inconsistent results. Could I connect directly to the spring, in any configuration, to trigger this button in a simple way?
I believe it is worth trying to connect directly to the spring: you may get more consistent triggering (I would expect it, but no guarantee).
I'd use a small ceramic capacitor instead of a direct connection though, to protect the capacitive sensor from DC and 50 Hz AC currents which may appear in your circuit. It should be anywhere in 1-100 nF range, one terminal connected to the spring, the other one to your relay. I actually expect such capacitor to be present on the PCB in series with the spring, but extra protection can't hurt.