Im replacing the buttons on a Bluetooth speaker so that it can be controlled by Arduino.
I've been able to work out that a direct short to ground from one of the pads will effectively activate the button.
I've tried using the analog write function and some N channel transistors with some limited success, however, the Bluetooth control is very voltage sensitive and can easily cause the controller to reset or bounce quite a lot.
Does anyone else have some other suggestions to try?
That implies you want to drive the opto LED with it's absolute max current, which is bad.
A 1k resistor in series with the opto LED is more than enough drive current to switch the current of a button with the opto transistor. Make sure the opto transistor is connected with the right polarity.