capacitative coupling

I am seeking ideas, or thoughts, on how to make a cable connected touch pen invisible to the capacitive touch screen of a modern smartphone without mechanically lifting the pen (see illustration). This is for a two axis (no lifting!) planer lego robot I am building to teach machine learning, in...

I am constructing a filter unit that is intended to allow visualisation (on a scope) and measurement of high-frequency interference on 240v UK mains by means of a high-pass filter to remove the 50 Hz component. What would be the best (and safest) circuit to isolate the unit from the mains while...