i am developing application in which i have to detect if smarphone is inserted into narrow(1cm/0.4inch) slot. So far i was using a switch whit long lever, that was pushed if the phone was inserted. But now i want to make the thing more "high end" and avoid scratching the phone whit switch, so i am thinking of adding an capacitance or inductive sensing idea, to detect phone or its battery field. I was thinking that if i would design a pcb whit a coil printed on it, whit a small ferro element in the middle of coil, could that detect phone? In theory the coil should detect the battery`s field, right?
I cant use any optical transmitter and detector since i have room only on one side of the slot to install electronics.
In the attached picture is a drawing of the slot where smartphone is inserted. There is no metal or anything beetwen sensing pcb and the phone, only thin plastic layer.
Thanks in advance for any help.