Disclosed are compounds of Formula or a salt thereof, wherein \u201cHet\u201d, Ra, and Rb are defined herein, which have properties for blocking Nav 1. It comprises also input signal means for bringing an electric input signal to the keypad, and signal conversion means for responding to a key activation by selectively converting the input signal into an electric output signal the value of which depends on the activated key. The waveguide includes the counter-propagating optical signals or some characteristic of the signals modified or effected by an external parameter caused by or indicative of the event to provide modified counter-propagating optical signals.