Light switch sensing

[George]

Thanks, tho I'd like to build myself, just for the experience if nothing else! I'm thinking of setting up an oscillator which will send a tone down the line at every zero crossing, however I'm not sure on how to receive the tone on the other end due to mains potential. So basically I have a triac powering - just before I fire the triac I'll be listening for the tone on the output of the lamp, if the switch on the lamp is turned off, no tone should come back.

Does anyone have any ideas how I can isolate that nasty mains potential either side of my the lamp circuit (bear in mind also that there will be rapid voltage transitions when in the middle of dimming.

Thanks