I want that my project detect 120V or 240V automatiquely. If I plug it on 120V a select automatic 120V or if I plug in 240V a select 240V.

I don't know how make that possible...
With a relay??? If I connect a 240V relay on 120V the coil buzz...

Suggestions?????
Thanks!!!!!!

 

autovolt power supply which detect voltge from 90vac to 260vac like comercial tv used......