switch on
1. [for something] to turn itself on. Exactly at midnight, the lights switched on. The radio switched on early in the morning to wake us up.
2. Sl. [for someone] to become alert or excited. The wild music made all the kids switch on and start to dance. About midnight, Ed switches on and becomes a real devil.
switch something on
to close an electrical circuit that causes something to start functioning or operating. Please switch the fan on. I switched on the fan.