Efficient method for isolated detection of AC voltage without a stepdown transformer
Here is how I have dealt with AC line detection in an efficient manner using an isolated opto-coupler.  Key attributes of the circuit:


The selected opto-coupler is one with a low forward current requirement
The circuit works on either 120 or 240VAC systems
The KSP44 transistor is high voltage type
The transistor function is to cut off drive to the opto-coupler input diode when the AC voltage rises above a certain level thus limiting overall power dissipation
The circuit design ends up outputting a pulse at each zero crossing
The design shown was used in a commercial product that passed safety inspections by a government approved safety inspection agency.