Typical opto has several kV isolation voltage (which is not always needed), has relatively big cases (TLP291 in 4-SOIC is one of the smallest as I know) and typical price up to 1 USD in small amounts (not cheap in my case).
In my case I'd be satisfied if I'd be able to transmit a signal with as low as 350VDC (250Vrms) isolation. There are no safety reasons (I just need to transmit the signal from neutral to line).
At the receiving side the signal should be something similar to a switch (like collector-emitter of the opto): "on" or "off". This signal will drive MOSFET driver. The speed is pretty low (1 ms would be enough).
Capacitor can be cheap and small solution, but it will not work on DC.
So if I don't need such big isolation voltage is there any alternative to optoisolator?