Detect presence of 230v - not measure

[retrolefty]

polymorph:
Using an optocoupler requires either a very large resistor and bridge rectifier, or a small transformer (to step down the voltage) and the optocoupler.

Actually not. With this circuit driving an interrupt, the mains cycles can be counted. A 1/4W type for the 1K resistor is more than adequate.

That is a valid design but it really should utilize a cap that is designed and rated for direct mains connection, not just that it's working voltage is high enough, but that if it does fail it fails in a safe manner.