sensing AC line continuity/current?

I'm interested in sensing whether household lights are switched on or not, without modifying the switches on the lights. I'll be building a lot of these, so I want to find the cheapest/safest/simplest solution. I have a few ideas:

1. Simple homemade AC-DC power supply in parallel with light, drives relay or transistor switch for the arduino to read. Pros: I know it will work. Cons: Needs lots of parts. I need in the least a transformer, bridge rectifier, caps, voltage regulator maybe?

2. Wall wart driving relay/transistor in parallel with light. Pros: wall warts can be had surplus for ~$1 and are already built. Cons: they tend to hold a charge for a while, so my signaling will be a little slow.

3. small 120V lamp in parallel with household light, shining on photo resistor read by arduino. Pros: inexpensive, simple and Arduino is effectively opto-isolated from household power. Cons: probably needs analog inputs, code is probably more complex. AC lights tend to flicker, might be difficult to get a clean reading.

Are there other possibilities here? Can I actuate a relay with AC maybe? 120V opto-isolators?

Thanks.

KISS. :stuck_out_tongue:
Just use a LDR. Nice and simple.

I’m not sure why you’d use a second lamp. Whats wrong with putting the LDR inside the globe holder?