Hall probe to measure current?

Using Arduino Project Guidance
#1

I want to use an arduino to tell when a mains-electric (220 volts) device is switched on/off.

Basically, I have a set of security lights in my yard that switch on/off automatically when something moves past them. I'd like to record when this happens. One option is to hack each light, and tap into the light-bulb circuit and note when 220 volts is present. This would require extra wiring, and involves playing with potentially lethal voltages.

The other option might be to place a hall-probe next to the supply cable and detect the magnetic field created when current starts to flow. Would this work? is it practical? Is there a better way? I've played with hall probes before, but the ones I've used only trigger when a pretty heavy magnetic field is present, would one detect the low magnetic field from a current-carrying cable?

Suggestions please

#2

How do those non-invasive current meters work?- the ones you clamp round the wire being tested?

Maybe hack one of those? Or better yet, maybe the components they use can be bought. Maybe they are Hall effect…

#3

A current transformer placed around one wire can be used to measure current. See http://openenergymonitor.org/emon/buildingblocks/ct-sensors-interface

Or use a light dependent resistor (LDR) near the lamp.

#4

How do those non-invasive current meters work?- the ones you clamp round the wire being tested?

I think they use hall probes - but they're quite expensive, I have 8 lights I'd like to monitor.

use a light dependent resistor

That's a possibility but would require running extra wiring out to each light.

The lights are currently all wired radially from a single point, so it would be a lot simpler if I can detect the current in each wire at the hub.

#5