Detecting change ic AC current

[MikehZA]

Detecting change in AC current

Hi, I have a PIR motion detector mounted in the garden. At night when it detects movement it switches on a 500W (220V) flood light hopefully to deter intruders. I have no way of knowing when the light is activated so I thought if I could detect the change in current I could make a circuit that would sound a small buzzer inside the house. Is it possible/practicle to wrap a few turns of wire round the supply to the PIR to get a voltage/current that could be useable for this?

 

[FvM]

You need a current transformer and it would sense one wire (either live or neutral) of the supply cable. If you wrap a wire around the cable, or one of both wires, the voltage induced by the magnetical field of the current just cancels out. There are different coil geometries, that would sense the current, but it's much easier using a core, making up a current transformer.

You also can buy ready-made multiple outlet strips, that have an inbuilt current relay, switching several slave outlets from a master socket. They are used e.g. for audio or TV home equipment.

 

[MikehZA]

Thanks FvM, so without some sort of core there will not be any induced voltage.

 

[FvM]

The magnetic field lines form circles around a conductor. To measure an induced voltage, the windings must span an area perdendicular to the field. A rogowski coil would be suitable to sense the current (or more exactly it's derivative dI/dt). A simple coil besides the conductor can also sense the field, but will be succeptible to interfering fields.
Rogowski coil - Wikipedia, the free encyclopedia