I have been reading many such articles and have found no way to simply detect flow through a mains wire at low current.

At .27A for one LightBulb, I was calculating .05mA out of commercial 1:2000 inductance sensors, and no IC Hall Effect sensors are rated for less than 1A.

I only want On/Off Resolution to feed into a transistor and trigger a relay, so I consider my options to be:

  1. Use a drill to wind a ~4000turn coil with very thin resin coated copper, wrap it around the wire and pray for o.1mA that I can amplify (Darlingtons? Is this possible? Is it stable?)

  2. A Hall Transistor, also with lots and lots of gain?

  3. With Capacitance Coupling "make the probe itself as a small coil and amplify the voltage dropped (not induced)" These will share a common Ground. (Is there some freaky way to make a magic resonant coil around the wire... yes i'm looking at you Tesla.) This seems like I am LOSING power to the mains, but at least I can sense it.

Anyway, if I get this working I will share share share cos there are a lot of frustrated forum posts around but no damn answers.

Please comment / recommend / crush my delusions of grandeur so I don't have to waste all my time failing around.


1 Answer 1


Sounds like you need something called a current transformer. Being a transformer, it also provides isolation between the line circuit and whatever is doing the sensing, which is a good idea.

  • 4
    \$\begingroup\$ You can increase the sensitivity of the transformer by passing the primary wire through the core more than once. \$\endgroup\$
    – Dave Tweed
    Commented Jan 14, 2013 at 2:37

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