Non invasive current monitor

[Pensioner]

Hi everyone
I imagine what I am after will be very simple to those with a good knowledge of electronics, mine is very sketchy. I want to monitor a 12 volt DC circuit (car) that will have a permanent voltage on it, when the circuit is carrying a current, which can be any value between 2 and 15 amps I want to detect that current without changing the circuit impedance and operate a relay or SSR. I'm not looking for an off the shelf answer, I want to build it just for the hell of it.

Regards Pensioner

 

[Minder]

Several non-invasive transducers available, Honeywell make a wide range, there is also the ACS712, available on its own or on a board on ebay for a couple of $$.
M.

 

[(*steve*)]

As a side note, you can't measure anything without having an effect on it. (Heisenberg was right). Even these ACS712 chips add some series resistance and inductance. Practically speaking though, there would be very few cases where it could be noticed.