Inductive sensor

Just wiring a relay in wont do it? As soon as the required voltage hits the relay coil it would let the load pass. Do you mean any current? Or a specific threshold?

 

Theres also this...
http://controlconsultantsinc.com/h8...WLQUrQQO31BuL5LRpn9s9h-FRF0pT1iBoCrSAQAvD_BwE

 

An auto electrician should be able to point you in the right direction for your particular make model ( whatever it is as not shown here). Also they will most likely be able to advise whether legal or not to do the modification in your particular country/ region whatever that also may be. Wiring of some negative return systems can make adding extra loads from the high beam a little difficult to say the least for the inexperienced.

 

DC or AC?

 

Only hall effect is useful in this case. Even then, unless you can make a coil of wire to sense from then a non contact solution isn't easy. What's the problem with using a sense resistor?

 

No reason why you can't tap into the harness. A resistor could sense the voltage and draw effectively zero current doing so - certainly so little that it won't affect other sensing circuitry. The transistor can then operate a relay.

 

My brother did this. He just wired the lights from the battery through a switch. Battery to switch to lights. No relay needed

 

As I said, the negative return switching will cause headaches unless you know what you are doing. The lights in the majority of these vehicles are constant line feed so even tapping can cause feedback. Without relays you will cook wiring and existing switching which were never designed for the additional loading. In some instances this extra loading can reach in excess of 16 amp. Not to mention voltage drop problems.

 

There are the ACS712 modules on ebay for a $1.00 select 5a, 20a, or 30a version.
If you want completely non-invasive types there is the Honeywell CDSA series that sense current level or just the presence of.
M.