# circuit for detecting 12v dc presence to trigger a relay

Hi there:
I do not know what I am doing. Is they any simple way to detect 12v dc current presence in a wire ( induction ) without interfering with that circuit what so ever to trigger a relay. In many of the newer cars, there is data transmitted in the circuit, so changing resistence, etc is a problem. Just want to see if I can add secondary lights to my high beam circuit. So when the cars existing hi beams come on I can trigger some secondary lights to come on without splicing into that circuit. Any help would be much appreciated.

Lee

No its dc.

Why not use a photodetector to detect the light coming on ?

well the low beams would trigger it also. I should explain that it is a projector beam and both low and high beam come from the same bulb

Perhaps wind the insulated wire leading to your circuit around a reed switch.

They operate on magnetic flux.

How many turns you require would depend on the load current.

I’d think you could use a resistor divider circuit of sufficiently high values to to detect this without messing with the signal to the light to any appreciable degree. Is the problem here more one of not wanting to cut into existing wiring or something else?

Keep in mind that the electrical environment in cars is pretty messy and noisy, complete with pretty nasty negative voltage spikes, plus 12 volts is only nominally what they run at.

I should explain that it is a projector beam and both low and high beam come from the same bulb

If the projector runs off AC you can make a short extension cord that breaks out the hot lead and use one of these to measure the ac current with the bulb OFF
and then again with it ON and use the AC voltage to calculate the difference in the power and then recalculate for 12V to get the dc current.

no , I have no problem digging into the harness. My understanding is that with this particular truck (2013 dodge ram) are now using a network , and also, you cannot even put a brighter bulb as the ECM will detect a difference in resitance and give you a warning. So , as I am told by several different sources, I cannot change anything in the circuit. As for the mention of a resistor differnce, can you explain more in layman terms. I am also looking into the reed switch as someone mentioned. Thanks for all you input.
This is and will only get worse as the cars andvance become more of an issue. Surprised no one has marketed something for this problem.

In that case there should be a diagnostic port for the network.
Hacking the code may be an option

There are after market devices which use that for diagnostic purposes.

Beware its an instructable.

raschemmel:
If the projector runs off AC you can make a short extension cord that breaks out the hot lead and use one of these to measure the ac current with the bulb OFF
and then again with it ON and use the AC voltage to calculate the difference in the power and then recalculate for 12V to get the dc current.

By projector I think he means hi beam and dip which are dc I think

If HID it could work though

A resistor divider circuit consists of two resistors connected in series, end to end. The input voltage is connected to one end of the pair and the other end goes to ground, normally. You tap the connection between the two resistors. The voltage you get will be in proportion to the values of the two resistors. For example, use two 10k ohm resistors and your tap will get half the input voltage, use 10k and 90k ohm resistors and you'll get 90% of the original signal. Add up the values for the two resistors, divide that by the voltage, and that's how much current you'll drain off the original circuit. For example 12/(10k +10k) = less than a milliamp, negligible for a circuit powering headlamps.

What you'll want to do is select resistor values that will produce a voltage high enough to trigger the input line, but not so high that it will damage it.

Also, consider using a diode to protect your circuits from those nasty reverse-voltage spikes I mentioned. These may or may not be a problem in newer cars but they certainly were in my 69 Opel. The field collapse associated with any motors in the car, such as a fan or starter, even the started solenoid, when they are turned off, are to blame. This blew out all of the 12v LEDs on my instrument panel.

Is they any simple way to detect 12v dc current presence in a wire

Maybe what the OP is really asking is not how to measure the current, but simply how to detect the 12V on the load. If this was all he wanted to do I would have expected him to ask how to detect voltage instead of how to detect current. Detecting current without measuring it is the same as measuring the voltage, which is as simple as the explanation in the previous post. Detecting current using induction is quite a bit more involved.

Many late model vehicles use earth return switching and power the complete headlight system continually so detecting the voltage may not be an option.

There are ways to add the extra lighting and I can sketch you out a circuit if you like for those systems that you can try.

You didn't like the reed switch idea then?

I do not see the argument about extra resistance in the circuit fowling up the system, I think it's a bit of bull they fed you actually.

If all you want to do is turn on another lamp, why use an Arduino? Seems to be an overkill.

As has just been mentioned many use earth switching. Simply use this to turn on a transistor or MOSFET to operate the main high current relay.

Weedpharma

Looking at the op post I still think he does not want to make a connection to the circuit.

Is is a modification which can invalidate his insurance which may be why.

I can relate a first hand experience where a badly fitted trailer coupling caused a vehicle fire.

The insurers refused to pay out.

Thank you all so much for the info. I think I will personally splice into the wire and trigger a relay and see what actually happens to the system. If it fails or gives me a warning. If so, then will pursue one or more of these options. I will let you know the result. I am just looking to detect voltage. Sorry for my lack of knowledge. And the idea of only bleeding off 1 millamp for a trigger sounds good. I will do more research, and keep you posted.

Lee

leeswreackcreation:
no , I have no problem digging into the harness. My understanding is that with this particular truck (2013 dodge ram) are now using a network , and also, you cannot even put a brighter bulb as the ECM will detect a difference in resitance and give you a warning. So , as I am told by several different sources, I cannot change anything in the circuit. As for the mention of a resistor differnce, can you explain more in layman terms. I am also looking into the reed switch as someone mentioned. Thanks for all you input.
This is and will only get worse as the cars andvance become more of an issue. Surprised no one has marketed something for this problem.

It appears that the data circuit loading is the issue. Any data used can only be used to turn on a relay to actually carry the large current. If you connect into the circuit on the switched side you have either a Gnd or 12v applied. Either side needs to have a voltage divider as the voltage of either will be 12v at some time.

Remember that 12v is nominal and will get over 14v.

Weedpharma