Hi everyone,


I’m working on an automotive project where I need to detect ignition and seatbelt signals using a tracker device, but I want to avoid cutting or disturbing the original wiring.


I was exploring the idea of building a non-contact voltage detector using a transistor-based high impedance circuit (similar to how AC voltage tester pens work). The idea is to place a sensing wire near the target cable and detect whether voltage is present without physically connecting to it.


However, I’m unsure if this approach is practical for automotive 12V DC systems.


My questions are:

1. Is it feasible to detect 12V DC in a car wire using a non-contact method (capacitive or electric field sensing)?
2. Can a simple transistor-based circuit (high impedance input) reliably detect such signals?
3. Has anyone successfully implemented a non-contact DC voltage detector for automotive use?
4. Would noise and shielding in vehicle wiring make this approach unreliable?
5. Should I instead use a direct but non-destructive method like back-probing or an optocoupler-based voltage sensing circuit?

If possible, I would really appreciate if someone could share a practical schematic or suggest a robust approach for this use case.


Thanks in advance!

 

Sensing ignition system pulsing by capacitive coupling to a wire wrapped around the swiitched ignition coil primary wire should be easy, although it may require a bit of amplification. Sensing the status of the seatbelt switch wil be more complex, and probably require "back probing" one of the wires to the seatbelt-in-use switch. In all of the cars that I have worked on, back-probing the electrical connectors has been rather simple and easy, with adding tape over to keep the probe wire in place.

 

require "back probing"

What is "back probing"?

 

Back probing is forcing a skinned section of wire into the part of the connector where the wire to thepin enters the plastic housing. It unfortunately does not work with molded on connectors.
In most cases it is important to wrap some tape around the wires behind the connector to keep the inserted wire from coming loose.

 

Hall effect sensors come to mind.

 

Many of the wires that would be monitored do not carry much current, and so any magnetic field will be rather small. AND hall sensors require amplification because their output is low. AND they require external power. I would sayHalleffect sensors are a poor choice for MOST current sensing applications. Not ALL , but most.

 

A wire wrapped a few times around a spark plug wire is a common and simple approach as noted by @MisterBill2 . I have one on my boat engine, and it turns on an hour meter, so I have an accurate measure of engine on time.

What exactly do you want to measure?

 

A wire wrapped a few times around a spark plug wire is a common and simple approach as noted by @MisterBill2 . I have one on my boat engine, and it turns on an hour meter, so I have an accurate measure of engine on time.

What exactly do you want to measure?

I was not thinking aboutsparkplug wires. Consider that if the installation was pperfect, still, loss of the spark plug loading would put a lot of voltage capacitivly coupled into that wrapped wire. For those folks who don't believe that, disconnect one sparl plug and grab the wire well away from the open end. Grab the wire while the engine is running, of course.
Certainly capacitors block the DC voltages, but that fast rie-time pulse couples a whole lot of enery thru the capacitance. In many 12 volt wires in the car there is plenty of lower level noise to operate a simple sensor circuit.

 

We (or the TS, anyway) needs to distinguish between sensing that the ignition is on (key turned to the ON position, but engine not running), and detecting that the engine is running.

There are numerous IDC connectors intended to tap into an existing wire. I used a bunch of them when I added things to my old Bronco. At first I was concerned that the need to nick the wire would cause them to break, but I well over a dozen (possibly two dozen) when all was said and I never had a single failure. Small-sample, so YMMV.

 

There areat least two different styles of tap ins, and one that I have repaird the damage from. It is OK with some stranded wire sizes, but it puts deep nicks into solid wires. The electronic scheme for sensing that the engine is running uses a quad opamp with two stages used to boost the ignition noise on the circuit, then a rectifier stage, to trigger the last opamp as a comparator. It works fairly well tapping into some points on a 12 volt system. If there is alternator whine on the line it works very well.
So the question is about what sort of system?? Often, "back probing" the right circuit connection is simple and easy.

 

Can someone please explain or provide a simple schematic diagram? It would be very helpful.


I want to detect the ignition signal without cutting the car’s ignition wires, as the vehicle is still under warranty. I’m looking for a solution that can safely detect or obtain a 12V–14V signal using a split-core or non-contact method.


The goal is to use this signal for my tracking device. A simple and practical schematic would be greatly appreciated.

 

I looked at post #1 again. Do you need to sense actual engine RPM, or just that the engine is running??
In many applications a scheme to monitor the engine oil pressure warning light switch is used, and that circuit should be accessable either at the switch or at the bulkhead feedthru connector. At least those points can be accessed on the cars that I am familiar with. AND, for the seatbelt use sensing, there is usually a connector under the front seats that can be accessed.

 

The goal is to use this signal for my tracking device.

I think I'd focus on the tracking device itself. Can we assume it's determining and broadcasting its gps position? Suppose in "idle" mode it determines its position every few minutes and broadcasts maybe once an hour or whatever you think is appropriate. You could use detection of motion to trigger a faster sampling rate and more frequent broadcasts.

Focusing on movement would eliminate any issue of the car idling but not moving. An accelerometer would be another approach.

 

NowI wonder just what does "detecting the ignition" do for the "tracker Device?"

 

the most robust option is to connect to circuits of interest - directly...

since all wiring inside vehicle end up on connectors, one can avoid altering existing harness.
just follow device of interest to nearest connector, take picture of both ends, and get the compatible connectors to make your own "extension cable". then you have something that is easily reversible and does not compromise original vehicle harness.

 

It is not only possible, but also quitelikely, that many of the connectors in a recent car are simply not available at any reasonable price. AND, ONCE AGAIN, I suggest that for all of the none-molded connectors, pushing a second wire in along side the original wire, (Back-probing) is the way to go. If doing that is hard to understand, that is unfortunate.

 

Not sure i understand....

You are suggesting to just give up on the simplest and best possible option without even trying to take a look at connectors, because of a chance this may be costly?

I dont recall cost being mentioned as a factor. In fact very little is known. That includes model, make and year of the vehicle. So what kind of advice is "do not look"/"do not share"? May i remind you that this is acis is a public forum? Inquiring and sharing are core concepts. Why are you trying to shut it down? And if you do, what alternative do you recommend?

probing is fine for diagnostics but i would not think it is a practical way to tap into circuits for any other purpose. My understanding is that TS is looking for a way to connect (and keep connected) circuits more or less permanently - also while vehicle is in use. I sure would not want to hold probes while driving a car. Or did i miss something?

Jamming wire next to crimped pin may be ok for temporary connection though i have doubts on reliability, specially in a vehicle. There is a reason wires in automotive plugs are using crimped. Connections.

 

Even if a " direct but non-destructive " method were chosen there would still be the issue of designing a proper circuit.

Which is what the TS has asked for from the beginning.

 

Even if a " direct but non-destructive " method were chosen there would still be the issue of designing a proper circuit.

Which is what the TS has asked for from the beginning.

That is correct, that the TS requested "a method of detecting without cutting wires". Since both of those functions are part of the wire harness in most cars, tapping in at a connector is a logical approach. And acessing circuits by carefully adding a wire into a connector has been done since before I learned about it long ago.(1964)

As for the price and availability of mating connectors, The prices were far from what I would call reasonable, since one must purchase not only the plastic bodies of the connector , but also the terminals that attach to the wires. AND then somehow crimp all of those onto some wires, and correctly insert them into the connector bodies.

As for "the design", making a direct connection to a circuit does not require much of a design effort. Certainly "cutting wires" is a direct connection method that does not require much design.

Ofcourse, ANY addition or attachment to a vehicle electrical system requires enough knowledge and understanding to avoid causing damage or incorrect operation. That reality should be understood prior to beginning.

 

It's unclear to me what the actual goal is. On one hand it sounds like the purpose is to monitor seatbelt compliance, to identify periods where the car is being driven but the belt is not in use. But on the other hand there is mention of a tracking device, which to me means a radio broadcasting current (GPS) coordinates.

What are we trying to accomplish?