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Relay Trigger Interference Problem

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Peace9Love9Hope

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I am an MECP installer. I am trying to get a relay negative trigger from a circuit board in the headlight turn signal switch of a car. It is a data/low voltage system. When I hook a relay negative trigger to the negative trigger on the switch, I get back feed/interference on the headlight switch, because there is positive current grounding through it from the relay, and it changes the resistance/data in the circuit on the switch; which changes the signal to the computer and causes weird things to happen in the car. Once the relay is disconnected, the system goes back to normal.

I need to know how to isolate this or figure out another way to get this negative trigger. This switch seems to be the only way (after inspecting All Data diagrams for the vehicle), because after this constant signal is sent to the computer of the vehicle, the computer changes it to a non-constant/pulsing signal that I can't use.

My intent is to get a constant trigger for when either of the turn signals are started; so that I can switch an LED circuit to the turn signal circuit, instead of the parking light circuit it is at normally. On older cars you could hook into the negative trigger at the Flasher. These newer cars use a computer to flash the blinker which makes it more difficult.

Any ideas?
 
Draw a sketch of the voltage you would see at the point on the circuit board you are trying to tap into.

What is the voltage level with the ignition off?
What is the voltage level with the ignition switch in the accessory position?
What is the voltage level with the engine running?
What happens when you move the turn switch?
 
Do you want the information requested with the relay connected or not connected (the circuit board at its normal status)?
Relay not connected. You should be able to measure the voltage with just a DMM, with the possible exception of the pulsing....
 
Okay so, the voltage fluctuates between high and low. I will give you the min and max of each ignition setting:

The right blinker:

Lock: 93.1 - 115.8 mV
Accessory: 97.1 - 116.5 mV
Ignition: 94.6 - 118 mV
Running: 94.2 - 119.8 mV
Signal on: 3.7 mV (not fluctuating)

The left blinker:

Lock: 66 - 78.4 mV
Accessory: 68.2 - 79.4 mV
Ignition: 66.1 - 80 mV
Running: 32.4 - 79.8 mV (might have been a mistake on the lower one, I can recheck)
Signal on: 3.7 mV
 
Its a very rough sketch, but that is what it looks like. If you want I can get a picture.
 

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Is that measured with respect to chassis ground? I'm surprised that all of the voltages are so small. I would have expected signals that switch from near ground to near battery voltage...
 
Then you are not connecting to anything useful inside that circuit board. I think all you are seeing is ground drops, so your positive meter is not where you think it should be, certainly not where anything interesting is happening...
 
Headlight and turn signal drivers also sense current and low voltage drop to alert when expected load changes due to an open bulb or filiment ( mine has two filaments in parallel like DTR & brake filaments and the higher current one always fails 1st.

this gives redundancy giving one standby turning indication at lower intensity but still alerts dash with lamp error)

Therefore any sensing cannot draw or supply current.

i would use a comparator with threshold set to 10mV with 1k differential RC filter on input and open collector // reverse diode to drive 100mA relay or better a smart MOSFET switch.

As usual, protect against reverse voltage and 2x V+ surges

This comparator 36Vmax, 2mV=Vio, 20mA will do the job with suitable MOSFET . Scale 9 to 14.2V down to 10mV and compare with local ground nearest with twisted pair.(or UTP shielded?) or for more precision, use any LDO or any LED at 2mA for reference V to scale down to 10mV. e.g. Red =2V to 10mV needs 200:1 divider or ~ 1M to 5K to ground , other side 5 k to ground. 0.1 uF in between +-. And diode cap for Vcc to chip.

https://www.digikey.com/product-detail/en/LM339AN/296-6605-5-ND/372806

Need more details?, just ask the forum.
 
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Wow, awesome. Thanks Tony. I just ordered the comparator you suggested. However, I have never actually used a comparator. Does the comparator at least come with a diagram? Assuming its engineering I'm sure I could figure it out if I had a diagram.
 
There are many concepts missing in your training. How to use any IC. Physical, logical, functional, analog, electromagnetic principles beyond my commit to training here.

In electronics, a comparator is a device that compares two voltages or currents and outputs a digital signal indicating which is larger. It has two analog input terminals
84b6cffdc7c4a5c0ccb6e49e4da4aebb.png
and
6a45753779425261ae50f0e557c4bd23.png
and one binary digital output
98ad0411b5c7aee5181f13d70dea2518.png
. which like a switch to ground =0 or an open switch with the load being external part in series to V+ , limited by the driver IOL output low current, typically 10 to 100 mA.

Better chips offer low input offset ( low mV) and come in 1,2 or 4 per package and can run with a single or bipolar supply with the input voltage comparison possible near ground even with a single Supply.

Perhaps someone else can volunteer a layout on a protoboard,,.
 
Okay, Thank you for all of your help thus far though. Is there perhaps a textbook you recommend for these more advanced circuits? I am in the process of expanding my knowledge and going to school. So any books or guides would be great. This car will be a good learning experience.
 
I used to read every application note and magazine design corner notes in the library before I graduated.
You're in school to learn how to learn.
Do it.
 
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