I would be extremely grateful if someone could point me in the right direction on building a safe/reliable circuit for my bikes brake light.

Thread Starter

C25

Joined Mar 15, 2024
3
Hello everyone, im reaching out in hopes to make this work as the manufacturer of the indicator lights says it is incompatible with my bike.

I would like to add 2 of these aftermarket LED 3 function indicator lights near my license plate in conjunction with retaining my main rear tail lights. The 3 functions are: TURN SIGNAL (12 V - 1.1 W) / TAIL LIGHT (12 V - 0.5 W) / BRAKE LIGHT (12 V - 0.9 W) The 4 leads are sealed, 1 shared ground and 3 for the individual functions. All very easy for me EXCEPT the brake light function requires a positive signal and I believe my brake signal from the bikes computer to the rear tail light is a ground signal. im hopeful that i can come up with a MOSFIT solution or solid state relay that doesn't risk damage to my bikes control unit.

led cluster.jpg
I'm not even remotely close to knowledgeable on electronic circuits, but I believe the reason the manufacturer says it is incompatible is because the signal going to my sealed rear light cluster for the brake light is a ground signal rather than a 12v powered signal.



This is a crop of the circuit from the manufacturer of my bike (12) is tail light, 2 functions lamp and brake light.
rear lamp.jpg
With my lights on I used a circuit test probe to find the wire to terminate to and was surprised as the moment I tested (2)(-s) my brake light came on without applying brake but the test light did not light up. I assume I just gave it the ground it was looking for. (3)(+) is powered on with my head lamp as it supply's power to both functions of the tail lamp. (1)(-) is always grounded.

1 terminates at the other grounds that my license plate and blinkers do.
2 terminates at the bikes control unit
3 terminates at the positive line that also runs to the license plate light.

Plug to tail light
plug.jpg

Sealed LED tail lamp, top row is lamp and bottom row is brakes.
tail lamp.jpg

I just spent the last couple hours watching videos on MOSFIT's and relays used to convert a ground signal to positive signal.

This is maybe one option:
ground trigger.jpgGround trigger source would be from my bikes control unit (ECU/ECM/DME)

My number one goal is something that has no chance of damaging my bikes control unit or disrupting the function of my primary brake light.
My thoughts are that there is a way to just tap into (-s) on my schematic and use it as a ground trigger source without the possibility of putting any stray voltage into that line that could cause damage.

Is there a smaller/modern/safer/more reliable solution than a standard automotive relay?

I have been studying MOSFIT and maybe that is a solution or maybe solid state relay?

Thank you very much if you made it this far and if there are details I should add or clarify im happy to do my best. I have a multi meter if any other values are required.

I hope this made sense.
Thanks again.

Weather is great and im just banging my head into the wall trying to get this figured out.
wide view.jpg
 
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Mussawar

Joined Oct 17, 2011
95
Hi C25,
First of all MAKE SURE that your brake light is controlled by negative signal. If this is the case, a general purpose 12V SPDT relay should be able to handle your light. If your original brake light is driven directly by your control card then it should be able to drive the relay too. Since desired light is only 0.9W therefore no need to use any high power relay. Diode is just for protection back to control card.
Please see attachment.
Regards
 

Attachments

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crutschow

Joined Mar 14, 2008
34,704
The relay suggested by Mussawar should work fine.

If you would rather use a transistor, then either a P-MOSFET or PNP BJT should work.
Below is an example circuit using a P-MOSFET:

It supplies 12V to the LED when the bike computer signal goes to ground.

The MOSFET can be any P-channel type with a voltage rating of at least 40V, an on-resistance of no more than a 1Ω, and in a package that you can work with.

1710520088958.png
 
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Thread Starter

C25

Joined Mar 15, 2024
3
Thank you both so much!

When I disconnect the tail lamp plug (circuit in diagram), I get warning lights on my dash. The bike does have some CAN bus circuits, but they are represented with a weaving line and x1/x2 for color codes on the manufacturers wiring diagram.

rear lamp.jpg
When I ground (-S) my brake light comes on. (that was reckless i know)

How would I determine that -S is just a simple signal that's a ground circuit?

-S is just a straight line to computer, doesn't look like CAN in the diagrams.

Would measuring just resistance or ohms alone on the signal (-s) be enough to prove that?

My guess is that if I patched my multimeter into that it would show OL and then give me resistance when activating the brake lever. If that is the case, I should be good to proceed?

I have two of these LED's @ .9w each.

Thanks so much.
 

crutschow

Joined Mar 14, 2008
34,704
What signal is -S?
Just measure the voltage when the brake light is on and when it is off.
That should tell you all you need to know about that circuit.
 

Thread Starter

C25

Joined Mar 15, 2024
3
What signal is -S?
Just measure the voltage when the brake light is on and when it is off.
That should tell you all you need to know about that circuit.
Thank you.

Bike on, I get 9 volts from (-s) signal wire. I did not expect that.
When i hit the brakes I get .005 volts from the signal wire.

Maybe time to scrap the idea, unless i crack open the light housing and see whats going on inside that.

Thanks everyone.
 

Mussawar

Joined Oct 17, 2011
95
Thank you.

Bike on, I get 9 volts from (-s) signal wire. I did not expect that.
When i hit the brakes I get .005 volts from the signal wire.

Maybe time to scrap the idea, unless i crack open the light housing and see whats going on inside that.

Thanks everyone.
Seems that brake light is not just controlled by a simple negative line. If this is the case, I would not recommend to operate a relay directly from your computer card. This may overload computer circuit and damage. Circuit posted by @crutschow with MOSFET would be a good choice then, as it will not overload your computer output circuit by any means. You can use it without danger of computer destruction. But it's still experimental.
Alternately you can arrange a separate line for brake light by taking input from brake lever without fingering the sensitive original circuitry.
 

Ya’akov

Joined Jan 27, 2019
9,247
Welcome to AAC.

Just a note that if I was going to wire anything directly to an ECU or other vehicle computer I would certainly use an optoisolator for galvanin isolation. The opto is just a small component with an LED and phototransistor inside. The LED is connected to the controlling signal and the phototransistor to the controller.

This way it is impossible for anything you do on the controller side to harm anything beyond the phototransistor half of the optoisolator which would represent a couple of dollars of destruction at most compared to frying the I/O on your bike’s computer.

Once you have the opto connected, whatever circuit you want to build to actually be the controller is left open, but you won’t be smoking anything that will make you feel like an idiot. Good luck on your project!
 
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