Wire extra 3rd brake light by sensing current/voltage in rear brake lights-how to do this?

I have a 2019 ford ranger pick up that I just added a bed cap to. The cap has a third brake light that I need to wire. I would like to use the wire that lights the rear brake lights but not to tap into the wire directly. I would like to be able to sense when the brake lights are on and then light this new third brake light.

the new light is an led that uses .3 amps at 12.6 volts. the truck lights are led. I assume that this should be pretty simple to do. I am not an electronic guy but I know a fair amount about electric.

yes there are a few ways to wire the new light. I just like the idea of doing it this way. I may use this method to do some other wiring in the future.

first post, so I hope I am doing this correct. I am an old guy.
thanks!

To sense the current to the existing 3rd brake light, you need to how much current it takes. Is it one of these:-
https://www.ebay.co.uk/itm/225509307058 ?
That one uses LEDs and is rated at 0.9 W so it takes around 75 mA, which is quite hard to sense without tapping into the wire.

It could be different in a 2019 Ranger, but LED third brake lights are not new. I had a car with a stock LED third brake light that was built last century.

75 mA is a very small current to try to sense without tapping into the wire. There are Hall effect sensors, but they are usually for currents of 5 A or more.

Is there provision on the Ranger for an electric trailer brake controller? If so, there will likely be 4 wires to that, and they will be +12 V (probably black), ground (probably white), trailer brakes (blue) and the switch signal (probably red). If you connect the extra brake light between that red switch signal and ground it will work fine. The ground can be anywhere, you don't need to wire that back to the trailer brake connector, although you can if you want to.

A hall sensor to detect current in wire and a relay driver or SSR, or a low end micro would do the job.




Regards, Dana.

Diver300 said:

To sense the current to the existing 3rd brake light, you need to how much current it takes. Is it one of these:-
https://www.ebay.co.uk/itm/225509307058 ?
That one uses LEDs and is rated at 0.9 W so it takes around 75 mA, which is quite hard to sense without tapping into the wire.

It could be different in a 2019 Ranger, but LED third brake lights are not new. I had a car with a stock LED third brake light that was built last century.

75 mA is a very small current to try to sense without tapping into the wire. There are Hall effect sensors, but they are usually for currents of 5 A or more.

Is there provision on the Ranger for an electric trailer brake controller? If so, there will likely be 4 wires to that, and they will be +12 V (probably black), ground (probably white), trailer brakes (blue) and the switch signal (probably red). If you connect the extra brake light between that red switch signal and ground it will work fine. The ground can be anywhere, you don't need to wire that back to the trailer brake connector, although you can if you want to.

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Thanks a lot for the response. As I posted, I know there are other ways to wire the new brake light on the cap. I am just hoping to be different and I like the challenge.

danadak said:

A hall sensor to detect current in wire and a relay driver or SSR, or a low end micro would do the job.

Current Sensing Techniques using Different Current Sensors

In this article, we will discuss how to measure current using various current sensing techniques with their advantages, disadvantages and applications.

circuitdigest.com

Hall-Effect-Based Linear Current Sensor IC - ACS712 | Allegro MicroSystems

The Allegro ACS712 provides economical and precise solutions for AC or DC current sensing in industrial, commercial, and communications systems.

www.allegromicro.com

Regards, Dana.

Click to expand...

Thank you for the response. I will check out the links. Looks like you got the point of my question!
happy new year
frank

Lostsoul said:

Thanks a lot for the response. As I posted, I know there are other ways to wire the new brake light on the cap. I am just hoping to be different and I like the challenge.

Click to expand...

I had thought about using Hall effect sensors but to sense 75 mA is difficult. The lowest current Hall effect sensor that I could find that didn't need the wiring disturbed was this:-
https://www.lem.com/sites/default/files/products_datasheets/ctsr_1-p.pdf
and that has a sensitivity of 1.2 V / A, which would give you about 90 mV, if that figure for the existing brake light of 0.9 W is correct.

That may be useable, but it would be quite a small voltage to sense. It would give better sensitivity to put more turns of wire through the current sensor. As there has to be just one of the +ve or -ve wires going through the sensor, putting more turns would need the wiring extended, and I was under the impression that you didn't want to disturb the existing wiring that much.

Take a look at this table :


Sensitivities down to << 1 mV / A

You could always use a IA module and where lights plug in make an adapter
to take IA shunt R connections.



Regards, Dana.

Lostsoul said:

Thank you for the response. I will check out the links. Looks like you got the point of my question!
happy new year
frank

Click to expand...

if your truck has the "towing package", there should be a connector behind the number where you can connect a pigtail for the trailer tear lights. Perhaps it would be very easy to tap into that.

Note that some vehicles (including Fords) are using CANBus or VIN Bus serial controls to turn lights on/off. Tapping into a tail light module controlled by a serial bus communication may be more challenging than you planned.

danadak said:

Take a look at this table :

https://www.digikey.com/en/products/filter/current-sensors/525?s=N4IgTCBcDaIBYEMA2SAEBnApgO3QewCcQBdAXyA

Sensitivities down to << 1 mV / A

You could always use a IA module and where lights plug in make an adapter
to take IA shunt R connections.

Amazon.com : current sensor module

Regards, Dana.

Click to expand...

The OP is probably looking for a sensor with a sensitivity of 10 V / A or more, as the current being sensed is only about 75 mA. The figure of 75 mA isn't certain as it came from a power rating on an image on Ebay of a brake light from a different model year, but in the absence of firm data it's a reasonable working assumption.

The sensors with a large sensitivity,1 V / A or more, all have multiple turns of wire on their current inputs, so to fit them to the truck, the stock wiring would have to be cut and the current routed through the sensor.

As I understand it, the OP was trying to sense the current without tapping into the wire. If the wire needs to be cut and extended, he might as well cut and tap in and measure the voltage, which will be 0 or 12 V, which is far simpler than measuring small currents.

The wire probably terminates in a connector.....? Male to female or vice versa ?

So just create an adapter to insert there between the mating connectors ?

That way one can remote the sensor into available space.

My mistake on sensitivity, I had it ass backwards, these might do :

5V / A and 25 V / A.

But hall pricey, the shunt link I posted small $$$$. One could also use a high G IA module,
G 's up to 1000, to get range needed.



Regards, Dana.

Hi guys I just talked up a storm below using the voice typing if you wanna really go ahead. I just thought it would be fun to try to Why are the brake light on the cap by sensing the brake light electronically and not actually by piercing the wire. I really can just tap into the brake light at the tail light without any really problem.

hi guys I wanted to thank you for your responses I am having trouble with one eye and it is difficult to see the screen clearly so I am the voice function to do the typing let’s see how this works I’ll tell you the Ranger story my Ranger story quickly I hope so and August I bought a 2019 ranger lariat super cab
I am 60 and just retired plan to use the Ranger to tow a camper who is the wife I wanted to get a cap too.
I have been reading and doing some posting on the Ranger 5G forum and these new trucks are very complicated electronically. The brake controller it’s a complicated affair the brand red arc and Ford have made the brake controller specific for this truck.
they took the red arc usual brake controller manual and added another box to interpret the third brake light signal to send to the brake controller itself so rather than have one box you have two boxes. I bought the brake controller from Ford and need to install it. The brake controller hooks up to the third brake light high in the cab. They call it the chms.
You have to remove the trim in the backseat and go all the way up to the headliner and connect with a sort of T connector so there’s no splicing and run that wire back to the brake controller under the dash.
After reading many forum posts it seems that Ford had some trouble
with wiring harnesses and had to replace them so I think they have you go all the way to the third brake light instead of splicing in somewhere along the line to make it plug and play.
Hooking up the brake controller you also have to find a couple of wires on the dash that are just hanging there they’re made for the brake controller but they don’t have a connector on button and break controller kit you get a couple of connectors to make this place it doesn’t seem all that complicated but Ford sort of dropped the ball I guess by not making the whole thing just plug and play.
The one thing I did learn is you should use the Ford red or brake controller kit because of Ford‘s emergency braking if you just ordered a regular brake controller and installed it when the truck slams on the brakes in an emergency situation it wouldn’t apply the trailer brakes. I am saying when the truck applies the brakes not the driver and there’s this can happen in some emergency situation.
Some guys looked at Ford’s information for body builders I guess for when you just ordered a truck without the bed on it and the BCM which I assume means body control module they have a diagram where you can actually it shows where you can Splice in for the brake signal I’ve only seen one guy who actually did it and he didn’t use the signal for anything he just modified the signal with some box to make his lights flash and do fun things.
so getting back to the cap, if you add a cap on the bed you’ll have a third brake light and you need to wire it. I guess for a long time people would just wire the third brake light on the cab to the trailer connector lights sometimes you need a box to separate so you don’t get a flashing brake light when you make it turn this truck has separate light bulbs for break and turn signal.
This truck is a smart truck and you can’t tap into the trailer connector at all because he thinks you have a trailer and does those different things because of that so it’s not a good idea to do that.
So really it’s very simple you just pull a tail light out and put a tap on the wire for the brake light I’m used to scotch locks I don’t know if there are more better connectors these days so yeah the simplest thing is to take the tail light out two bolts tap into that wire and wire up the third brake light on the Cap, no biggie.

The guys on the forums seem to be very reluctant to tap into Brake light wire at the tail light. I’m not sure exactly why maybe they were just worried about corrosion. I just thought it would be fun to sense the current of the brake light and then use that without actually physically piercing the wire to light the wire for the Cap.

I originally ordered a cab from a dealer locally from ARE the cap came in but there was some problem with it and they refunded my money they wouldn’t even tell me what the problem was I just got my money back kind of bizarre. Two days later I found a used cab that was pretty much what I wanted and the color was almost perfect life is funny sometimes.
The new Cap that I ordered I paid $50 for a harness to wire the brake light on the Cap but I have no idea where it was supposed to be hooked up to and when I asked the dealer they didn’t really know either so I don’t know what would’ve happened had I got the Cap I know it sounds bizarre.
You’re probably falling asleep reading this.
I will post this and maybe go search on the forum for a link for the BCM. I know this is all rather trivial stuff and I appreciate your help thanks a lot