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# DIY Trailer Hitch Wiring Challenge

Posted June 24, 2009 10:12 AM by frankd20

Trailer hitch wiring doesn't sound like it should be complicated, but it can be quite a mental challenge as you will see.

I recently completed a DIY trailer hitch for my car and needed to wire the lights to go with it. The problem is that my car is one of those that have separate bulbs for the brakes and the blinkers. My trailer has only four wires going to the lights, ground, running lights, and a left-and-right light, which act as both brake and blinker lights.

At the time of wiring these lights, I had the taillights of my car apart and didn't want to run to the store to find a solution. Instead, I simply wired four diodes, which would isolate the outputs of the brake and blinker and allow me to combine them. This circuit worked, and low and behold, the blinkers blinked and both brake lights came on when prompted.

Shortly after, that I realized that the simple circuit had a flaw. The issue was that when the brakes were on while the blinker was on, the blinker wouldn't blink. (Despite the flaw, my simple circuit did work well enough for my trailer to pass inspection since they never had me hit the brakes and turn the blinker on at the same time.)

The normal function of the lights should be that when you hit the brakes, both lights come on; if you turn on the blinker with the brakes on, that side should blink, while the other side stays steady on. The question became how to accomplish this with as simple a circuit as possible.

Instead of just posting the solution, I would like to make this into a little challenge: design as simple or as elegant a circuit as you can for the application above that would make the lights work correctly. By simple or elegant, I mean least number of parts and/or lowest cost to produce.

I have a number of what I feel are good, simple solutions that would work and I think there are likely many good solutions I don't have. I also have some designs that seem more complicated, but would work. So please, post your solutions. Next week I will discuss and post the solutions I have – if they haven't already been guessed.

By the way, I ended up buying the circuit at the store, which cost me about \$15. The low cost doesn't really make it worthwhile to make this (should cost about \$2 in parts), but its fun to know how it works.

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#1

### Re: DIY Trailer Hitch Wiring Challenge

06/24/2009 11:07 AM

Having had to face this more than once I'm going to shut up and watch the answers for a bit.

And then I'll post how to handle this when constantly faced with other peoples trailers!

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#2

### Re: DIY Trailer Hitch Wiring Challenge

06/24/2009 12:18 PM

With dual element bulbs, one wire for running lights and license plate light, one for brake, one for each directional. Then run an quick disconnect auxillary from the trailer chassis to the car chassis for ground.

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#3

### Re: DIY Trailer Hitch Wiring Challenge

06/24/2009 12:48 PM

The problem is that my car is one of those that have separate bulbs for the brakes and the blinkers. My trailer has only four wires going to the lights, ground, running lights, and a left-and-right light, which act as both brake and blinker lights.

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#4

### Re: DIY Trailer Hitch Wiring Challenge

06/24/2009 11:26 PM

Simple, move here to Oz :)

Aust has had mandatory separate Brake/Blinkers for many years, so unless you bring your car with you there will be no problem.

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#6

### Re: DIY Trailer Hitch Wiring Challenge

06/25/2009 5:09 AM

Same in the UK re separate lights. Also, indicators are amber, stop & tail lights red. Indicators must also have repeaters on sides of the front wings. Is this the same in Oz?

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#5

### Re: DIY Trailer Hitch Wiring Challenge

06/24/2009 11:52 PM

Eh? I've rewired a seven pin in the dark, in the dark rainy night, without a light in sight and without help. And it's a challenge

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#7

### Re: DIY Trailer Hitch Wiring Challenge

06/25/2009 7:38 AM

"twas a dark and stormy night as I plugged the seven pin into the trailer and watched my truck lights go out."

OK, it was actually cold and snowing - but...

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#8

### Re: DIY Trailer Hitch Wiring Challenge

06/25/2009 8:43 AM

is why you get the big bucks

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#9

### Re: DIY Trailer Hitch Wiring Challenge

06/25/2009 11:20 AM

I don't remember the particulars, but trailer wiring on Detroit cars is easy due to the dual element lamps and the switch in the steering column. The arrangement allows the running lights, brake lights and turn signals to function as they should. Japanese cars are wired different and require diodes to prevent backfeed into the opposite side. A simple module purchased at any auto parts store does the trick.

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#10

### Re: DIY Trailer Hitch Wiring Challenge

06/25/2009 12:06 PM

While all these are nice comments, no one has attempted to come up with a solution to the question asked.

The trailer has four wires and only four wires. These wires go to ground, a running light, and a left and right light. The question does not allow for adding extra wires or connectors or modifying the trailer lights in any way.

Yes of course you can buy an adaptor, but that isn't the point, I want you to put your thinking caps on.

My question is if you were designing that adaptor, what is the circuit design you would use?

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#11

### Re: DIY Trailer Hitch Wiring Challenge

06/25/2009 1:13 PM

a simple 'low tech' solution would be to power brake lights through two 12Vdc NC SPST relays with the coils powered by the turn signal voltage. a more 'elegant' solution can be achieved using SCR's

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#12

### Re: DIY Trailer Hitch Wiring Challenge

06/25/2009 1:18 PM

I have done this several times for motorcycle trailers.

I make a board with 5 relays on it (one spare for repairs) (1.Running lights 2.Brake lights 3. left turn signal 4. right turn signal). Motorcycles don't need a back up light. I wire the power directly from the battery and have the relay turned on by the original signal thru a diode able to handle the current to the relay activation. I typically use a 1n4004 because I have a bunch of them. The fifth non-relayed wire is ground. For some reason motorcycle trailers use a 5 wire system with a different color code by each trailer manufacturer.

When a small car trailer is being modified for motorcycles this system works quite well. You just have to add a ground wire to the four wire system. I don't trust a greased trailer ball for a frame ground.

For your system I would add two exclusive or gates on the input with the two inputs being brake and turn signals. The turn signal will cause the output to pulse in this case.

Of course being an electronics instructor I have easy access to all kinds of parts cheap.

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#24

### Re: DIY Trailer Hitch Wiring Challenge

06/27/2009 10:35 AM

GA for the XOR, but how to do it elegantly?

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#13

### Re: DIY Trailer Hitch Wiring Challenge

06/25/2009 9:31 PM

It's an interesting problem, of course it can be solved using relays, but what is the absolute cheapest way to do it? Can it be done just using diodes?

Question 1 Do the signal lines get switched between 12V & 0V or do they float (12V when ON & N/C when OFF).

Question 2 Is the grounded side of the lamp available (are there 2 wires or 1 from the lamp).

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#14

### Re: DIY Trailer Hitch Wiring Challenge

06/26/2009 8:14 AM

I hope I understood the requirements for the USA Trailers, here is a simple version from me, only three relays are required with no Diodes, no Transistors, so no voltage drops.... Its only a very rough and simple sketch, so if anything is not understood, just ask.

Signals:- G = Ground or frame, LI = Running lights, L = Left Blinker, R = Right Blinker, B = Brakes, + = +12 volts. Right half on car, left half on the trailer.

Components:- RL = Left Blinker Relay Coil, RR = Right Blinker relay Coil, RB = Brake Relay Coil.

Relays need to be 12 volt Car quality with contacts that will handle the amps for the Brake/Signal lights. There may be a need for small snubbing capacitor across the contacts to prevent arcing....time will tell...

All signals are assumed to be nominal 12 volts + when active.

If the brakes are put on, 12 volts is applied to both the RB relay and the normally closed contacts of RR and RL. Also removing 12 volts from the second contacts of RL & RR, therefore supplying the braking light bulbs with power via the normally closed contacts of RR and RL.

If a turn signal goes on, it pulses either RL or RR, interrupting the power to the brake/turn light on that side only.

If only turn signalling is needed, +12 volts is applied to the NO contacts of both RR and RL via the NC contact of RB, so long as the brakes are not applied, the relevant signal results in blinking the relevant signal bulb by interrupting the 12 volts at either RL or RR.

This was the simplest solution I could find, it also being quite robust as no electronics are involved.

If clicking is heard on the car radio, a snubbing diode should be wired in reverse across each coil, but it will work as shown without either snubbing diodes or capacitors......

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#15

### Re: DIY Trailer Hitch Wiring Challenge

06/27/2009 2:51 AM

Nice job!

I wonder where Guest would put the SCR?

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#16

### Re: DIY Trailer Hitch Wiring Challenge

06/27/2009 3:13 AM

My fist design was with a logic chip, then once I knew what I needed, I could see that it was not necessary touse electronics and 12 volt car relays are very robust and no 5 volts needed.....

SCRs in a DC situation are a pain in the neck as they lock on.....Darlingtons are better as they will still turn off when not driven....FETs are also good, but all REALLY need snubbing diodes......my final circuit will work without, though some car radio interference (mostly on MW & LW. FM should be OK on a good radio) may be heard.....

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#17

### Re: DIY Trailer Hitch Wiring Challenge

06/27/2009 3:14 AM

Have i won the prize for the simplest answer? See post# 14.

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#18

### Re: DIY Trailer Hitch Wiring Challenge

06/27/2009 3:32 AM

Prize for the only good answer so far.

I had a car with relays in the trunk for the rear light set.

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#19

### Re: DIY Trailer Hitch Wiring Challenge

06/27/2009 4:27 AM

I would say thats the best place for them, in some sort of box, easily removed for repair if required.....If I was installing such a unit, thats where I would put it too.....

In Europe, we have mainly 3 different ways to connect a trailer. The old European 7 Pin connector that is in use almost everywhere. It had a minor modification some years ago (I don`t remember quite what), but is otherwise unchanged for many years.

When more connections were required, the UK went a very simple way, it retained the old 7 Pin and introduced a new extra 7 pin (incompatible to the old one to stop accidents) and if you need more connections, you now have two connectors used at the same time. It simply means that a small trailer can be still used and/or a larger caravan, without having a gender changer to pull one or the other.....

In Germany they went to a 13 pin connector (with smaller pins that can carry less current than the old 7 pin), so when using a 7 pin trailer, I need to have a gender changer to convert from 13 to 7......

There is another connector here (that I have never seen used up to now) that accepts both an old 7 pin or a 13 pin ( not the same 13 pin just discussed, a different one!)....

"Clever" manufacturers like VW only put a few pins in the 13 pin harnesses, which means that you cannot charge a battery onboard a caravan!! Nor can you buy extra pins to add the needed wiring either!!! They think they are God!!

Thank God I have a Mitsubishi and it has the 13 pin connector implimented fully!!! Much better.

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#20

### Re: DIY Trailer Hitch Wiring Challenge

06/27/2009 7:25 AM

The US has many, many interfaces as they are almost always fitted after the dealer (manufacturer).

For caravans and such there is a heavy duty seven pin, now being replaced by a seven "blade" for more contact?

For things like boats a six pin was popular along with a latching receptacle.

Now light trailers are settling on a four pin "flat" molded rubber - more disposable crap.

So if you own a pickup ("can you help this weekend" vehicle) you end up carrying around a zillion adapters and hope the keying/wiring is conventional.

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#25

### Re: DIY Trailer Hitch Wiring Challenge

06/27/2009 1:04 PM

The car was manufactured with power to the trunk for the audio amplifiers and control relays for some of the rear lights.

1988 Dodge Dynasty.

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#21

### Re: DIY Trailer Hitch Wiring Challenge

06/27/2009 7:26 AM

What he said - best answer so far

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#22

### Re: DIY Trailer Hitch Wiring Challenge

06/27/2009 7:50 AM

Nice of you to say, but I was hoping FrankD20 would also say something....

Also I was hoping that someone else would come up with a better idea/less components or something along those lines......me being a bit competitive.....!!!

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#23

### Re: DIY Trailer Hitch Wiring Challenge

06/27/2009 10:33 AM

Stay tuned .....

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#26

### Re: DIY Trailer Hitch Wiring Challenge

06/29/2009 10:10 AM

I have a number of designs I will discuss later this week, one of them does use relays, although its possible to reduce the relay count from what you have.

Most of the designs I have don't use relays, but I can see you did put some good thought into this to come up with your design, well done.

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#27

### Re: DIY Trailer Hitch Wiring Challenge

09/15/2009 11:16 PM

I'd run the lic plt and running lights in serries with the turn bulbs for less power.

separate and issolate with diodes and run the works with relays just to prevent any ill effects on the car light cct.

That way running lights are ~8volts and regular brake or turn get full 12 minus the .7 for the diode.

total is 3 bulbs single filament, on my trailer my plate is in the middle.

4 diodes.

2 relays.

by far the cheapest and the nicest!

Not sure how to embed the drawing... but I'll try to write it out:

on top row you have car light bulb signals: Left, Brake, Licence(and running), Right and ground.

ok we'll look at the Lic. cct... 12v is on one end of lic bulb other is bridged to two anodes of two diodes that run through the L and R bulbs, then to ground. Thus running lights in both branches are two fillaments to ground, producing a dim light on all 3 bulbs. the lic light will be slightly brighter. Also each bulb is isolated from reverse current by a diode. so if turn signal is on, no effect seen on other two bulbs, simply the reversed biased diode with not conduct.

Also if the lic light is not powered there will be no effect on any lights, stop or turn.

ok now 12v brake goes to two anodes of two diodes one to the left and one to the right each then through a NC relay contact and then to the turn bulb through to ground. So hit the brakes and power is forward biasing both diodes and lamps light with 12v... ok 11.3 ish Volts.

Now turn on the left signal and the power switches the relay on and off blinking the left lamp... with or without the brake.

left lamp sig wired to the coil then to the lamp (also tied to cathode of diode for lic light) then to ground.

Right same just on right.

and ta da!

juliusz_22@hotmail.com

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#28

### Re: DIY Trailer Hitch Wiring Challenge

09/16/2009 7:28 AM

Sorry, no diagram makes it hard to understand (like impossible?).......

Make a diagram in .jpg form and add it using the litlle camera icon....its self explanatory really.

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#29

### Re: DIY Trailer Hitch Wiring Challenge

09/17/2009 12:01 PM

ok I responded to the 2nd part of the blog and my pic was posted... but I can't.

here is the link which is fixed.

even simpler and pretty.

2 diodes

2 relays

1 resistor.

and that's it.

Cheers

Juliusz

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#30

### Re: DIY Trailer Hitch Wiring Challenge

09/17/2009 1:00 PM

Two problems I believe I see:-

1) When braking, no indicator anymore, except maybe a tiny blink while the relay changes over, with normal filament bulbs, probably not noticeable. As when turning, you almost always need to slow down, this is a significant problem, you need to allow blinking AND brake light......

To fix it, you need a further relay that allows voltage through to the bulb(s) while indicating, and no braking......then it would work fine.

2) Feeding filament bulbs through diodes drops voltage/current, so the brake lights will not be as bright as they really could be. Again a not insignificant problem.

3) Feeding the bake lights via resistor as parking/side lights means that your resistor will get quite hot. A further problem is if one of the brake lights burns out, the other one will suddenly get much brighter due to getting a higher voltage......depending on the value of the resistor used, this could get easily almost as bright as when braking.....

I still feel that you made a good attempt in the full idea of the original poster. I believe we are the only ones........you and me both!!!!

I personally do not like using either diodes or resistors as reliability is maybe affected, but thats only my personal take.....it was certainly allowed.

Have a great day.

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#31

### Re: DIY Trailer Hitch Wiring Challenge

09/18/2009 10:10 AM

Although a drop if .7 volts seems high, in reality the brightness is fine... I've got a set up like this running on mine.

Also the stop and turn together would work fine as the turn signal is interupting the brake at 50% duty cycle typ. only on the one side. (or both if hazards are on)

How much better can you get?

simple and cheap.

oh and diodes are quite robust but I quadruple the requirements to prevent troubles over the years.

As for thr resistor getting hot... true if only one.

I put 3 in parrallel and they were (power) bearly warm. and again tough for their duty.

So my friend... I think I took the cup from ya...lol

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#32

### Re: DIY Trailer Hitch Wiring Challenge

09/18/2009 12:34 PM

Not quite correct. Let me explain why and hopefully you will then FULLY understand it.

I Hope that also a few others will take the time to check through what I said is correct....

Lets take the RH lamp as an example. Here is your diagram where everyone can easily view it:-

The blue arrow going up is going to battery +12 volts.

If the RH indicator is used, then the RH lamp pulses at the frequency of the turn signal (between 60 and 120 per minute if I remember correctly is the allowed frequency.), going on as the relay pulls in and going off when it drops out, because there is no voltage on the lower contact at this time.

Fine. It works correctly for signalling.

But now put the brakes on.

Now the lamp is getting +12 volts from the braking on the other side of those relay contacts, via the diode.

So now the lamp is getting +12 volts FROM BOTH SIDES OF THE RELAY AT ONE AND THE SAME TIME, NO MATTER WHAT POSITION THE RELAY IS IN.

As the relay may only take a few milliseconds to switch over from one position/contacts to the other, the loss of voltage for a couple of milliseconds will simply not be noticed as filament lamps take probably at least 0.25 of a second to cool down, or 250 milliseconds.

I gave you the fix in my last post. It seems you both do not understand the problem (build the circuit EXACTLY as you have shown in your diagram, then you will understand far far better what the problem is), nor do you understand the simple fix I gave you either !!!

So my friend... I think I took the cup from ya...lol

Was FAR too soon!!! I therefore still retain the cup, before, now and forever......and you remain 2nd in a race of two people......

By the way, adding an extra relay, the quick way to make your circuit work properly, , means that you now need the SAME number of relays as I used!!! And I did not need any resistors or diodes either........!!!!

So if anyone can "crow" now (I am demonstrating here a similar disregard for polite manners as you did before!), it ain't you Buddy!!!

Its clever ol' AG again!!!!

Its clever ol' AG again!!!!

Its clever ol' AG again!!!!

Its clever ol' AG again!!!!

Its clever ol' AG again!!!!

Its clever ol' AG again!!!!

Its clever ol' AG again!!!!

Its clever ol' AG again!!!!

Its clever ol' AG again!!!!

Its clever ol' AG again!!!!

Its clever ol' AG again!!!!

Its clever ol' AG again!!!!

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Anonymous Poster
#33

### Re: DIY Trailer Hitch Wiring Challenge

10/06/2009 5:15 AM

Not nice, either one of you.

You both have skills but please don't let it get to your heads. Ruins the community. Who wants to deal with arrogance, even if it's accurate?

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#34

### Re: DIY Trailer Hitch Wiring Challenge

10/06/2009 5:44 AM

I was actually replying (not in my usual manner) in the same vein that the other person was using, even though he cannot see his errors (in both electricity and good manners), in spite of me mentioning it twice in as simple a manner as possible....

.....though I think he has now seen/agreed that he screwed up big time and has long gone!!

Have you never ever "crowed"? Then you have never lived!!! How boring for you life must be.........I am sorry that I brought you out of your deep boring sleep.......I'll try and be better mannered the next time, just 4 you!!

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