Trailer Stop & Turn Converter Schematic 2

Trailer Stop & Turn Signal Converter

If you ever wondered why your trailer Stop/Turn signal lamps are so dim you are not alone. The answer is simply that the typical Stop & Turn Signal Converter/adapter drops a whopping 2.5V! This is VERY significant on a 12V electrical system. This DIY project provides the information you need build a simple, inexpensive converter that reduces this drop to a mere 0.5V @ 2A, and even has sufficient guts to power a 7.5A load. While this documents the most common 4 way (4 wire) system, it may be easily incorporated into more complex systems.

Why is a converter required?

Most automobiles manufactured today have separate Stop and Turn signals — the stop signals are Red, while the Turn signals are Amber. Unfortunately, most trailers do not include the luxury of amber Turn signals, but use the Stop signal also as a Turn signal. The converter takes both input signals and generates one output signal for the trailer. The logic for accomplishing this is simple, but tricky—check out the following truth table. Curiously, when the Stop and Turn signals are simultaneously applied, the trailer Turn signal is “inverted” so that when the vehicle Turn signal is On, the trailer Turn signal is Off, and vise versa.

Truth Table

Trailer Stop & Turn Converter Truth Table

Fortunately, it can simply be performed the following circuit:

Trailer Stop & Turn Converter Schematic 2

How it works

Both left and right sides of the circuit are identical. The 12V power from both the Stop and Turn signals are OR connected by high current schottky diodes (D1 & D2) and fed to the drain of a P-Channel MOSFET (Q1). To turn Q1 on requires that either the Stop or Turn signal to be off. Zener diodes D3 & D4 (applied as rectifiers) make a negative OR connection so that whichever lamp is not powered applies essentially zero volts to R2 that in subsequently applies a negative voltage to the gate of Q1 thus turning it on. When both Stop and Turn signals are simultaneously powered (high), there is no negative gate drive signal available and Q1 turns off via R1.

Overvoltage protection

Note that the rather unconventional application of zener diodes as low voltage rectifiers. The purpose of this is to provide transient voltage protection for the Schottky rectifiers.

D9 is a zener that protects the MOSFET gate from overvoltage transients — its low cost is essentially an insurance policy in protecting against potential MOSFET failure. Automotive electrical systems can be very “dirty” when it comes to generating transients — e.g. what happens when you have your foot on the brake when you start the engine? The starter current is not the problem, but the termination of starter current can generate high transient voltages due to the series inductance of the starter wiring.

Failure modes

While I believe that the potential for circuit failure is low, it is important to recognize the symptoms. Otherwise, even a very experienced mechanic may have difficulty making an accurate diagnosis. The symptom for D1 or D2 failure is “crosstalk” between the vehicle Stop and Turn signal lights. In other words, when the brake pedal is pressed, both Turn signal and Stop signals light simultaneously; or when the Turn signal is activated, both Turn and Stop signals light simultaneously. This will occur with or without the trailer connected.

MOSFET failure is indicated by absence of the trailer Turn signal when the brake is pressed.

Feasibility prototype

I did not build the complete assembly — that will come later — what I tested was a feasibility prototype that I “haywired” together — see photo. This provided all the information needed to demonstrate feasibility and performance. At 2A, the MOSFET ran cool — no heatsink required.

7.5A Stretch version

If applied at 7.5A, the MOSFET power dissipation (Rdson = 0.07Ω) calculates to 4W. At this power level, a small heat sink is required. I recommend a heat sink with a thermal resistance of about 10°C/W. If the assembly is potted, flat pieces of aluminum may be used to spread the heat evenly throughout the assembly. The diodes may also require some heat sinking as the power dissipation calculates to 3.3W — the cathodes of each pair of Schottky rectifiers could be soldered to a common sheet of copper.

On second thought, while 7.5A may be possible, I do not think I would push it beyond about 4 to 6A.

stop & turn haywire

Test data

Trailer Stop & Turn Converter Data

Bill of Materials

Trailer Stop & Turn Converter BOM

Link for useful information regarding trailer wiring and connectors etc.

Why is the existing technology so poor?

The standard circuit that is available from numerous vendors including U-Hall is similar in topology except that it uses standard silicon rectifiers and PNP Darlington transistors. The Darlington transistor accounts for most of the voltage drop because Darlingtons saturate poorly. All components are assembled into a potted assembly. Reliability is poor. I did not know that mine had failed until I rented a trailer from U-Hall and found that the lights did not function. As a result, I was forced to purchase a new U-Hall converter unit that they expertly installed for me. At $17, they did not overcharge for the item. On the negative side, the convenient LED indicators that were molded into the trailer connector failed within a few months.

Typical specifications for existing technology

Stop/Turn signal current is limited to 2.1A (one standard lamp)
Tail/Clearance lights is limited by the connector and/or wiring to 7.5A.

U-Hall Converter

For the future

Relay topology Stop and Turn signal converter (lowest voltage drop of all)


Join the conversation!

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  • JRichardson

    I’m working on Tom’s Circuit above and am having an issue getting it to work correctly. Has anyone else used this schematic to do this? I like the idea of using a single IC and a few resistors and FETs to achieve this so I would really like to stick with this solution.

    What seems to be happening is the tail light seems to work fine, but when I connect one of the brake outputs (without a brake input), the brake light turns on and the other side goes dim.

    Any thoughts?

  • Adam Daniels

    Will this circuit survive a trailer side short-circuit (think of physical damage to wiring due to wear, pinch, impact, etc.) – this actually happens. Suppose the car’s fuse didn’t blow.

    Can you provide schematic for a self-resetting short-circuit protection feature? Regards.

    • Jim Keith

      This circuit will likely survive a short circuit by blowing the fuse –this is due to the high current rating of the series components. If the fuse does not blow, the wiring harness will likely melt down. A Raychem resettable fuse in each series line could do the job if sized properly so that it could handle the normal load current properly.

      The circuit provided by Tom could be powered via a current source regulator –will post a circuit for such.

  • Jim Keith

    Cool circuit Brian! Ideal for adding trailer lights without increasing the load for the existing vehicle signal lights. This, of course, requires running an unswitched 12V power source to the unit –not as simple as it may seem in most vehicles and must be fuse or breaker protected against short circuit.

  • tom

    oh for Peat’s sake, I missed a pull-down on the open side of the tail light gate(pin 8).
    Built circuit and it seems to work:-)

    • Brian

      Excellent. I will take your word for it. I was able to eventually get the original schematic to work but I ended up going a different route with my vehicle. I might however look in to this again when I get a trailer with LED tail lights. Thanks!

  • tom


  • tom

    Not sure if my PDF loaded so here are jpegs.

  • tom

    1. This circuit is wrong. Why is it still up?
    2. Use a high voltage exclusive nor with 3 “P” channel mosfets
    (one for running lights).
    if you want a circuit I can probably whip one up.

    • Brian

      What is wrong about it? I eventually got it to work with my crude circuit building skills. Please whip us up something.

  • Jack Brown

    Very informative looking over the schematic and reviewing the comments. Unfortunately I do not have the skills capable of creating a unit such as this. Still hoping I can get some help with a problem I am having.

    Trying to wire a m/c with side car for tail lights. M/c has functioning leads from signals, tail light and brakes. I am wiring these to converter and then out to tail lights. Tried 3 different converters so far and cannot get power out. Keep in mind there are only two lights in total not the usual four with a typical vehicle and trailer set up.

    I’m guessing without the vehicle tail lights in the circuit the converter will not work. Is there a way to trick the converter into thinking the vehicles lights are present? Maybe adding resistors? Keep in mind I am far away from an electronics expert!!

    I realize this is not the purpose of this thread but thought I’d take a shot any way.

    Jack Brown

    • Jack Brown

      Hey Jim,

      Can’t thank you enough! I have been messing with this issue for two weeks and mucho frustration. I also have 4 converters.

      I added 12 volt bulbs to the input side of the converter and miracle of miracles it works. I had to add a bulb for each input lead for complete function.

      So now I’m off to Radio Shack in hopes they might still sell resistors.

      The inter web is a wonderful thing!! Thanks again.

      Jack Brown

    • Jim Keith

      Try adding a normal 12V light bulb connected between the input and chassis –if that works, then replace the bulb a 100Ω 5W resistor.

  • DanDuda

    Thanks to these instructions I was able to make some operating units. However, there were some points of confusion due to errors in both the schematic and your description of how the circuit works. The arrow in the MOSFET points the wrong direction for a P channel. What is indicated in the schematic is an N channel.
    In the “How it works” section you said, “The 12V power from both the Stop and Turn signals are OR connected by high current schottky diodes (D1 & D2) and fed to the drain of a P-Channel MOSFET (Q1).” According to the schematic the connection from diodes D1 & D2 is to the source – not the drain – of Q1. Once I figured that out, I was able to get the circuit working.

    • Jim Keith

      Thanks for the feedback–my apology for the poor MOSFET symbol & description error–my schematic software is limited in this regard. Glad you got it working OK.

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