Trailer Side Indicator Lights

[Imsolidstate] is working to add side turn signals to a trailer. These orange clearance lights are illuminated when the vehicle’s headlights are on to increase a long trailer’s visibility. They also blink along with the turn signals on the back of the trailer. A standard 6-pin lighting harness doesn’t support this functionality so the trick is to add them without altering the towing vehicle in any way. He’s using an ATtiny24 microprocessor to interpret the logic from the vehicle and then translate the turn signal and tail light data into a signal for the additional side indicators.

29 thoughts on “Trailer Side Indicator Lights

  1. You can’t wire turn signals for the side of the trailer to the LT/RT trailer signals. The turn signals are also the brake lights on most trailers. If you wire up turn signals for the side of a trailer from the left turn or right turn of the trailer wiring harness they will come on whenever the brakes are on. So when you are driving down the road both of your turn signals come on when you put on the brakes. And big rigs have dedicated turns signals that are independent of the brake signals. Go pull a trailer sometime and you’ll figure it out.

  2. I’m all for increased safety, but how long exactly is this trailer? It seems like a lot of work for something where you are going to have an indicator in the middle (rear of tow vehicle) anyway.

  3. There are applications where using a microprocessor instead of logic gates makes sense, but I don’t think this is one of them. The necessary logic could be done with just an AND circuit wired to the brake/turn bulb wires. If both bulbs are on, the output goes high and energizes a relay coil. Then you just wire the side indicators through the normally-closed contacts, so they’re inhibited from lighting up whenever the relay energizes.

    I kind of feel like people who got into electronics after the debut of microcontrollers go straight there because they don’t know any other way to do it.

  4. It’s certainly commendable when public safety is improved through innovative approaches like this. Yet when safety is the *first* priority, it is advisable to also be cognicant of reliability issues. Reliability and robustness often go hand-in-hand with simplicity, which means keeping component count, solder joint count, transistor/gate count, etc, to a bare minimum. In other words, eliminate as many potential points of failure as possible.

    I cringe when I see a micro in an application like this, and not just because it’s perhaps overkill. Take this from someone who *must* always consider liability issues, i.e. Elec. Engineer for 25+ years.

    Anyway, that aside, and without looking terribly closely at this, I’d first consider discrete diode/transistor logic. Best of luck.

  5. As far as I know, this is a legal requirement in the UK. When a tow ball is installed on the back of your car/van/whatever, a socket (sealed) is also installed next to it and wired into the car’s electrical system. When you hook up your trailer, you just insert the plug from the trailer into the socket on the vehicle, and all your brake, indicator, etc signals are reflected in the signals on the back of the trailer. It’s a standardised plug and socket, so any trailer will fit to any car.

    Maybe this just doesn’t happen in the US?

  6. @EdZ

    It’s basically the same thing in the US: 4- to 7-wire standardized plugs for passenger vehicles towing. For larger vehicles (Class VII in particular) there are additional turn indicators in the middle of the trailer where the 4- to 7-wire connector doesn’t have a provision. That’s what this guy is trying to add.

  7. Thanks EdZ! Most people aren’t getting it. Most people obviously haven’t worked on a trailer.

    Hey Orv: Your AND gates sound nice and simple, but they fail; when the brakes are on and the turn signal is on (in the off part of the flash) you are flashing the turn signal for the wrong side. Not to mention on/off brake activity while the signal is flashing makes the side signal flash erratically. That’s why micros are better, I only need one chip to do this and I can reprogram it whenever I want to make changes. Do that with logic gates.

    JDN: it’s a prototype. Calm down.

    BCK: read my article. It doesn’t work that way.

  8. Adam: I see… different systems in the US compared to europe, here we have separate lights for running, stop, and each turn signal, like on our cars…
    this way, when you connect the – to the 5w/LED side marker it lights up when the turn signal lamp is dark, and when the turn signal-bulb is lit, the side marker gets + on both sides, and goes dark, until the turn signal is lit again…

  9. Not a bad writeup. I agree, it’s a little overkill, but hey, you actually have the ability to program exactly what happens.

    I’m just really glad he used a custom setup instead of an A******. By the way, for those who think MCU’s are unreliable, they’ve been used in the automotive industry for years without fail. Heck, this is simple compared to most of the controlling systems in a vehicle.

  10. like the above complaints that a microcontroller is overkill, i would just use a few relays.
    power coming from the running lamps pin and through the switched side of the relay.
    turn signal wire goes to the relay coil.

    if he was using the microcontroller to make the trailer sidelamps blink down-the-line, then thats ok and i wont complain; but because its not, RABBLE RABBLE RABBLE.

  11. arduino? no? then i’m not interested! :p

    Overkill but still, it fills up a few minutes of the day for us to read about it and a few minutes/hours of a boring day for him to make it.


  12. This isn’t necessarily a comment for this specific project but more about the “overkill” comments a lot of posts get. It seems a lot of people have some sort of hate going on for micros and prefer to use “simpler” approaches. What I don’t get is why people feel the need to scream “overkill! I could do that with X number of relays/gates/what have you” when someone uses a micro?

    These days micros are dirt cheap (probably cheaper than buying the individual and/nand/etc chips you would use), easy to work with and offers an easy way to expand on the original idea. Where’s the harm? I understand it when people use a $40 Arduino to blink an LED but using a small PIC/AVR that you can buy for next to nothing I don’t see the point in complaining about it. Not nerdy enough or what?

    Wow, that rant went on a lot longer than I planned… sorry.

  13. The problem, as I see it, is that how do you tell between the flashing of and the indicator turning off at the end of the turn? A state-machine with a timer would do this, but then you/re back to a fair whack of logic, or a micro. I don’t know if this meets the applicable traffic codes in your state/territory/country, but how about exclusive or-ing the tail and blinker circuits? shure the lights will blink out of phase, but..

    a) they will blink and
    b) thet will return to a known state when the blinker turns off.

    74HC86 is you friend. Plus a transistor or two. and a relay or two.


  14. Good to see that I’m not the only one who uses the ATtiny24. It’s a much overlooked chip that has just about the perfect number of i/o’s for just about any small project like this.

    Overkill or not – I think it’s a great start. With some creativity, this could be extended to do much more…

  15. what i want to do is get my runing light on my trailer to flash when i put my indercater on and run as normal side light then indercater are off how would i do this and i got a 16 ft catering trailer towed my a 7.5 ton lorry please help as i need to get this working

  16. etrailer sells item 118158, they sell it to hook an RV 2 wire Brake/turnsignals to a towed vehilce 3 wire system, (Turn signals seperate from brake )

    works great if you want to add turn signals to trailer

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