Using 12v at base of transistor

Dang, I just realized a logic error in my design!! Noooo!!

Consider brakes are on at an intersection, and turning right.

I just tested this on the 4-wire tailer harness in the car.

In that case the left light stays on, and the right light flashes - and by that I mean there's steady 12v on the left wire, with 12v/0v on the right wire.

Using my logic, when both lights are on, the red brake lights will be on (good),

BUT when the flashing right light is off, the left light will be the only one on, which using my logic, will make it show the left orange arrow...

so then the result looks like "red brake lights" intermittent with "left orange turn light"... giving the impression that we're turning in the opposite direction that we're actually going.

Dang!

This just got a whole lot more complicated!

Anyone have any brilliant ideas?

Maybe a large capacitor can keep the "signal" high for enough time to allow the other side to flash without switching to the orange arrow. But then the right turn would only flash the red light, not the arrow. Which is no different from how a regular trailer does it.

I could also run an extra line from the car for the brake signal, but that would be non-standard and the trailer would still have to handle the case where some other vehicle is towing it.

I'm wondering if I need to step this up to microcontroller control. The microcontroller can be kept powered on from the trailer battery. But then the failsafe (controller off or dead) needs to still give some sort of proper indications.

For the time being I might just wire up the arrow and the red part together, with the side effect being that the brake signal will include both orange arrows on.

Thanks,

-Nico

ps, this is the circuit I had:

And a video showing how all works well until you press the brakes (both switches on) while blinking one side:

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RE: Using 12v at base of transistor

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Hello Nico,

Here's my scheme:

The relay can be any 12V coil double pole change over (often called DPDT)  you fancy. The coil resistance should be at least 100R if you want to use up your pn2222a transistors.

The relay will only be energised when both turns inputs are high and the brake lights get the power. If only one turn input is high then the turn indicators get power.

With R1 and R2 as shown the turn on threshold for Q1 on the LT input is only about 0.8V, you could reduce R2 to 100R which would set the threshold at 6V or so.

The transistor only switches the relay coil current and is protected against supply spikes and relay coil inductive spikes by Z1.

A small capacitor (100nF ceramic) from Q1 base to emitter wouldn't hurt and might reduce sensitivity to electrical noise.

MK

Thanks Michael! That looks really good, much better than my sketch   Are you using special software for that?

For the resulting behaviour, am I looking at it wrong, or would it have a similar problem as my first prototype? - what happens when the brakes are on and the blinker is on at the same time?

For example, brakes are on, and the left turn signal is on. That means TP2 stays at 12v and TP1 alternates between 0v and 12v.

First, both TP1 and TP2 are on, the relay is energized and both red brake lights are on.

Then TP1 is off, TP2 is still on, meaning the relay is off and the right indicator orange light is on, the left brake and signal lights are both off.

The result is that the taillights alternate between having 2 red brake lights and the orange right turn indicator (opposite of the actual signal indicated by the tow vehicle).

I'm hoping I'm overlooking something

Thanks,

-Nico

Hello Nico - you are correct - I hadn't registered that problem correctly.

I think I can tweak it so that it's fixed.

The software I use for pcbs is EasyPC - by far the best cheap PCB CAD that I'm aware of - it doesn't do everything but what it does it does very well.

I'll post mk2 in a bit.

MK

Hello Nico,

Here's a better design - two extra diodes but the relay only needs to be DPST so will be cheaper and easier to find.

I think this nearly does what you want - if only one input is high the appropriate turn light comes on, if both inputs are high the relay energizes and the brake lights come on (but so do the turn lights), the brake lights stay on until both inputs go low because the c terminals on the relay hold it on so long as there is power through either diode.

I don't really like the turn lights coming on at all when braking but this gets much harder since you need quite a lot of 'statefullness' in the controller - are you up for a micro ?

MK

That's really good, Michael!

You're right about the turn lights coming on when braking being a bit annoying, and might be confusing to other drivers.

It's similar to hooking up the wires directly to the red and orange lights together, but gains the benefit that signals are only using the orange.

...this gets much harder since you need quite a lot of 'statefullness' in the controller...

That's the conclusion I came to as well.

It seems that the only way to tell if it's a turn signal is by the flashing action.

are you up for a micro ?

But of course! That's probably where I'm most comfortable. My worries are that the micro would have to start up very fast, or else be kept powered up by an alternate source. I'm guessing that a capacitor would keep it running long enough between blinks though. After that it should be possible to determine when a light has been on and off, and then flash the orange indicator on the transition from 0v to 12v, maybe missing the first flash, but that's ok.

ps, the trailer does have it's own 12v power source too, so that could be used to power it, however, the failure mode (ie, no trailer power available) should provide safe function still (maybe just using the red lights, as would be the usual for trailers).

Maybe the easiest way to handle it is to add two lines from the car turn indicator wires to directly control the orange turn indicators.

Thanks,

-Nico

Hello Nico,

Simple micros with on chip RC oscillators can be up an running in under 1mS quite easily - I don't think you need store power between blinks because if the brake is on there is always at least one turn signal high to provide power.

Do you have  a micro preference - almost anything would do (PIC, AVR, ARM) and do you need it to be in a DIL package or are you OK with SO which would need a board. The whole thing will be much more reliable with a board of course.

MK

Hi Michael,

...up and running in under 1mS quite easily

That's really good to know.

For the between-blinks power store I was thinking that would be needed for the case where it's just a turn indicator flashing (no brakes). Out of curiosity I might test to see how long one will run on a capacitor.

Do you have  a micro preference

I do have a micro preference, mostly due to laziness   Because my stuff is pretty much all hobby I've decided to focus on Arduino and Raspberry Pi - I found that otherwise I end up spending my time learning new tools instead of building projects. I also have TI MSP430, and CCS isn't too bad, but Energia makes using those super simple too.

I'm still at the DIP stage. I have a few surface mount ATTiny13 chips because I needed one for an art project (it's glued to a rock and "wired" with conductive glue).

I haven't created any PCB's yet. A lot of my projects never go beyond breadboard, and the ones that did I've used perf board - a bit messy but worked well enough.

I do have some nice little 3x2x1 inch project boxes that include perf board in it, that I bought from Radio Shack when they were clearing out all that stuff before they became "The Source" here in Canada. One of those might work well for this.

In terms of controllers inventory, I have a couple of DigiSparks (ATTiny85 based) that might work well for this, or maybe one of the TI MSP430 chips because their DIP format would make them easy to add to a board.

Cheers,

-Nico

Note to self, because I'll probably need it later...

Removing 5 second startup delay for Digispark

Note to self, because I'll probably need it later...

Removing 5 second startup delay for Digispark