Using 12v at base of transistor

Short answer - no.

The base of  a transistor is current driven - think of the base emitter junction as being a diode (the arrow on the emitter shows the direction of conventional current when forward biased). When the transistor is operating the base emitter voltage of  a silicon transistor like the pn2222A will be about 0.6V (actually it will vary according to the base current).

R2 controls the base current so for 500mA in the load you'll need about 10mA in the base so make R2 = 1k2.

But actually, don't do this at all - use a relay !

The pn22222 is rated 500mA max so it will struggle to turn on a 2W light bulb (might be enough - you don't say what the load is - but remember that normal light bulbs draw perhaps as much as 10x the normal working current at switch on).

The pn2222 is rated for 40V max so it should be protected form car power supply transients.

If the load is shorted the transistor will fail.

Relays can cope with all the above - but don't forget a diode across the coil (cathode to +ve supply) to stop it generating huge voltage spikes when it is turned off.

MK

Thanks Michael!

To be more precise with the details, they are to switch the LED taillights on a camper trailer. I should test how much it draws. The description said 3W, but that might be for the whole unit.

The wiring from the car has 4 wires (ground, lights, left turn, right turn), which is designed for taillights that just have the red light: dimmer red for lights, brighter for turn blinker or brakes (brakes = left and right both on).

But, being that things have to look prettier than that, we bought a nice looking set of LED taillights that have separate orange turn indicators.

So I thought I'd use some logic to make the orange turn light go on, unless the other orange turn light is also on, in which case both red lights go on instead.

I noticed cars have cheap-and-plentiful relays. I think the one I need is called a 5 pin SPDT? the relay has a default path, and when the coil is energized it takes the other path. So the default path goes to power the orange arrow, and the coil of the relay is connected to the orange arrow of the other side, which would switch it to power the red part.

It doesn't really matter if the orange turn lights flash a bit when the brakes are pressed - gets people's attention

Here is a picture.

A big point of restoring this old Trillium trailer we bought was for my wife to paint it up (thus the nice colours and "goat girl googoo goggles" window cover). This is why the "regular" brake lights were "too normal"

-Nico

Picture perfect!

If you get a big, Zener protected MOSFET, and drive it with your dinky bipolar, you will have a smaller and quieter soln. than a relay, don't forget a heatsink and a shut-off resistor.  For an example:http://www.st.com/content/ccc/resource/technical/document/datasheet/5e/25/44/34/36/e7/49/93/CD00045633.pdf/files/CD00045…

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

Hello, I think I can do the right thing with one transistor and one relay but it needs to be double pole changeover.

When both turn lines are high the relay will energize because its coil is connected between  LT and a transistor switched on by RT. When the relay is on it routes LT and RT to the brake lights, when it is off LT and RT go to the turn indicators.

I'll scribble a circuit for you later today if I get a moment (or too bored with the real work !!).

MK

Hooray!  Naomi is cheering you on from the sidelines!! (or from the side of the Trillium)

I'm thinking the problem has moved to just controlling the yellow arrows, and having the red brake lights be directly wired up so they do the default as a regular trailer would.

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