Flash controller for 12V brake/tail system

Hi all,

I am trying to build a controller to flash an automotive LED brake/
tail light. The light has a 3-wire interface: white is ground, while
black/red select output brightness level. All LEDs are lit,
regardless of brightness. I'm not sure how this is accomplished
internally (the light is a 3-wire black box to me, I can't crack it
open).

During operation, I want the light to alternate between low and high
output modes (frequency/duty TBD...most likely something in the 2 Hz -
5Hz range). The simplest way as I see it is to connect the 'low' wire
to +12V and then have the 'high' wire get switched on/off by a simple
timer (a 555 in astable mode seems right for the job). I haven't
measured the current through the device yet, but I believe that it
will be more than the ~200 ma the 555 output can handle.

I want to avoid a relay if possible...so a power transistor (as a
switch) seems to be the way to go. The designs I have seen show the
(NPN) transisitor emitter connected to ground and the load connected
to the collector...clearly no good for this application since the
switching must be done on the 'positive' side of the LED.

This is a simple problem, so I'm sure it has already been solved.
Does anyone have any ideas (or links to good online resources)?

Cheers,
Dave

 

An NPN transistor will work just as well if you connect the load to the
emitter and the collector to +12V.

 

Yes it can be done that way.
Since your common led can go to the + side , the remaining 2 leads
need to be pulled to common..
using the transistor method with the emitter to common and the
collector to a remaining lead, you're simply pulling the circuit
to common when you switch on the transistor.
But then again, if the common wire to the LED package isn't designed
for + and is for - connection.
You then simply connect the collector to the + source, base with a
small resistor from the 555 timer out, and the emitter will drive a leg
of the LED package.

 

Yes it can be done that way.

Thanks! I'm new at this and don't 100% understand every element of
the circuit...or why most designs have the load connected to the
collector. As long as I get the required voltage drop/current flow,
I'm happy.

 

You might be able to do it using a flashing LED as a timing device, which would be easier, I think. I'm not sure, but I guess that a flashing LED cuts the current when it is dark.

If so, you can use a power PMOS, a resistor, and a flashing LED in the following configuration:

12V---+-------------.
| |
| |
2.2k |--|D PMOS
| G |
+----------|--|S
| |
| |
BLINKY '-- to HIGH LED thingy
LED
|
0V----+---------------- to LOW LED thingy


The PMOS should be able to handle at least 10W, so you'll also need a heatsink.

You can get it all at radio shack.

If the blinky does not work, a 555 should work just as well. However, I have heard that automotive environments are very noisy, which might cause false triggering problems. Remember to use a cap from the CV input to ground, and a fairly large bypass cap between the gnd and vcc pins.

I'm also unclear as to why there are three inputs to your LED. Can you post the mfgr and part number?

Regards,
Bob Monsen

 

With the common-emitter (load connected between the collector and the
supply) configuration, you can get voltage gain. That's important if
you're building an amplifier, but if the transistor is to operate as a
saturated switch and the input signal switches between (nearly) the supply
voltage and ground, it doesn't matter which side of the transistor
the load is on.

 

With the common-emitter (load connected between the collector and the

I want the transistor to act as a switch...either full-on or full-
off. I am using 12V - 13.2V nominal batteries (currently powering a
couple of halogen MR16 lights for cycling at night). I'm not pleased
with the light output of those little red 'blinky' LEDs powered by a
couple of AA batteries, so I decided to try a 12V LED-based stop/tail
light.

Sorry for any confusion, my electronics terminology is weak (I'm more
of a software guy).
The part is marked as a VS-L17R. I found a link, but the details are
non-existant:
http://china.autoparts007.com/manuf...l=TA0026607.jpg&Comname=VALVES COMPANY LIMTED

It is a 13-LED light that has a 3-wire automotive interface (white,
black, red). The white/common lead connected to the negative terminal
of my battery (effectively my system ground). Connecting the red lead
to +12V turns the LED array on, and connecting +12V to the black lead
causes the LEDs to become extra-bright (which would normally happen
when applying the brake)

I currently have this wired up with a manual switch (to select normal
or high brightness). What I would like to do is keep +12V connected
to the red lead (normal brightness, always-on) and use a 555-switched
transistor to pulse +12V to the black lead (high brightness).

Now I'm not sure if I should be using a power BJT or not, as the 555
will provide enough output voltage to turn on a MOSFET. I don't fully
understand the differences between MOSFET/BJT yet (or advantages/
disadvantages), so I'm off to review...

 

Now I'm not sure if I should be using a power BJT or not, as the 555

Just did some 'testing' and discovered that the light isn't current
regulated, and draws about 250 mA at 13.2 V. Jumps to 333 mA at 15
V. At 3V a single center LED turns on, and at 6V all LEDs turn on.
It's a simple low-battery indicator! My eyes are still hurting from
that 15V test.

If I knew how to get the red lens cover off I'd try to build the flash
controller inside the unit, and perhaps bump up the intensity of the
'low' beam (only 25 mA @ 12V). Hopefully a heat gun will do the
job...I can't see any other mechanical attachments.

 

I reversed the S and D labels on the picture below... beware. For PMOS, the source (S) should be connected to the 12V, and the drain (D) should be connected to the device.

Regards,
Bob Monsen

You might be able to do it using a flashing LED as a timing device, which would be easier, I think. I'm not sure, but I guess that a flashing LED cuts the current when it is dark.

If so, you can use a power PMOS, a resistor, and a flashing LED in the following configuration:

12V---+-------------.
| |
| |
2.2k |--|D PMOS
| G |
+----------|--|S
| |
| |
BLINKY '-- to HIGH LED thingy
LED
|
0V----+---------------- to LOW LED thingy


The PMOS should be able to handle at least 10W, so you'll also need a heatsink.

You can get it all at radio shack.

If the blinky does not work, a 555 should work just as well. However, I have heard that automotive environments are very noisy, which might cause false triggering problems. Remember to use a cap from the CV input to ground, and a fairly large bypass cap between the gnd and vcc pins.

I'm also unclear as to why there are three inputs to your LED. Can you post the mfgr and part number?

Regards,
Bob Monsen