1. Home
  2. Hobby Electronics Basics
  3. Flash controller for 12V brake/tail system

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

Reply to
David Bonnell
Loading thread data ...

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

Reply to
Stephen J. Rush

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.

--
"I\'m never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5
Reply to
Jamie

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.

Reply to
David Bonnell

=_NextPart_000_002B_01C8041E.5FD3B8A0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable

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
Reply to
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.

Reply to
Stephen J. Rush

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:

formatting link

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...

Reply to
David Bonnell

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.

Reply to
David Bonnell

=_NextPart_000_00A5_01C80506.643385E0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable

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.

- > 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. >=20 > 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. >=20 > 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)? >=20 > Cheers, > Dave >

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
Reply to
Bob Monsen

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.