Re: Blinking third break light

hboothe_at_gte.net
Date: 03/16/05 

Date: 15 Mar 2005 19:57:55 -0800

Bryan, there have been a few posts in reply to your question but other
than to raise the quasi relevant question of legality (I've seen them
flash on motorcycles) they're not really addressing your question.

The good news is 1) I know how to do it, 2) I've seen and have a
schematic for something really similar. The bad news is that I'm not
expert so my experiences and knowledge are limited (kinda the deaf,
dumb, and blind leading the blind. :)

That said, I read this schematic about 5 years ago and it was for
flashing the brake light on a motorcycle. There are two ways you can
decide to have that light flash; continuously while the brake light
circuit is energized or to have it flash some number of times and then
latch on (blink - blink - blink - stay on....). Either of them will be
simple to implement.

Let's put into words that which is easier said than done; locate the
wire that runs to the Elizabeth Dole light (third brake light),
preferably somewhere inside the car instead of inside the housing (for
space, appearance, and accessibility considerations). Bascially, just
cut the positive wire that goes to that brake light. The raw end of
the wire from the chassis is now your source voltage; it should have
voltage only when the brake light circuit is energized. The other raw
end, that goes to the Elizabeth Dole light is the output that you'll
hook up to your circuit you're going to build.

The circuit I use consists of basically 8 parts: one potentiometer
(100k), one capacitor (10 micro F), one 4049 hex buffer, one 4017
decade counter, one 680 Ohm resistors ,x number of 4148 diodes (the
number of outputs from the 4017 you intend to use), 1 2N4401
transistor, one more 4148 diode, and finally one relay.

It's a really simple thing once you've played with it and seen it
(believe me, electronics isn't my area of expertise and yet I think
it's pretty simple). The 4017 decade counter does just what it seems
like it would do; it counts input pulses. It sends one of 10 outputs
high (on) depending on the number of input pulses it gets. I use the
4049 hex buffer to generate the pulses (I have no idea how it does it,
I just read it in the original schematic I read). I tried to use a 555
timer to produce the pulses but could never get it to work right.
Anyway, the capacitor is used with the 4049 hex buffer I presume as a
filter, and the pot is used to adjust the output pulse frequency. The
4017 receives the pulses as input and sends each of its outputs high in
turn. You tie the output through a diode to the resistor. Then,
finally, you tie in the transistor, final diode, and relay. I don't
know why they used the transistor, but I suspect it's because the 4017
might not be able to sustain the load of the relay on it's outputs.
Here's where it gets fun.

If you want it to blink on and off three times and then stay on you
simply tie outputs 1, 3, 5, through diodes, to the resistor which is
tied to the transistor. Then you tie output 7 to the resistor AND to
the Clock Enable pin on the 4017 (tying its own output #7 to the Clock
Enable) which "freezes" the whole thing. It'll stay locked on like
that until the whole circuit gets reset (when the circuit loses power
when the brake circuit de-energizes as the pedal is released). The
output trips on the transistor which applies power (from the up-stream
brake circuit) to the relay (any NO automotive relay that can handle
the load (check out a horn relay for a 1998 Dodge Ram 1500 at an
autoparts store for around 8 bucks and which has nice-sized legs for
female blade connectors)). You use the second 4148 to kill any spike
voltage from the coil in the relay.

Now, I've never tried to see if one will reset when you cut power to
it, but the original schematic that I saw worked that way, I *think*.

Unfortunately, it's near impossible to try to describe to you what the
connections/schematic looks like in type. This link is now dead, but
if you could find some web archive site they might have it (I never
looked at this site/page before so I don't know if it would do what you
need or not):
http://www.electronics-lab.com/projects/automotive/002/index.html

Here's a link to a, I think, much more complicated setup, but the
schematic is nice and clean:
http://www.uoguelph.ca/~antoon/circ/flashing.htm

Unfortunately I don't have a site that would be available long enough
or reliably enough to post it in this reply. I can draw something up
and e-mail it to you, if you'd like, just let me know and post your
address and I'll see what I can do.

In the meantime, here's a simple idea...if you don't mind it flashing
constantly: http://www.oldengine.org/unfaq/sig1.htm

Super simple, it looks.

Anyway, hope that helps.

Take care.

--HC

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