In article , Larry wrote:
(GregS) wrote in news:gv1m1f$dqm$1
:

A thought. That circuit should allready have the necessary capcitance
on the output. It "should". Adding input filtering may be required
to the domes. Boat power may be noisey, but we are just guessing.
Gradual dimming is usually the result of driving too hard. They also
short term dim when too hot.

greg



Why not just use a 7809 regulator IC, or something heavier duty if it draws
more than 1A? The output will be EXACTLY 9VDC, no more no less, no matter
what the battery does, with 5.2V of overhead space to play with. You won't
even see the engine start dimming them unless the battery is dead or all
the terminals are corroded up.

On the cheap, let the regulator drive a big NPN power transistor if you
need more current cheaply. The LEDs will run fine on 8.4VDC steady from an
Emitter follower with 9V on its base.

What a boat DOESN'T need is another digital electronic transmitter like a
DC-DC converter for stupid simple lighting making a racket in every RADIO
aboard.....Let's keep it analog and QUIET.

7809's are about 20 cents.....2N3055 power transistors give you 15A for a
couple bucks....Remember the case is at + battery voltage on the collector.

Its all about efficiency. The best way is use 4 high power devices in series
and you will need nothing else. Might be too bright.

greg