How to power a single LED from a 12v supply?

[Pete Verdon]

Over this coming winter, I'm planning a complete refit of my small
boat's electrical system. One of the things I would like to add is a
tiny light at the masthead, to illuminate at night the flag I fly there
to tell the wind direction. This light needs to be just bright enough to
make out the flag immediately above it - it should be invisible from any
appreciable distance. This is because the anti-collision regulations lay
down a complex (but logical) system of lights for identifying different
types of vessel, and having a random superfluous masthead light would
interfere with that. I haven't tested yet, but I suspect a single
standard LED might be all that's needed for dark-adapted eyes to pick
out the mostly-white flag nearby.

This light would be powered by a feed from the "official" navigation
lights further down the mast. These run at a nominal 12v - perhaps up to
14.5 when the engine is running.

How would I best power a single LED from a 12v source? My electronic
learning stopped when I left school, so I don't really know what I
should be looking for. Are there standard voltage convertor chips which
would be suitable?

Like many sailors without a shore power hookup, I'm twitchy about power
usage, so something that doesn't gratuitously waste energy into a big
heatsink would be good, even if compared to other loads the question is
more psychological than practical.

Thanks for any advice you can give,

Pete

(For any fellow sailors reading this, who are used to seeing a Windex
via the overspill from a tricolour, note that this is a
traditionally-rigged boat with a plain truck masthead, so that doesn't
apply.)

 

[Rich Grise]

On Tue, 05 Oct 2010 23:23:54 +0100, Pete Verdon wrote:

Over this coming winter, I'm planning a complete refit of my small
boat's electrical system. One of the things I would like to add is a
tiny light at the masthead, to illuminate at night the flag I fly there
to tell the wind direction. This light needs to be just bright enough to
make out the flag immediately above it - it should be invisible from any
appreciable distance. This is because the anti-collision regulations lay
down a complex (but logical) system of lights for identifying different
types of vessel, and having a random superfluous masthead light would
interfere with that. I haven't tested yet, but I suspect a single
standard LED might be all that's needed for dark-adapted eyes to pick
out the mostly-white flag nearby.

This light would be powered by a feed from the "official" navigation
lights further down the mast. These run at a nominal 12v - perhaps up to
14.5 when the engine is running.

How would I best power a single LED from a 12v source? My electronic
learning stopped when I left school, so I don't really know what I
should be looking for. Are there standard voltage convertor chips which
would be suitable?

Like many sailors without a shore power hookup, I'm twitchy about power
usage, so something that doesn't gratuitously waste energy into a big
heatsink would be good, even if compared to other loads the question is
more psychological than practical.

Thanks for any advice you can give,

(For any fellow sailors reading this, who are used to seeing a Windex
via the overspill from a tricolour, note that this is a
traditionally-rigged boat with a plain truck masthead, so that doesn't
apply.)

I just googled "12V led light" without the quotes and got "About 564,000
results"

Good Luck!
Rich

 

[Jamie]

Pete Verdon wrote:

Over this coming winter, I'm planning a complete refit of my small
boat's electrical system. One of the things I would like to add is a
tiny light at the masthead, to illuminate at night the flag I fly there
to tell the wind direction. This light needs to be just bright enough to
make out the flag immediately above it - it should be invisible from any
appreciable distance. This is because the anti-collision regulations lay
down a complex (but logical) system of lights for identifying different
types of vessel, and having a random superfluous masthead light would
interfere with that. I haven't tested yet, but I suspect a single
standard LED might be all that's needed for dark-adapted eyes to pick
out the mostly-white flag nearby.

This light would be powered by a feed from the "official" navigation
lights further down the mast. These run at a nominal 12v - perhaps up to
14.5 when the engine is running.

How would I best power a single LED from a 12v source? My electronic
learning stopped when I left school, so I don't really know what I
should be looking for. Are there standard voltage convertor chips which
would be suitable?

Like many sailors without a shore power hookup, I'm twitchy about power
usage, so something that doesn't gratuitously waste energy into a big
heatsink would be good, even if compared to other loads the question is
more psychological than practical.

Thanks for any advice you can give,

Pete

(For any fellow sailors reading this, who are used to seeing a Windex
via the overspill from a tricolour, note that this is a
traditionally-rigged boat with a plain truck masthead, so that doesn't
apply.)
not knowing what type of LED you plan on using... I'll give you some

rough numbers.

470 ohm R at .5 watt or better (common value)..

attached it in series to one of the legs, and attach to 12 volt source.

if the LED does not light, reverse connections from the 12 volt source...
The + should be leading into the anode side of the LED.

Bye..

 

[Pete Verdon]

Rich Grise wrote:

I just googled "12V led light" without the quotes and got "About 564,000
results"

And did you notice that all those results were referring to readymade
LED light fittings, "bulbs", or other assemblies that are unrelated to
my query?

Pete

 

[Pete Verdon]

Jamie wrote:

470 ohm R at .5 watt or better (common value)..

attached it in series to one of the legs, and attach to 12 volt source.

Hmm, so simple

Presumably, if 2v is being dropped across the LED and the remaining 10v
across the resistor, and they're both passing the same current, then
five times the energy is being wasted in the resistor. I guess that
can't be helped?

Pete

 

[John Larkin]

On Wed, 06 Oct 2010 00:38:36 +0100, Pete Verdon
<news@verdonet.organisation.unitedkingdom.invalid> wrote:

Jamie wrote:

470 ohm R at .5 watt or better (common value)..

attached it in series to one of the legs, and attach to 12 volt source.

Hmm, so simple

Presumably, if 2v is being dropped across the LED and the remaining 10v
across the resistor, and they're both passing the same current, then
five times the energy is being wasted in the resistor. I guess that
can't be helped?

Pete

You could use two or three LEDs in series and reduce the current
proportionally. That would multiply the electrical efficiency.

For example, one led + 470 ohms uses about 20 mA. Electrical
efficiency is 2/12 = 0.16

Two leds + 800 ohms uses 10 mA and makes the same amount of light.
Efficiency is 0.33.

But both are pretty small amounts of current. A 40 A-H battery would
supply 20 mA for 2000 hours.

John

 

[DJ Delorie]

It's possible to make a very small switching power supply from only a
few parts (IC, inductor, two caps) to supply "just enough" voltage for
the LED and a small current limiting resistor, if you're really
concerned about a few milliamps. However, there are also special ICs
designed to drive LEDs - including white LEDs - at a constant current
very efficiently.

A quick perusal of Digikey (search for "white led" then choose
step-down) found the ZXLD1366 series, for example. 6-60V input,
0-100 mA output, up to 97% efficient.

 

[Rich Grise]

On Wed, 06 Oct 2010 00:34:15 +0100, Pete Verdon wrote:

Rich Grise wrote:

I just googled "12V led light" without the quotes and got "About 564,000
results"

And did you notice that all those results were referring to readymade
LED light fittings, "bulbs", or other assemblies that are unrelated to
my query?

Sorry. I guess I assumed that by "How to power a single LED from a 12v

supply?" that you wanted to power an LED from 12V.

Those LEDs run off 12V, they've got their current regulator built-in.

What's the real problem? Are you looking to light your boat, or are you
looking for a construction project?

Thanks,
Rich

 

[petrus bitbyter]

"Pete Verdon" <news@verdonet.organisation.unitedkingdom.invalid> schreef in
bericht news:i8g8hr$kat$1@news.eternal-september.org...

Over this coming winter, I'm planning a complete refit of my small boat's
electrical system. One of the things I would like to add is a tiny light
at the masthead, to illuminate at night the flag I fly there to tell the
wind direction. This light needs to be just bright enough to make out the
flag immediately above it - it should be invisible from any appreciable
distance. This is because the anti-collision regulations lay down a
complex (but logical) system of lights for identifying different types of
vessel, and having a random superfluous masthead light would interfere
with that. I haven't tested yet, but I suspect a single standard LED might
be all that's needed for dark-adapted eyes to pick out the mostly-white
flag nearby.

This light would be powered by a feed from the "official" navigation
lights further down the mast. These run at a nominal 12v - perhaps up to
14.5 when the engine is running.

How would I best power a single LED from a 12v source? My electronic
learning stopped when I left school, so I don't really know what I should
be looking for. Are there standard voltage convertor chips which would be
suitable?

Like many sailors without a shore power hookup, I'm twitchy about power
usage, so something that doesn't gratuitously waste energy into a big
heatsink would be good, even if compared to other loads the question is
more psychological than practical.

Thanks for any advice you can give,

Pete

(For any fellow sailors reading this, who are used to seeing a Windex via
the overspill from a tricolour, note that this is a traditionally-rigged
boat with a plain truck masthead, so that doesn't apply.)

I doubt a simple cheap white low power LED will light much more then just
itself. This ones have a forward voltage drop of between 3Vdc and 4Vdc can
handle a maximum current of 20mA. For a try out take three of them in series
with a 180R or 220R resistor, depending on the forward voltage of the LEDs.

Beware: LEDs can handle only a limited reverse voltage. The LEDs mentioned
above usually 5V. So connecting them the wrong way you may blow them. A
Schottky diode bridge will prevent this and you can lower the resistor to
180R or 150R.

In the unlikely event that the LEDs produce too much light, replace a LED by
an extra series resistor. If you need more light, you'll have to find other
(more expensive) LEDs. They draw more current and so produce more light.
There are tens or even hundreds of types so you may need some time to find
out.

petrus bitbyter

 

[Tim Wescott]

On 10/05/2010 03:23 PM, Pete Verdon wrote:

Over this coming winter, I'm planning a complete refit of my small
boat's electrical system. One of the things I would like to add is a
tiny light at the masthead, to illuminate at night the flag I fly there
to tell the wind direction. This light needs to be just bright enough to
make out the flag immediately above it - it should be invisible from any
appreciable distance. This is because the anti-collision regulations lay
down a complex (but logical) system of lights for identifying different
types of vessel, and having a random superfluous masthead light would
interfere with that. I haven't tested yet, but I suspect a single
standard LED might be all that's needed for dark-adapted eyes to pick
out the mostly-white flag nearby.

This light would be powered by a feed from the "official" navigation
lights further down the mast. These run at a nominal 12v - perhaps up to
14.5 when the engine is running.

How would I best power a single LED from a 12v source? My electronic
learning stopped when I left school, so I don't really know what I
should be looking for. Are there standard voltage convertor chips which
would be suitable?

Like many sailors without a shore power hookup, I'm twitchy about power
usage, so something that doesn't gratuitously waste energy into a big
heatsink would be good, even if compared to other loads the question is
more psychological than practical.

Thanks for any advice you can give,

Pete

(For any fellow sailors reading this, who are used to seeing a Windex
via the overspill from a tricolour, note that this is a
traditionally-rigged boat with a plain truck masthead, so that doesn't
apply.)

A single LED wants to run at between 1.5V and 2.5V, depending on color
(there may be some 3V ones out there, I dunno). This means that if you
want to run _one_ LED from 12V you're going to have to either drop a
whole lotta volts through a resistor (turning those volts into heat), or
you're going to have to run a switching converter (which you probably
don't want to undertake).

So:

1: Why not use several LED's in series, enough to make about 6V
nominally (check the volts vs. temperature, and go for "works right when
it's really, really cold). Use little ones, and run less current --
same light, less current, same voltage, good.

2: Instead of shining a light _up_ to reflect all over from a white flag
(or to just plain reflect poorly when things get dirty or wet), why not
put a wind vane up there with LEDs along the bottom, shining _down_.
That way instead of most of the photons from the light being scattered
every which way, most of them will land on deck, meaning that far more
of them will land on your retinas for the amount of energy burned off
the battery? It'll require a slip ring or some similar cleverness, but
it'll be loads more efficient.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

 

[Winston]

Pete Verdon wrote:

Over this coming winter, I'm planning a complete refit of my small
boat's electrical system. One of the things I would like to add is a
tiny light at the masthead, to illuminate at night the flag I fly there
to tell the wind direction.

(...)

How about a self-glowing flag made with photo luminescent nylon thread?
http://lightlead.com/Photoluminescent_Products/Glow_Yarns.htm

Shine a little UV light at it and it will glow for hours.
http://www.ultravioletledflashlights.com/NightShark.html

--Winston

 

[Dan]

On 10/5/2010 11:48 PM, Winston wrote:

Pete Verdon wrote:
Over this coming winter, I'm planning a complete refit of my small
boat's electrical system. One of the things I would like to add is a
tiny light at the masthead, to illuminate at night the flag I fly
there to tell the wind direction.

(...)

How about a self-glowing flag made with photo luminescent nylon thread?
http://lightlead.com/Photoluminescent_Products/Glow_Yarns.htm

Shine a little UV light at it and it will glow for hours.
http://www.ultravioletledflashlights.com/NightShark.html

--Winston

I found out by accident a red laser pointer can darken a piece
glowing sew on tape. I have a flashlight that has UV, visible and a
pointer. I found the laser makes a dark spot on the tape and I could
actually write on it.

It doesn't have anything to do with the topic, just an interesting
discovery.

Dan, U.S. Air Force, retired

 

[Winston]

Dan wrote:

(...)

I found out by accident a red laser pointer can darken a piece glowing
sew on tape. I have a flashlight that has UV, visible and a pointer. I
found the laser makes a dark spot on the tape and I could actually write
on it.

It doesn't have anything to do with the topic, just an interesting
discovery.

Whoa! 'Nothing new under the sun', huh?

An enterprising young engineer could use that to create a demonstration
of xerography / laser printing for a deserving outfit like the
Children's Discovery Museum.

Cool!

--Winston

 

[haaaTchoum]

Hi,

You can try a constant current generator with two transistors :

http://img829.imageshack.us/i/26686413.gif/

In this example, you are sure to get around 20mA between 8 and 15V as input
voltage. Of course, this is an example to modify according to your LED
power.

Cdlt,
F


"Pete Verdon" <news@verdonet.organisation.unitedkingdom.invalid> a écrit
dans le message de news: i8g8hr$kat$1@news.eternal-september.org...

Over this coming winter, I'm planning a complete refit of my small boat's
electrical system. One of the things I would like to add is a tiny light
at the masthead, to illuminate at night the flag I fly there to tell the
wind direction. This light needs to be just bright enough to make out the
flag immediately above it - it should be invisible from any appreciable
distance. This is because the anti-collision regulations lay down a
complex (but logical) system of lights for identifying different types of
vessel, and having a random superfluous masthead light would interfere
with that. I haven't tested yet, but I suspect a single standard LED might
be all that's needed for dark-adapted eyes to pick out the mostly-white
flag nearby.

This light would be powered by a feed from the "official" navigation
lights further down the mast. These run at a nominal 12v - perhaps up to
14.5 when the engine is running.

How would I best power a single LED from a 12v source? My electronic
learning stopped when I left school, so I don't really know what I should
be looking for. Are there standard voltage convertor chips which would be
suitable?

Like many sailors without a shore power hookup, I'm twitchy about power
usage, so something that doesn't gratuitously waste energy into a big
heatsink would be good, even if compared to other loads the question is
more psychological than practical.

Thanks for any advice you can give,

Pete

(For any fellow sailors reading this, who are used to seeing a Windex via
the overspill from a tricolour, note that this is a traditionally-rigged
boat with a plain truck masthead, so that doesn't apply.)

 

[petrus bitbyter]

----- Original Message -----
From: "haaaTchoum" <a@a.com>
Newsgroups: sci.electronics.basics
Sent: Wednesday, October 06, 2010 11:14 PM
Subject: Re: How to power a single LED from a 12v supply?

"Pete Verdon" <news@verdonet.organisation.unitedkingdom.invalid> a écrit
dans le message de news: i8g8hr$kat$1@news.eternal-september.org...
Over this coming winter, I'm planning a complete refit of my small boat's
electrical system. One of the things I would like to add is a tiny light
at the masthead, to illuminate at night the flag I fly there to tell the
wind direction. This light needs to be just bright enough to make out the
flag immediately above it - it should be invisible from any appreciable
distance. This is because the anti-collision regulations lay down a
complex (but logical) system of lights for identifying different types of
vessel, and having a random superfluous masthead light would interfere
with that. I haven't tested yet, but I suspect a single standard LED
might be all that's needed for dark-adapted eyes to pick out the
mostly-white flag nearby.

This light would be powered by a feed from the "official" navigation
lights further down the mast. These run at a nominal 12v - perhaps up to
14.5 when the engine is running.

How would I best power a single LED from a 12v source? My electronic
learning stopped when I left school, so I don't really know what I should
be looking for. Are there standard voltage convertor chips which would be
suitable?

Like many sailors without a shore power hookup, I'm twitchy about power
usage, so something that doesn't gratuitously waste energy into a big
heatsink would be good, even if compared to other loads the question is
more psychological than practical.

Thanks for any advice you can give,

Pete

(For any fellow sailors reading this, who are used to seeing a Windex via
the overspill from a tricolour, note that this is a traditionally-rigged
boat with a plain truck masthead, so that doesn't apply.)


Hi,

You can try a constant current generator with two transistors :

http://img829.imageshack.us/i/26686413.gif/

In this example, you are sure to get around 20mA between 8 and 15V as
input voltage. Of course, this is an example to modify according to your
LED power.

Cdlt,
F

Excellent advice... Assuming the OP to be able to understand the schematic
and build the circuit. Using a 12V battery, you can take two LEDS in series.
Twice the light for the same energy cost. Even three LEDs may be possible
depending on the forward voltage of the LEDs.

petrus bitbyter

 

[default]

On Tue, 05 Oct 2010 23:23:54 +0100, Pete Verdon
<news@verdonet.organisation.unitedkingdom.invalid> wrote:

Over this coming winter, I'm planning a complete refit of my small
boat's electrical system. One of the things I would like to add is a
tiny light at the masthead, to illuminate at night the flag I fly there
to tell the wind direction. This light needs to be just bright enough to
make out the flag immediately above it - it should be invisible from any
appreciable distance. This is because the anti-collision regulations lay
down a complex (but logical) system of lights for identifying different
types of vessel, and having a random superfluous masthead light would
interfere with that. I haven't tested yet, but I suspect a single
standard LED might be all that's needed for dark-adapted eyes to pick
out the mostly-white flag nearby.

This light would be powered by a feed from the "official" navigation
lights further down the mast. These run at a nominal 12v - perhaps up to
14.5 when the engine is running.

How would I best power a single LED from a 12v source? My electronic
learning stopped when I left school, so I don't really know what I
should be looking for. Are there standard voltage convertor chips which
would be suitable?

Like many sailors without a shore power hookup, I'm twitchy about power
usage, so something that doesn't gratuitously waste energy into a big
heatsink would be good, even if compared to other loads the question is
more psychological than practical.

Thanks for any advice you can give,

Pete

(For any fellow sailors reading this, who are used to seeing a Windex
via the overspill from a tricolour, note that this is a
traditionally-rigged boat with a plain truck masthead, so that doesn't
apply.)

Choose a high efficiency LED for lots of light at little current.

A single led isn't usually a reason to get all sophisticated with
drive circuitry since you want it fairly dim... and even a 9 LED
pocket torch will run for >20 hours on a few AAA batteries.

I'd mount a single 20 milliamp LED, wire it in series with two
resistors, one to limit the current to 20 milliamps and one variable
wired as a rheostat to adjust the current between 100% and 10%.

On a 12V battery that would be ~470/500 ohms fixed and 5,000 ohms
variable in series with the LED. I'd use a 5 watt wire wound
variable.

Choose a color to match the wind telltale or use white.

Use a narrow angle LED to keep the light only on the flag. 6 degree
Led aimed up at the flag shouldn't be too visible to others, and you
can always shield the light so it only illuminates the flag.
--

 

[Pete Verdon]

petrus bitbyter wrote:

From: "haaaTchoum" <a@a.com

You can try a constant current generator with two transistors :

http://img829.imageshack.us/i/26686413.gif/

In this example, you are sure to get around 20mA between 8 and 15V as
input voltage. Of course, this is an example to modify according to your
LED power.

Excellent advice... Assuming the OP to be able to understand the schematic
and build the circuit.

I can certainly build the circuit straight from the diagram, although I
don't necessarily understand it well enough to make substitutions (eg a
different transistor due to availability, or different resistor values
for different LEDs)

This does look like a very interesting possibility.

Pete

 

[Pete Verdon]

Tim Wescott wrote:

2: Instead of shining a light _up_ to reflect all over from a white flag
(or to just plain reflect poorly when things get dirty or wet), why not
put a wind vane up there with LEDs along the bottom, shining _down_.

That's a nice, logical solution to this specific problem, but not really
appropriate to the boat generally. This is a traditional-style
gaff-rigged boat with a wooden mast, and a wind-vane would look out of
place compared to a traditional burgee on a bamboo staff.

Thanks too for all the other suggestions posted.

Pete

 

[ehsjr]

Pete Verdon wrote:

petrus bitbyter wrote:

From: "haaaTchoum" <a@a.com


You can try a constant current generator with two transistors :

http://img829.imageshack.us/i/26686413.gif/

In this example, you are sure to get around 20mA between 8 and 15V as
input voltage. Of course, this is an example to modify according to
your LED power.


Excellent advice... Assuming the OP to be able to understand the
schematic
and build the circuit.


I can certainly build the circuit straight from the diagram, although I
don't necessarily understand it well enough to make substitutions (eg a
different transistor due to availability, or different resistor values
for different LEDs)

This does look like a very interesting possibility.

Pete

It's a current regulator circuit. Here's a brief explanation:
R3 (33 ohms in the circuit) sets the current that will go through
the LED, regardless (within reason) of the voltage source.

R2 applies + to the base of Q1 making Q1 conduct and allowing the LED
to draw current through R3 and Q1.

When the current through R3 causes a voltage drop across R3 that equals
about .6 to .7 volts, Q2 conducts and creates a voltage drop across R2,
lowering the drive to the base of Q1, which in turn limits the amount
of current Q1 can conduct.

Since we know R3 is 33 ohms, and that Vbe for Q2 is around .6 to .7
volts, we can figure the amount of current that will be allowed by
using ohms law: I = E/R so I = .7/33 or about 21 mA. If we figure
based on .6 volts, I = .6/33 or about 18 mA

Provided we supply the circuit with a reasonable voltage, the current
through R3 (and therefore the LED) will be constant.
Reasonable in this case means a minimum voltage high enough to light
the LED in the circuit, and a maximum voltage low enough so that
the transistors maximum rating is not exceeded. Your 12 volt supply
is fine.

As to substituting parts: for a typical LED, you can use any NPN
transistors you have on hand. The typical Vbe will be around .6 to .7
volts. There is nothing critical about R2 - it is chosen to keep Q2's
collector current well below maximum. R3 is not critical either, but
it is chosen so that the LED maximum current is not exceeded.

Now, if you were to use a high power white LED, you need to select
components for that higher power - you can't just use whatever
you have on hand - and a heat sink for Q1 may be needed.

There is an even simpler circuit using an LM317 regulator IC:

-----
+12 -----in|LM317|out---+
----- |
adj [R]
| |
+---------+
|
[LED]
|
Gnd --------------------+

The value for resistor R is computed by the formula R = 1.25/I
where I is the current you want the LED to draw. Say you use a
typical LED and you want the current through it to be set about
20 mA. A 62.5 ohm resistor would provide that, and a standard
value of 62 ohms, or 56 ohms or 68 ohms would be close enough,
yielding currents of ~ 20.16, 22.32 and 18.38 mA, respectively.

You can also use the LM317 or the two transistor circuit with
LEDs in series.

Ed

 

[Rich Grise]

On Wed, 13 Oct 2010 00:39:17 +0100, Pete Verdon wrote:

Tim Wescott wrote:

2: Instead of shining a light _up_ to reflect all over from a white flag
(or to just plain reflect poorly when things get dirty or wet), why not
put a wind vane up there with LEDs along the bottom, shining _down_.

That's a nice, logical solution to this specific problem, but not really
appropriate to the boat generally. This is a traditional-style gaff-rigged
boat with a wooden mast, and a wind-vane would look out of place compared
to a traditional burgee on a bamboo staff.

Thanks too for all the other suggestions posted.

Well, if you want a floodlight illuminating your burgee, why are you

making it into such an ordeal? Just get a 12V LED thingie, and slap
it up there.

http://www.superbrightleds.com/
http://www.theledlight.com/
http://www.superbrightleds.com/cgi-bin/store/index.cgi?action=DispPage&Page2Disp=%2FBA9S6_specs.htm
http://www.futurlec.com/LED_Lamps.shtml

Or any of thousands more.

Or you can get a plain ol' white LED,
http://www.google.com/search?hl=en&q=white+LED&aq=f&aqi=g10&aql=&oq=&gs_rfai=

and slap a series resistor on it: the resistor value will, of course,
be:

(12V - VLED) / (ILED)

But I can't understand what it is that's making you reject all those
wonderful other suggestions - if you want to save power, you could
use a simple PWM circuit.

But, if none of these suggestions are good enough for you, then do
whatever you wanted to do in the first place, and quit sniveling.

"I want a circuit!"
"Suggestion A"
"No, that's not it."
"suggestion b"
"no, that's not it."
"suggestion c"
"no, that's not it."

Just tell me what answer you want, and I'll be happy to tell you what
you want to hear. ;-)

Good Luck!
Rich