Ignorant LED questions (low power LED usage)

Igor, juvenile hoodlums will thank you. Leaning out of a car window clutching a baseball bat while going fast in the dark makes it very hard to see mailboxes. Having one lit up by LEDs would be a blessing.

So much for bad humor:

A switching power supply would be the way to go. I doubt that any are made specifically for LED supplies, and finding one that'll be efficient at the low power levels you're looking at will be a challenge. So you have your work cut out for you -- but you're known to be energetic.

On a sunny day (Mon, 12 Feb 2007 12:02:25 -0600) it happened Ignoramus21592 wrote in :

Well, you could have put some LEDs in series on that 12V, and create a constant current source with some FET or transistor. This is the classic low component count LED constant current source:

k a k a k a k a ------ LED1--LED2--LED3..... LEDn--------- PLUS 12 V | |--- ---->| JFET | |--- | | select | [ ] R for 25 mA | | ---------------------------------------------------- MINUS

See above. More efficient even is a switchmode current regulator ....

Well light at night, sun during the day, you need at least a battery of sorts.

Long story, many solutions, late at night, whatdoyouknow. Over to somebody else.

There are some chips made for the purpose but they tend to be hard to find, costly, and are primarily for high power leds.

Easiest way to save some power is to just pulse the LED with a much higher current for a much shorter duty cycle - or use one of the flashing LEDs or get a hold of a LM3904 and use that to flash your leds.

Solar powered - you'd want it to shut off while light outside.

Could one say that impedance matching is required?

Match load and supply or use something acting like an imaginary DC to DC transformer (ex. smps). D from BC

LED's are efficiently driven by switchmode current supplies. There are now dozens of different chips available from just about every major player.

Readily available from a dozen manufacturers.

-- Many thanks,

Don Lancaster voice phone: (928)428-4073 Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552 rss:

Please visit my GURU's LAIR web site at

So there are. A quick web search got me many relevant hits.

I know what you mean, but so far, all my mailbox accidents were NOT due to juvenile hoodlums. They were due to people not knowing how to drive. Once it was a snow plow, and another time it was some odiot who left his or her car mirror lying next to the mailbox.

Thanks Tim... I am still hoping that this is a standard problem...

That's a great idea, very simple and elegant. Thanks.

Definitely, yes. Battery plus solar charger.

Also very wrong. The efficiency would be negligible.

The trick is to provide JUST ENOUGH VOLTAGE to allow a desired current to flow through both the LEDs and their (hopefully very small) current sensing resistor.

On a sunny day (Mon, 12 Feb 2007 13:35:12 -0700) it happened Don Lancaster wrote in :

Not really, a few volts drop with 12V and a few more when battery goes way up say to 14 V or more, will protect the LEDs, keep teh current constant very well. It is more constant and more efficient then each LED a resistor. It is not as efficient as _most_ switchmodes. And it is not wrong. The only thing that is wrong is that your prediction I would have LED lights in my house by now never happened. :-) And will not happen in the coming years. LOL

Greetings Don! Perhaps you missed that there were many LEDs in series? Although not as good as a switcher, it would be many-fold better than a single LED resistored to 12v.

Cheers, James Arthur

...

I've seen ordinary bicycle retroreflectors on mailboxes. They take zero power. :-)

However, if the vandal is driving with no lights, I guess it wouldn't help much. ;-)

(and if you put a solar panel on top of your mailbox, it would probably get stolen. =:-O )

Good Luck! Rich

I don't know where you live, but in snow country I've seen mailbox posts set back 4-6' from the shoulder, with the box on a cantilevered arm, like a fence gate. It stays in place by ordinary friction, or maybe there's a piece of breakaway wire. This does two things - the snowplows don't plow the post down, and if the box gets hit, it just swings out of the way.

Hmm - this suggests an additional scenario: the box bounces off the front of their car, swings around, and breaks their back window. ;-)

Cheers! Rich

it could be, in fact, much closer to 100% than to 0%. Depending on voltage drop per diode.

That's exactly what I did.

Cat# SPL05 is a solar powered, 3 LED lite - charges in daylight, turns on automatically at night. $4.50

I don't think you can beat that. Even if it is less power efficient than some pulser or switcher design, it's a cheap "done deal" that seems to fit your needs. Besides it provides the solar cell you'll need if you build one.

Ed

Ed, looks very interesting (the number is actually SPL-05). Thanks.

some sort of "switcher" eg "joule thief" (google)

those cheap solar led lights often use a similar approach too.