Electronics gurus...how can I build this?

[Breadfan]

So, for an upcoming project I want to work on, I want to use some LED's. In my collection of electronic gooides, I have a fishing tackle box full of assorted LEDs I've collected over the years. Some are new that came in electronics "grab bag" assortments, some are pulls from equipment I found that I dissassembled and stripped for parts, and plenty are from my computer modding days (which aren't really done with, just statisfied at the moment.)

Annnnnyway...I have the split up by color, size, and type. But, I had this elusive set of about 50 or more LED's that are clear, varying sizes, and I don't know what color they will shine at. (Some could even be infrared)

I want to start testing them, but since LEDs will run at different voltages based on color and size I don't want to blow any up.

I know a save voltage to apply is about 2.5v.

What I'd really REALLY like, is to build a 12V powered rig that is capable of varied voltage. So, a 12V input with a rehostat would work.

BUT, how would I account for the amp draw of the LEDs? Would a rehostat of sufficient amperage capacity work? (BTW, LED's only take mA's of power, so any rheostat should work...but what if I want to user this for slightly higher powered items?)

To get true varied voltage, is a simple rheostat sufficeint? Or should I try to build something more complex with a true adjustable voltage regulator in it?

My thought is that according to Ohm's law, when I set my rheostat and see I'm using 2.7v to power something, and then I put something of higher amperage, I will not see 2.7v for the same resistance, right?? I could be way off here.

I'm getting a bit rusty on this stuff I guess. :) Anyway, any ideas the best way to build this, hopefully with gutted items? (I have boxes of electronic junk so I planned to build it off stuff I can find there.)

Thanks!

BTW for what it's worth, this LED project I want to do is actually Marauder related...

[jawz101]

I'm not too familiar w/ these kinds of projects but I bet the tinkerers at Make Magazine's forums (http://forums.makezine.com/)would be all over this.

[fastblackmerc]

Zack might be able to help you also.

[Sully008]

The only thing I know is if you're going to run 12v to the led, you need to put a 300 ohm resistor on one of the leads going to it otherwise you'll blow the led. Ask me how I know...

[Breadfan]

The only thing I know is if you're going to run 12v to the led, you need to put a 300 ohm resistor on one of the leads going to it otherwise you'll blow the led. Ask me how I know...
Oh yeah, I do know. And when I know the voltage and aperage stats of an LED I do the calculation with Ohms Law to find out the resistance value I want to use to match it up as close as possible.

What i want to use is a rheostat, which is basically a variable resistor. So I can "dial in" the right voltage for LED's that have unknown specs.

But as metnioned, I wouldn't mind using it for other projects that require voltage somewhere between 0 and 12 v DC. But between there the amp draws may differ...

I'm wondering if in these instances a simple rhestat would suffice of if I should build something more complex?

For instance I believe a voltage regulator will deliver the desired voltage regardless of the amp draw.

Thanks for the tips thusfar!

[shakes_26]

I use a DC power supply, with adjsutable current and voltage. With current set to off, I adjsut the voltage to 2.2v (that usually the best value to start with), then bring up the current, once I get the thing to the brightness I desire, I make a note and do the resistor value calculations.

Now you got the right idea, a pot on the 12vdc output to set the voltage down to 2.2 or so, at thsi level the current drop wont hurt anythign so you shouldnt need any current limit resistor.

Remember I= e/r, so 2.2v/600ohm = about 36mA, usually safe for most LEDs at least long enough to see what color they are and such.

you might start with a 3.3v regulator, a little RC filter on the output to stiffen the supply, and your pot to adjust the output.

the above from the school of down and dirty double E

[Breadfan]

Thanks! That's what I was looking for.

Yeah, a supply with adjustable voltage and amps would be best, that'd really be nice to have.

I found one online in a quick search... http://www.web-tronics.com/00addcreposu.html

I think it'd be kinda fun to build, maybe I'll research for some plans and what components are needed.

[Wires]

http://www.we-todd-did-racing.com/wetoddimage.wtdr/wMTI0NDA0MDZzNDEzZGZkMzF5NTQx. jpg


Instead of building a voltage regulator, you could use a voltage regulator IC to make a current regulator. For just testing diodes, it doesn't make much difference, except that once you set the variable resistor for a good current, you'd get that current for any diode you checked, regardless of its forward voltage.


The fixed resistor, R1, sets the maximum current (when variable resistor is at zero) Use 33 ohms for a max of about 38 ma, enough for high current diodes, and not so much to fry most others if you quickly turn it down.

I had a 1K for the variable resistor, which gives a minimum (at 1K) of 1.3 ma. A higher value resistor would give you a lower current at max.

The supply could be 12, 9 or even 6 volts, it just needs to be about 3 volts greater than the forward voltage of the LED to account for some voltage drop in the regulator.

Ignore the AM1, that's the current meter I used in the simulator to find the resistance values. Your test LED goes here.

[Dr Caleb]

What they said. The voltage drop across any LED is .6V

Run a 12 supply, put a 1k resistor in there, and you're good to go. IR LED's will need more current, so the 1k won't overload them.

One thing you'll have to pay attention to from your goodie box - colour varaiation. Each manufacturer and different batch will produce slight variations in the output colour of the LED. You'll want to try to match them before you make anything permanent, or it will look 'off'.

[Cheeseheadbob]

Mark, that's right. You can't spell GEEK without a double E!:D


the above from the school of down and dirty double E

[BigCars4Ever]

A quick down and dirty safe way is to buy a 3V coin cell like a CR2032. These small coin cells can only source about 10ma so it would be safe for most of your led's. Blue and white led's would be the only exception since the Vf is usually in the 3.5 to 4.5V range. All other colors will be somewhere between 1.2 and 2.1V.

[magindat]

+1 on batch color. make sure you test for color in the dark. ambient light will skew the wavelength.

Once you figure out the forward voltage of a given set of LED's, you can divide by 13.5 (alternator voltage) to see how many in series run on a '12v' car circuit. That means no need for heat-generating resistors once installed.

[drgnrdr33]

When testing the LEDs start with the rheostat or pot @ 0 V and increase slowly. LED forward drops vary with color and technology. Some are as low as 1V and as high as 2.1V. If you go over the forward drop you will blow the junction = toast.

Also, since you might have infrared LEDs that you wont see, use a current meters to see when current flows. If current flows but no visible light then you have an IR LED.

Like you said, LED current is minimal so any old pot should work off a 12V supply as long as it's 1000 ohm (ballpark) or more max.

[Wires]

Of course, you could also get completely ridiculous, as I did here:

(Picture barely shows glowing diode, this is one of those diodes with a narrow viewing angle)

http://www.we-todd-did-racing.com/wetoddimage.wtdr/wMTI0NTY3MzZzNDEzZGZkMzF5NTQx. jpg

(Knobs on lower right tell the story - 100 ohm series resistor, 4 volts across resistor and diode. "Base step generator" is not used here, and is not connected to anything. )

[Breadfan]

Thanks for all the replies guys, I think I may still look into building a variable voltage and current test rig for my bench.

Last night I solved my LED quandry, just decided to go with new LEDs to ensure they match, so I hit up RadioShack after realizing my local one actually still sells a few electronics parts.

Got some 2.1v green LEDs with 30ma current draw. Shooting for 13.5v source I wired two sets with three in a series and a 270ohm 1/2w resistor on each set.

They're only 630mcd output each though, a bit less than I was hoping for, so we'll see how it goes. Running on 12v with that resistor I didn't get to see their full output anyway.

I used an old ATX computer power supply as my 12v power source. Origianlly I was going to just use a 12v wall-wort power adapter, but found out it was putting out 18.5v, not the 12v listed!! I knew a computer power supply would be much more accurate for 12v and 5v (3.3v too if you crack it open) sources.

They make great bench power for DC current, just jumper pins 14 and 15 and it'll run without being plugged into a motherboard. (pin 15 and 16 are at the clip portion of the ATX connector, 14 just to the left, 14 is usually green and 15 usually black.)

So I wired them into some breadboard getting them bunched in a cluser of three.

More to come about where they are going.

Thanks again!

[magindat]

I've used the same ones. Put 6 in a series and run your car's voltage straight to the ends.

[MENINBLK]

I always thought LEDs were 5V ?

[Breadfan]

Some are, but not all.

It depends alot on the size, light output, and color.