mini variable power led

[cmecha]

i am looking to create a mini led circuit that has a variable power controller on it. it does not have to be bright but if i am going to use a potentiometer i would like it to be small. i basically want to control the brightness on a 3mm led it does not have to be bright. any ideas and sourcing parts would be awesome. also i want this to be powered by 1x2032 battery. i have a battery holder and led already please let me know if and where i need to get hte other components.

 

[Noctilucent]

(links in this post are indicative only - not affiliated with the sites)

You'll at least want a switch, if you haven't got one already, to just switch the entire assembly off when you're not using it. You can find slide-type switches intended for PCB mounting pretty much anywhere, which would work well for your intended use.
http://www2.conrad.nl/goto.php?artikel=708020

Push/hold buttons are usually a bit more expensive and tend to be more bulky as well (due to the mechanisms required for the toggling behavior).

The next question is whether or not you'll need a minimum resistor value for your LED. If you know the details of your LED, you can use an online LED/resistor calculator. If you don't.. then it's usually 'safe' to presume a forward voltage (Vf) of 3.0 for white and blue, 2.6 for green, and 1.8 for red LEDs. Those are gross estimates established years ago that no longer conform to each and every LED, though - so if you do have exact details, that'd be better. Otherwise, just err on the safe side (as you don't have brightness high on your list).
http://led.linear1.org/1led.wiz
Resistors themselves are a few cents each and, again, can be picked up pretty much anywhere they'd have the other components.
Note that a 2032 button cell itself has some internal resistance and will only discharge at low milliAmp levels. That's why you can usually just hook up any LED, even 1.2V red ones, directly to one of these without the current-limiting resistor requirement.
But if you'd decide to upgrade to 2 AA batteries, checking into the above would be prudent.

For the potentiometer, just pick up a 10mm 'trimmer' potentiometer that takes a spindle.
http://www2.conrad.nl/goto.php?artikel=431958
( exactly article is a 15mm 'vertical' version - look for the 10mm version instead. vertical vs horizontal will depend on how you'd want to wire it. )
Note that this article is a 'closed' potentiometer - that is to say: the track is inside the housing, which is at least dirt-proof and mostly splash proof. There's 'open' models as well, which obviously wouldn't be good candidates for outdoor use.

Then source a disc-type spindle. This lays almost flat against the potentiometer, taking up the least space, while giving quite good control.
http://www2.conrad.nl/goto.php?artikel=426040

There -are- potentiometers with that type of spindle already included, but they tend to be more expensive; they're easier to source sometimes, though, and don't require manual assembly.
http://www.westfloridacomponents.com/mm5/graphics/00000001/p148.jpg
( Edit -> note that this is an 'open' type, where dirt and such can fairly easily get to the track )

One rather important question is what value potentiometer you'll need. 1M Ohm is typically going to do the trick to dim down to the lowest level (if the LED doesn't just cut off entirely), but may be too difficult to control as the LED's output curve is not linear, while the potentiometer is (there's logarithmic potentiometers, but rarely in that form factor, and often the wrong way around).
If you happen to have any resistors laying about, you can just try and see what light output you get from putting those in series with your LED. Find the one you like best dimmed all the way, and get a potentiometer closest to that value (a typical series is 1, 2.5, 5.. so if you find that a 330k Ohm resistor gives you what you need, a 500k Ohm potentiometer would be your most likely bet).

That's all you'd need for a -very- basic, but slightly energy-wasting, setup. Just put all of those components in series and off you go.
The potentiometer could be glued to the bottom of your battery holder - the switch can probably be glued to the 'flat' side of the potentiometer, making things very compact indeed.

A slightly more advanced option is to use pulse width modulation, so that the light blinks on/off really quickly to dim. This saves energy (although for a 1 LED setup driven from a 3V button cell, that's debatable) as you're no longer just wasting energy through the potentiometer. Unfortunately, any circuit for that is going to be fairly bulky or relatively expensive.
In addition, PWM does not dim well to very low brightness. The reason for that is that PWM always switches between a given brightness (usually full brightness), and no light at all. Even though LEDs can be switched very quickly, at some point you're just going to lose out to constraints of the circuit driving the LED and the best you'll achieve is a very quickly flickering LED. It might look dimmer, but any movement of the light at all will quickly show this flickering in effect.
( http://www.reuk.co.uk/LED-Dimmer-Circuit.htm )

So although PWM is a superior method as far as battery life goes, I doubt it's worth the bother in what you describe. What is the application, though?

As far as sourcing these components go, newark, radioshack, etc. will all have these things, but typically in varying models. Especially the potentiometer I pointed to - Piher brand - seems to be less popular in the U.S. where other types are favored; except those don't take spindles. Hence the pre-wheeled versions possibly being easier to source.

 

[MikeAusC]

If you want to use only one 2032 battery then you'll have to use a Boost driver to get the voltage high enough - every 2032 White LED torch I've seen uses 2x2032 for direct drive.

 

[cmecha]

I plan on using this LED http://www.parts-express.com/pe/showdetl.cfm?partnumber=070-520&source=googleps

Any other parts i'm not concerned about. but i want this to be cost effective.
As for the application I recently Purchased a trijicon RX09 with a chem light port. well the chem light works great but i don't want to constantly feed the thing glow sticks so i want to make a mini back up lighting solution that plugs directly in to the chem light port. if this works i will be epoxying it into some sort of housing and attach it to a Picitanny mount.

 

[Noctilucent]

If you want to use only one 2032 battery then you'll have to use a Boost driver to get the voltage high enough - every 2032 White LED torch I've seen uses 2x2032 for direct drive.

That only really applies when you want to get maximum output - which he doesn't seem to be concerned with. Keep in mind that the Vf if often the -typical- value.. not minimum. a 2032 will happily light most white LEDs without any booster.. it just won't be particularly bright.
I've got a Vf(typ)=3.4V @ If=20mA one hooked up to a 2032 at the moment myself.. it's been going for 4 days now.. it's now dim enough to not be illuminating a heck of a lot, but it's still visible.

That said.. adding a 2nd battery is easy enough, but then you're likely back to the current limiting resistor.. and the battery holder might not fit 2.

A boost circuit is pretty cheap, but does add bulk, even if you get the el cheapo ones made with SMD components.

 

[Noctilucent]

I plan on using this LED http://www.parts-express.com/pe/showdetl.cfm?partnumber=070-520&source=googleps

See the other post on whether or not you might need a 2nd battery. However...

i want to make a mini back up lighting solution that plugs directly in to the chem light port.

It looks like that scope may be more fond of a green LED than a white one? A green LED would typically be brighter for the same power used, also a tiny bit cheaper.
I'm not sure if it would be cheaper than just using these glowsticks, though. In the long run, if you're just using it sporadically, then it probably is (since it's not running when you don't need it).
Another concern may be diffusing the light from the LED.. the glow stick provide illumination along their entire length, pretty much, while an LED is more like a tiny spotlight. A piece of regular paper goes a long way, but some tracing paper or matte plastic would be more efficient.

 

[cmecha]

the chem port is actually a fiber optic collector. so it doesn't matter where the light in the port is as long as there is light. as for color i do not believe it matters so i could in theory use green or red. i will try to wire a simple circuit using a 3mm red led and if gives enough light i would be satisfied. Using a common 3mm red led the maximum voltage is 2.6v while a 2032 battery puts off 3v so i would need a resistor to negate .5-1v of energy. sorry but like i said my circuit knowledge is elementary at best.

 

[MikeAusC]

as for color i do not believe it matters so i could in theory use green or red.

If you want to use a single 2032, then you'll be a lot better off using a Green or Red LED because of their lower voltage.

Cheap Keyring torches drive the LED directly from the battery - the internal resistance of the cell limits the current to a safe level.

See if you can find a Keyring torch that has multiple brightness levels - in one tiny cheap package you'll have the electronics to regulate the current as battery discharges to maintain constnat brightness.

 

[Oznog]

But green/white keychains end up using two 2016 cells.
As said, they rely on the internal resistance of the cells to limit the current. However, the current and resulting light level is HIGHLY variable, and they will always go very "hot" when using quality new cells. It's bright, but greatly exceeds the specified current for a 5mm LED, and the LED degrades very quickly. In my experience, those LEDs seem to degrade so fast the LED is shot after about an hour with new batts if used continuously.

If you were to put a 5mm LED on two 2032 batts, the internal resistance is much lower than 2016. Thus even though the nominal voltage is the same, if you don't add an external resistor or CC limiter the current will be much higher and exceed the LED's rating for almost the entire 2032 life, not just the first 30% of its charge or so. But, the good news is that an external resistor is stable throughout the batt's life, whereas 2016 resistance goes up as the charge decreases resulting in less consistent light output.