is there a way to increase the light of an LED with a dial?

You probably cannot increase it. Do you mean decrease the light? You could use a varible resistor to adjust the output to the LED.

Mike

You want a variable resistor. The value depends on the particular LED but you might try 2.5K or 5K.

Yes, James ie right. You don't want a varible resistor like I said. You want a variable resistor. haha. I'm terrible at typing.

Mike

It may not work with a simple variable resistor because at higher settings, the current will increase enough to probably just burn out the small piece of the pot track that's still in circuit. A fixed resistor will also still be needed to limit the full brightness current. In order to get a decent control range, without risk of over-running the pot's track and wiper, I would suggest that a small transistor needs throwing into the mix to control the LED current, whilst the pot is left the low-current job of controlling the transistor's base. If I get time today, I'll give it a try.

Arfa

Google on LED driver. That should keep you occupied for some time. ;-)

If you wish to increase the brightness beyond that a constant current driver can do you'll be into a pulse circuit.

--
*Sometimes I wake up grumpy; Other times I let him sleep.

    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.

OK. I just tried it with a few bits that came to hand. White LED. 200k pot.

82k resistor. 2N3904 transistor. 12v supply. LED current adjustable from 0 - 25mA. You could also do it using an adjustable regulator as a constant current source.

Arfa

Thanks for your help. Great to hear it worked. I don't want to go above the brightness limit of the LED, so that shouldn't be a problem,. Now all of the parts you just listed, are they some of the things I can find at a local Electronic store, Radio shack or something I need to find online? Also, what kind of cost am I looking at here?

You could buy one of the dollar stores lanterns with a variable output, and transfer the ckt. Its not going to fit inside the flashlight, but you might be able to size down the ckt.

greg

Parts values are not that critical. Any linear taper pot of around that value (150k, 200k, 220k, 470k), any resistor around that value (68k, 82k,

100k, 120k, 150k). Might be worth you getting a few resistors of around that sort of value, and just experimenting to see which one gives you best adjustment range with the pot value that you finish up using. The value of this resistor, along with the gain of the transistor, will determine the full on current through the LED. Start with the highest value resistor, and the pot turned right down to the ground end, and measure the current through the LED as you slowly turn up the pot. You don't really want more than 25mA through a standard white LED. The brightness did not increase much above about 18mA, with the example I tried. As far as the transistor goes, any general purpose NPN small-signal silicon transistor with a collector rating of 100mA or so, will be fine, so 2N3704 or whatever you can lay hands on. Any electronics component store should have these parts on the shelf. Prices are cents.

In case you don't know how I hooked this up, it was fixed resistor top end to +12v, bottom end to the clockwise (from the front) tag on the pot. Anticlock tag of the pot to ground (supply "-") together with transistor emitter. Transistor base to centre tag of pot. LED anode to +12v. LED cathode to transistor collector. Measure current in LED by inserting milliammeter in series with LED ( meter "+" to +12v, meter "-" to LED anode.

Arfa

Well I was assuming the original resistor was still in place, but yeah you do need it.

Well RS used to have stuff like that. Since they shut down my RS I don't get there much anymore.

I still didn't hear a full purpose of this. The good dimmers use pulses to control brightness, much more efficient. The switched dimmer as seen on the $10 lanterns at the dollar store and K-Mart is really nice. I took one lantern apart last spring with the intentions of beefing up the LED's, but the driver circuit will only handle so much current. I still have one still intact.

greg

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Ok, thanks for that. Iam going to try and find a dollar store lantern like you said, although I don't recall seeing them anywhere of late. I guess variable brightness would be just as good for this project.

Arfa, How much current are we talking about here? If this is what I'm immagining in my head with 2 or 3 led's it shouldn't draw much power. I guess he could have one of those 5w led flashlights, but that was not what I had in mind when he said dollar store.

Mike

Actually, I only want ONE LED in what I am building, so I guess that might check things a bit. Just one LED is all I need for this project.

Hi Mike. Well, if you reckon on a maximum current of say 20mA per LED, then for 3, that's 60mA. Depending on what rail voltage he's figuring on running it from, there's a good chance that if he was just using a pot on its own - even allowing for still using any original limiting resistor - that when he got towards the low end of his pot, he might be getting towards the dissipation limits of the track and the wiper structure. Most carbon track pots that you are likely to pick up at a 'general' electronics store, are only rated to a few mW, and are not designed to carry current. There are exceptions such as wirewounds, and some plastic tracked types. It just seemed to me that the addition of a 20c transistor guaranteed that the pot would not be damaged the first time it was turned up.

Someone mentioned pulse drive, which is the better way to do it, both from linearity of control and LED life angles, but you are then into either a specialist driver IC, or something like a 555 timer IC and a couple of extra Rs and Cs and *still* the pot as well.

Arfa

You might get away with just a pot on a single LED. I guess for the cost of one, it's worth giving it a try, but it does depend to some extent on what rail voltage you are going to use to run it from, and the pot might not be too long-lived if it is going to be regularly turned up and down. If it's just a 'set and leave' project, you might be ok.

Arfa

The 'usual' LEDs found can only be driven to about 30mA from a constant current source. So a 1/4 watt pot would likely be ok at the sort of voltages found in a torch.

--
*The statement below is true.  

    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.

I wasn't saying that your idea was bad, but just thought it might be a little over complicated considering the OP's apparent electronics knowledge. I agree that using a transistor would be best for reliability. The only reason I questioned you is that in the past I have used pots for exactly this, but then again that was before all of these high power blue / white LEDs exsisted too.