Single LED brightness adjustment

[HelpLess]

I wanted to set up a 3 volt single LED to be able to adjust the brightness from OFF to maximum brightness. I was using a pot switch, but read this is not a good idea. I would like to have the adjustment by turning a knob or pushbutton. I could not find the parts I need. Any help/advice or links to manufacturers would be greatly appreciate. I am pretty novice at this.
Thank you.

 

[Sceadwian]

The simplest method aside from using a micro controller would be building a simple 555 timer circuit and adjusting the duty cycle. As long as the frequency is above a few hundred hertz it'll appear to be on continously and the larger the duty cycle the brighter it will appear.

 

[HelpLess]

Would I be able to power it off with the 555 as well? Also do you have a website link that offers the components I may need?

 

[kchriste]

I was using a pot switch, but read this is not a good idea.

You can if the pot is designed to handle the LED current. Most carbon "Audio Pots" are not and will go intermittant after a while. To get around this, you can use the following circuit: (I didn't put any resistor values in there because you don't mention what voltage your supply is)

 

[HelpLess]

Thank you this is currently how I have it set-up. Unfortunately I won't the pot to serve as a switch as well and cannot find anything small enough. Any recommendations on where to get a pot switch that can handle the LED?

 

[audioguru]

If you use a 1M pot and a 6V battery then with the pot turned down the current in the pot is so low that a battery will last for a very long time, it shelf life. The transistor will need to be two connected as a darlington or use a darlington.

 

[kchriste]

Any recommendations on where to get a pot switch that can handle the LED?

You/I need to know what the max LED current and supply voltage will be first. For higher power you can use a wirewound pot, but the carbon pot / transistor method will be cheaper and easier to find parts. You can do away with the swicth on the pot if you wish, but this will means that there will be some current draw when the LED is off. Again, the carbon pot / transistor method will be better in this regard.

 

[HelpLess]

Power supply is 3 volts.

 

[Sceadwian]

You should be able to find pots that have an off detent, a lot of consumer electronics equipment use them. What resistance that off detent provdies I'm not sure, but it's going to at least be as good as 'far left' on a regular pot, and provide a physical detent preventing accidental turn ons.

 

[kchriste]

Both are 3 volts.

That will make it much more complicated. Can you go to 4.5V? The problem with using a 3V supply with a 3V LED is that there is no voltage left to drop through a resistor and thus you cannot easily limit the max LED current. You will need to go to a buck/boost regulator which I think may be a little too complicated for a beginner project.

 

[HelpLess]

That will make it much more complicated. Can you go to 4.5V? The problem with using a 3V supply with a 3V LED is that there is no voltage left to drop through a resistor and thus you cannot easily limit the max LED current. You will need to go to a buck/boost regulator which I think may be a little too complicated for a beginner project.

I can go to 4.5 V. I was just using 3 to keep the total enclosure as small as I can.

 

[Sceadwian]

Is this mains powered or battery?

 

[HelpLess]

Is this mains powered or battery?

It is Battery-powered

 

[Overclocked]

I can go to 4.5 V. I was just using 3 to keep the total enclosure as small as I can.

Then use a 9V. Its about the same size as 2 AA batteries.

 

[HelpLess]

Then use a 9V. Its about the same size as 2 AA batteries.

I can certainly do that as well....my main problem is finding the correct pot switch that will do the job and is small enough.

 

[kchriste]

The only problem with a 9V battery is that it won't light your LED for very long. (About 1/20th the time as 3AAs). Here is a circuit that'll work from apx 3.5-5V. It's a little more complicated that the previous one but will give you better brightness over the life of the batteries. The max LED current is apx 20-26ma (At low currents Vbe will be 0.5-0.7V) and is set with R1.

 

[Sceadwian]

The POT feeding the transistor is probably your best bet.

 

[HelpLess]

Alright fellas I have been trying to learn as much as I can about the 555 Timer Circuit (I know it is simple, but electronics is not my forte ). My question is what resistors (R1 and R2) should I use and what capacitor. I assume I need to be over 100 Hz at the minimum "brightness" setting and then what do I go up to.
If I set up the 555 Circuit should I put the pot in place of R2? Or is there a better way to adjust the frequency of pulses from the timer...push-button?
Thanks for all your help thus far.

 

[Hero999]

THe formula is on the 555 datasheet, also changing the frequency is useless, you need to change the duty cycle.

 

[audioguru]

If you use a 555 to make pulse-width-modulation to adjust the brightness of the LED, then with it turned down so the LED is turned off the 555 consumes a fair amount of battery power, unless it is a Cmos 555 driving a transistor to boost its power.

Kchriste's circuit draws nearly no battery current when it is turned down.