Dimming LEDs

Thread Starter


Joined May 27, 2007
I bought a submersible tube of 25 blue LEDs for my fish tank. I would like to have the ability to dim them. The light plugs into a standard wall socket. I was thinking maybe a potentiometer could do this (cheaply) or a dimmer switch for a light in the house (more expensive). Attached is a pic of the light tube.

I don't know a whole lot about electrical type projects, but I'm good with a solder gun, and live near a RadioShack. So if anyone can help out I would greatly appreciate it.




Joined May 16, 2005
You can learn how to do this by searching for "PWM" or "pulse width modulation." ;) The dimmers won't work very well (potentiameter) or at all (wall dimmer) for LEDs.


Joined Jun 30, 2006
Since it's all sealed up cracking it open would be a waste. What is the voltage supplied to it? I assume current regulation is done inside the water proof housing which may make it difficult to do if it's more than just resistors.

Thread Starter


Joined May 27, 2007
I will do a search on PWM, thanks!

You are correct, cracking it open would be a waste. Without knowing resistor values or how current regulation is setup, will make this a slightly more difficult project than I thought.

I guess I could try emailing the manufacturer and see if they could shed some light on how it's all setup.

Thanks for the info!!!!

This is a great forum!! I love tinkering with electricity and this place will help greatly!!!


Joined Jan 28, 2005
Yep. The AAC forum is "Tinker Central" for all your tinkering needs.

If you need a bit of introduction to theory on a level that makes electronics fairly accessible, visit the tutorial section of the AAC website at www.allaboutcircuits.com.



Joined Apr 27, 2007
Why resort to PWM? Why not using a potentiometer conected as a variable resistor, in order to control current. Althought LED brightnes is not proportional to the voltage across a resistor-LED network (or across a LED), it is proportional to the current biasing it (with some logaritmic tendency). You can confirm by reading Kingbright or Nichia datasheets.

Also, with a 555 timer you can make a pulse width modulator, but you have to use pin 5 (if I am not mistaken).


Joined Jan 22, 2004
yes you can use a 555 timer as your pwm as what cumesoftware has suggested but use pin 3 (output) to control a power transistor or a power mosfet.



Joined Sep 28, 2007
I recently used a 555 to control LED brightness for lighting a metal sculpture. I put a slow sine wave into pin 5 (control voltage) of the 555 to get a gradual brightening and dimming. It worked great, but I was only able to get 20-80% duty cycle swing. Anyone know what the ultimate limit is for duty cycle control via pin 5?


Joined Aug 8, 2005
The cheapest way to design this tube would be to put all LEDs in series (the Xmas tree effect, if one goes out they all do), with a single ballast resistor and a series diode/rectifier.

Now, based on that assumption, a standard light dimmer should work. You could try one and see, shouldn't hurt anything regardless.