Reducing LED Brightness?

[Tchail]

Hey Everyone,

I've been working on putting together my first model using LEDs and fiber optics.

(I'm almost ashamed to admit that I've played around with both over the years, but never actually committed myself to completing a project using these components.)

I've been working using Round 2's recent reissue of the Klingon Vor'Cha.

For the engines, I've decided to use two of the white LED backlight modules from Adafruit: https://www.adafruit.com/product/1626 placed back to back along the side of each nacelle, along with two 3mm white LEDs in the front of the nacelles.

(BTW - White LEDs are typically 3.3v / 20mA.)

I plan on using a 9v power supply, so according to an on-line resistor calculator, I need to use a 180 Ohm resistor if I'm wiring two LEDs in parallel.

However, these LEDs are awfully bright:







Granted, I'll be coating the inside of the nacelles with black paint.

But do all of you have any advice on reducing the brightness of the LEDs?

Or should I just get a breadboard and start playing with resistor values to see what works?

Cheers,

-Tc

[TShark]

If you want it just right then I would recommend a bread board. Not only is a bread board good to customize model electronics, but it allows you to run the components for a few hours to ensure they don't fail prior to model installation.

[Ant]

To make less bright, just reduce the current by increasing the resistance. Try doubling it to star with...

[brt]

Use a variable resistor/ trimpot.

https://www.google.ca/search?source=hp& ... Kx8nbDpnZM

[Tchail]

All good suggestions, thanks!

I’ve ordered a breadboard and will start by playing with the resistors I have on hand.

[swhite228]

Simple solution...go to the local theater supply house and pick up a sheet of neutral density theater gel.
From Lee Filters web page: http://lowel.tiffen.com/edu/light_controls/gels.html
Neutral Density (ND). Grey in appearance, they reduce the output of a light in progressive steps from 1/2 of a lens stop to 4 full stops without diffusing the light or affecting the color temperature. This is preferred to electric dimming of the light which will shift its color warmer.

It's easy to cut and a sheet at around $8.00 yields enough material to do several models.

[Andrew Gorman]

A coat of white paint can work well. If it’s still too bright, add another layer.

[b5ranger99]

LEDs are not linear devices, they do not act like resistors e.g if you drop the voltage 10% the current does not drop by 10%.
The LED needs 3.3 volts and before current starts to flow and an additional component is needed to limit the current to 20 ma.
So if you want to use 3 - 1.5v batteries (4.5 v) with the LED you will need a 65 ohm resistor in series with the LED.
Increasing the resistor to 120 ohms will not reduce the output of the LED by half. The brightness of the LED can be adjusted to some degree by varying the current through the LED. however doing so with a pot is EXTREMELY difficult and touchy.

The best way I know of to control the brightness of an LED is by using a circuit which pulses the LED with the proper voltage through a correct resistor and varying the duration (width) of the pulses (pulse width modulation). If the circuit is pulsing on half the time and off the other half the LED will appear dimmer. If the pulse rate is greater then 30 pulses per second the LED will appear to be dimmer and not blinking. The brightness can be adjusted by adjusting the ratio of on time to off time. Several simple circuits are available to provide this type of control. A Google search for a 555 circuit or PWM circuits should provide a number of possibilities. If the search fails send me a PM and I'll send you a couple circuits.

Good Luck.

[brt]

I've already used a trim pot in one of my models to adjust brightness. Works like a charm. I plan to keep doing it.

[Ant]

Yup, absolutely no reason a trimpot can't be used to reduce the brightness.

To be safe, use it in series with the resistor calculated to give your maximum allowed current through the LED - that way you will never accidentally wind it down too far and let too much current flow.

[brt]

That's what I did.

[MillenniumFalsehood]

Yeah, another vote for the trimpot. And I would add, after you're done adjusting it, it might be good to put a drop of paint on the knob so that it doesn't move as you're assembling the model. They can be easy to knock around, so I figure that little bit of insurance can't hurt. Alternatively, you can measure the resistance of the pot after you adjust it, then find an equivalent resistor to match it.

[TAC]

WARNING: Do not replace the variable resistor (VR) for the resistor inline. Otherwise you will run the risk of selecting a resistance lower than recommended, and blowing the LED. Always add the VR to the resistor.