potentiometer  to control LEDs - Page 1
Forum section: electronics / beginners

author: Grayfox
author_rank: Regular Contributor
timestamp: October 15, 2013, 10:24:20 am
content: HeyI want to make a simple RGB LED light that can have each color adjusted.I want to make it very simple/I tried using a potentiometer to control a sinle LED a while back.But it went from dim to bright with a 1/8 of a turn on a 270* B1K Linear pot.Is there a way I can adjust the brightness in an easy manner.

author: WarSim
author_rank: Frequent Contributor
timestamp: October 15, 2013, 10:37:52 am
content: The easiest is with a potentiometer.If the adjustment is in the wrong place on the pot, use series and parallel fixed resistors.Even better use a more appropriate valued pot and a series fixed resistor.I am assuming you pot is set up as a variable series resistor.If it is set up as a variable voltage divider, it is a waist of power and you will need a series resistor on the whipper.There are much more accurate/complex methods with PWM, LED drivers or programable constant current sources.

author: Grayfox
author_rank: Regular Contributor
timestamp: October 15, 2013, 10:44:49 am
content: My circuit was12v Battery----1Kohm Linear Pot-----LED---500Ohm resistor---Ground

author: madires
author_rank: Super Contributor
timestamp: October 15, 2013, 12:09:55 pm
content: You could try a potentiometer with a lower resistance (250 Ohms) and increase the value of the fixed resistor to get a finer control.

author: WarSim
author_rank: Frequent Contributor
timestamp: October 15, 2013, 01:41:08 pm
content: Any details on the LED like colour, type, purpose. Without an idea of the voltage and desired current span. Giving accurate values I'd somewhat impossible.

author: sleemanj
author_rank: Super Contributor
timestamp: October 15, 2013, 02:11:39 pm
content: Quote from: Grayfox on October 15, 2013, 10:44:49 amLet's assume that we are talking about the Blue channel, because that probably gives the largest Vf of around 3.2vSo with the pot dialled all the way back, you have 500R dropping 8.8v, 17.6mA, that seems a little low for "full bright".With the pot dialled all the way in, you have 1500R dropping that 8.8v, 5.8mA, that seems a little high for "full dark".So you might want to play with the values, reduce your fixed resistor, increase your potentiometer.Butdimming an LED just by limiting the current may not be the best way to do it, modern leds are fairly bright even at small currents so you'll likely find that there is a perception that the dimming isn't very fine, but rather granular (bright, not-quite-so-bright, pretty-much-off). PWM dimming will probably produce a better result. Grab a 555 timer, your 1k pot, and a couple of passives and make up a text book 555 pwm circuit. See how it compares.

author: casper.bang
author_rank: Frequent Contributor
timestamp: October 15, 2013, 02:36:13 pm
content: Quote from: Grayfox on October 15, 2013, 10:24:20 amThe problem with a discrete solution using potmeters and 555's, is that the 3 LED's needs different voltages and currents. On top of that, you can't assume that a 50% duty-cycle of one color matches the brightness from a 50% duty cycle of another color. While these things are tweak-able in hardware (resistors), you might want to try your luck with a microcontroller solution (ATTiny85 has 3x PWM out) which you can tweak very precisely in software.

author: G7PSK
author_rank: Super Contributor
timestamp: October 15, 2013, 02:53:01 pm
content: Use a multi turn pot perhaps with a vernier drive as well.

author: Grayfox
author_rank: Regular Contributor
timestamp: October 16, 2013, 12:51:34 pm
content: Plan on using a RGB LED strip off ebay.Mainly want to control each color.

author: casper.bang
author_rank: Frequent Contributor
timestamp: October 16, 2013, 02:04:24 pm
content: Quote from: Grayfox on October 16, 2013, 12:51:34 pmUnless you are in this strictly for the learning aspect, let me just point out that ebay has $10-$15 RGB LED drivers complete with remote control. I used one of those recently to pimp my wifes greenhouse:http://blog.bangbits.com/2013/08/how-to-pimp-greenhouse.html