Can LED be dimmed with volume POT?

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Can LED be dimmed with volume POT?

Postby Wezly » 18 Jul 2014, 17:55

Hey I'm building a really simple volume pedal, I'm wondering if it is possible to use one POT to fade an LED aswell as adjust the volume of my guitar.

This is the circuit I'm considering adding the LED to.. although I'm testing without the switch at the moment.

image.jpg


Thanks for your thoughts, Wez.
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Re: Can LED be dimmed with volume POT?

Postby Nocentelli » 18 Jul 2014, 18:55

you would need a dual gang pot, one set of lugs would be wired as per the diagram and the second set as an entirely separate variable resistor between the power supply and the LED. You would also need a resistor (maybe 1k) on series with the led to ground to prevent burning out the pot or led at maximum volume/brightness. You would also need a tapering resistor across the two lugs of the brightness gang to bring the maximum resistance down or it won't light at all until appproaching maximum volume.
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Re: Can LED be dimmed with volume POT?

Postby Wezly » 18 Jul 2014, 19:32

Hey, that sounds like a pretty simple solution! Guess I'll have to get my hands on a dual gang pot.
(any experience of turning two single gang pots in to dual?)

Regarding the tapering resistor - I have noticed this behaviour whilst researching dimming LEDs with POTs, Whilst testing my volume POT with my guitar i've noticed similar behaviour - when turning the volume up I very quickly reach full volume - would a tapering resistor help this too?

Cheers, Wez.
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Re: Can LED be dimmed with volume POT?

Postby Nocentelli » 19 Jul 2014, 22:57

Dual gang pots are a single unit with two sets of three lugs. They contain two separate resistive tracks with separate cold, wiper and hot contacts (aka lug 1,2, and 3 ): the two wipers are swept simultaneously by a single shaft. The standard dual pots you can buy easily have identical values for both potentiometers (e.g. 2 x 250kA). a typical application might be a stereo volume control.

dualgang.png


I should really have said "scaling resistor" rather then tapering resistor in my post: putting a fixed resistor across the lugs of a pot will scale it to a lower value and also affect the taper or feel of the pot's rotation. Here, you'd probably need to lower the value of the LED dimming pot on the right, maybe try a 100k for R1 and move downwards until the LED responded as required. The 470r sets the maximum brightness without burning out the LED but could be increased.

volume LED dimmer.bmp


Guitar volume pots are usually 250kA or 500kA, log means that you get a quicker change in resistance towards the end of the pot's travel, this ensures you don't have to wind it back to half way before you notice any reduction in volume. If log is too quick, try a linear pot. You can alter the taper (log, linear, antilog) of a pot to a certain extent by putting fixed resistors in parallel, but it always reduces the pot's value - you wouldn't want to do for a volume out, so just swapping the pot for a different taper in the same value is better.
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Re: Can LED be dimmed with volume POT?

Postby Wezly » 20 Jul 2014, 10:17

Hey thanks for the explanation that's really helpful!

Is there a reason that you have put your 470r resistor after the LED and not before? Every other circuit I look at has this between the LED and the power source? - I mean obviously we are putting a POT and a scaling resistor in front of it but still.. What is the logic behind putting it in between the LED and negative.

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Re: Can LED be dimmed with volume POT?

Postby Nocentelli » 20 Jul 2014, 13:19

Wezly wrote:Is there a reason that you have put your 470r resistor after the LED and not before? Every other circuit I look at has this between the LED and the power source? - I mean obviously we are putting a POT and a scaling resistor in front of it but still.. What is the logic behind putting it in between the LED and negative.

Thanks.


It makes no difference electronically if there is only one path "in" to the circuit section, and only one path "out". As long as you have a resistor permanently in the path of the LED between +9v and ground, it will limit the maximum current flowing through the LED, no matter if it is "before" or "after" the LED: Electricity doesn't care.

If i was doing a vero or pcb layout, i would order them whichever way around led to a smaller/neater/more elegant layout.
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Re: Can LED be dimmed with volume POT?

Postby Nocentelli » 20 Jul 2014, 14:39

OK, I've had a play around with this and the one i posted works ok with a 100k or 47k scaling resistor, but the taper is not ideal.

A simpler way that works better is the voltage divider arrangement shown below, not a bad sweep even with the log 250K I'm using and the LED goes from off to quite bright over the whole sweep.

LED dimmer_v2.bmp


However, it occurs to me that the brightness of an LED is not a very useful indicator of the pot position, since your eyes/brain quickly get accustomed to the brightness of the LED in one "position" and you will soon be unsure whether it can go any brighter, and the only way to tell is to turn it up, rendering the visual indication pretty pointless.

A much better visual indicator is to use a bicolour LED, or a three colour RBG CA (red/blue/green, common anode) LED, as shown below.

LED dimmer_v3.png


RBG CA LEDs have a single common anode (positive terminal) and three separate cathodes (ground connections): I used the 250k pot to "pan" between them using the outputs of the green and red portion only. At minimum, the green is lit, this fades toward the centre, and the red slowly lights up and gets brighter. You would probably want to tinker with the scaling resistors to match the colour variation to the volume increase to your taste. This is all done very easily in a few minutes if you have a breadboard - I'd advise getting one (if you don't already have one) if there is even a small chance you might want to continue building more pedals.

A red/green bicolour (which usually have a common cathode) might work even better, since these can go from green to yellow (red+green) to red as you turn the pot, but I've run out so couldn't try it.
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mmolteratx wrote:absolutely zero commercial use allowed. If I find anyone selling these, I'll fly to your house and kick you in the nads. And you may or may not find yourself in trouble.

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Re: Can LED be dimmed with volume POT?

Postby Wezly » 20 Jul 2014, 22:03

Thanks so much for testing that for me! - I'm gunna invest in a small breadboard (will be a blast from the past having not touched one since college!) - Thanks also for explaining about the positioning of the current limiting resistor.

The RBG LED is a GREAT idea, I will most definitely be getting my hands on some of those - might have a read of my old college books too, hopefully it will all come flooding back.

Cheers, Wez :D
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