Basic question: How to regulate LED brightness with ridiculously LARGE resistor

[KarenColumbo]

Yeah, yeah, I know. The basics But where should I look if not here? Google just gives me tons of Arduino stuff, I'd rather go the analog way. Hence my question:
- How can I regulate an LED's brightness from none at all to spec max with both 9V supply and a 100k pot (Wah).
- Background: State Variable Filter wah experiment -> defunct Cry Baby, hence single 100k pot -> great SVF schematics -> Vactrol (home-rolled).

The usual 1k - 4k7 I'm used to put into stompboxes for status LEDs vs. this 100k Wah pot I have.

In my unknowledge I suspect there is a way to somehow "transfer" those 100k to sensible LED-friendly values via a transistor amplifier. Isn't it?

Since I obviously (at least to me) need to control two resistances at once to sweep the center frequency in the SVF but only have ONE variable resistor available -> LDR/LED combi is asked for. (or maybe two "matched" FETs wired as variable resistors?)

Sorry - it's those silly simple things that trap me.

[antonis]

I think Wah pot wouldn't like DC across it..

[DIY Bass]

I would try first with a resistor that will give you max brightness in series with your 100k pot.  I think that 100k will reduce the current to the point where the LED will not light.  The other way you essentially have just the series resistance.

[KarenColumbo]

I think Wah pot wouldn't like DC across it..

Yeah, that is utterly possible That's why I blindly threw "transistor" into the arena.

[KarenColumbo]

I would try first with a resistor that will give you max brightness in series with your 100k pot.  I think that 100k will reduce the current to the point where the LED will not light.  The other way you essentially have just the series resistance.

Thanks, I will try just that to get a "feel" for what the LED(s) need.

[antonis]

I think Wah pot wouldn't like DC across it..

Yeah, that is utterly possible That's why I blindly threw "transistor" into the arena.

You could use 100k pot as high gain transistor bias resistor, lowering enough DC current through it..
(in relation to "direct" LED driving..)

[KarenColumbo]

Ah, sod it, I'll buy an expression pedal and put a dual gang pot in.
Thanks guys! This is obviously a dead end, else it would be done regularly

[Mark Hammer]

I believe Craig Anderton had an installment in Guitar Player years ago about using an LED/LDR arrangement to effectively "de-scratch" a wah or volume pedal.  I'll see if I can locate it (although I expect someone else here will probably beat me to the punch)

[Rob Strand]

All you need is a current source -google it.  Here's a text book type circuit.  Use an LM358 so the input can be adjusted down to zero volts.

https://electronics.stackexchange.com/questions/459323/constant-current-led-driver-using-opamp



The current through the LED is "the pot output voltage" /  R2.   You will want to increase R2.

You want to limit the maximum voltage to the opamp to something in the range of 2V to 4V.   The regulator ensures the behaviour is consistent with battery voltage.  You could toss it and just connect a resistor form the pot cw terminal to +9V to limit the voltage range.  A cap from the opamp + input to ground will help prevent glitches.

Now you can simplify this circuit by feeding the pot into the base of the transistor and ditch the opamp but then then behaviour depends on the transistor gain - it still works though.   You could even use a JFET instead.

Plenty of options.

[iainpunk]

PWM!!!!!!!!!!!!!!!!!!!!!!!!!

use a simple PWM circuit and switch the LED with a transistor,
https://www.electroboom.com/?tag=toothbrush
replace the motor here with the LED.

cheers

[KarenColumbo]

Whoa! That's the ticket. Thanks again!! I will try those immediately!!! And I'll search my "archives" for Doug's snippet, too!

[KarenColumbo]

I believe Craig Anderton had an installment in Guitar Player years ago about using an LED/LDR arrangement to effectively "de-scratch" a wah or volume pedal.  I'll see if I can locate it (although I expect someone else here will probably beat me to the punch)
[/quoteI think it's on R.G.'s ,,Technology of Wah Pedals"webpages

[tonyharker]

The trouble with LEDs is that they cannot be dimmed in the same way as a lamp. When you get below a certain voltage the LED will cut off abruptly.  The best way is with a PWM dimmer like this one https://www.ebay.co.uk/itm/DC-3V-35V-PWM-DC-Motor-Speed-Controller-Adjustable-LED-Dimmer-10khz-Fan-O2Z9/353421746582?hash=item524993e196:g:5tIAAOSwe-dfgiBy The LEW will still need a series resistor but should dim gradually to off.  The only problem with these is the switching frequency which may be audible and may inject noise into adjacent elecrtonics.

[Fancy Lime]

While you're at it with the PWM, use switches instead of vactrols as control elements, and you got yourself a Bass Crybaby 105Q. I have never seen a schematic but I read somewhere that is is based off an MXR envelope filter, and a quick look into my unit makes that at least sound plausible. There is a 4049 inverter that seems to be the oscillator and PWM control, a 4016 quad bilateral switch as the resistive control element, and two MC33174D quad opamps, which seem to handle the audio path, judging by their positions in what looks like a reasonably well organized layout. No guarantees, though, I haven't traced it and it's SMD with lots of unmarked caps, so...

Cheers,
Andy

[iainpunk]

The trouble with LEDs is that they cannot be dimmed in the same way as a lamp. When you get below a certain voltage the LED will cut off abruptly.  The best way is with a PWM dimmer like this one https://www.ebay.co.uk/itm/DC-3V-35V-PWM-DC-Motor-Speed-Controller-Adjustable-LED-Dimmer-10khz-Fan-O2Z9/353421746582?hash=item524993e196:g:5tIAAOSwe-dfgiBy The LEW will still need a series resistor but should dim gradually to off.  The only problem with these is the switching frequency which may be audible and may inject noise into adjacent elecrtonics.

well, a photo-resistor is really slow, in my experience less than 20Hz at its -3dB point when using to transfer signals, if you switch at above audio rates, there is no way that it would be audible, or affect audio at all.

cheers

[antonis]

20 or more items to drive an LED... 

(remarkably outrageous..) 

[PRR]

[KarenColumbo]

This one does it. Thanks so much!
Just gotta finetune it - when I move the pedal (pot) too fast upwards (toe position), there's some jumping around. Could be the pot, though. I really don't know how old the Cry Baby is. Gotta test it with a normal alpha pot. Maybe the LED gets too bright, too, which is easily avoided, I guess.

[KarenColumbo]

PWM!!!!!!!!!!!!!!!!!!!!!!!!!

use a simple PWM circuit and switch the LED with a transistor,
https://www.electroboom.com/?tag=toothbrush
replace the motor here with the LED.

cheers

Will do! I found this one (
https://gadgetronicx.com/wp-content/uploads/2014/10/pwm-led-light-dimmer.png
) at a quick google, and it doesn't work - connecting the pot seems to short something. But no matter. Gotta try meself a toothbrush

[KarenColumbo]

All you need is a current source -google it.  Here's a text book type circuit.  Use an LM358 so the input can be adjusted down to zero volts.

https://electronics.stackexchange.com/questions/459323/constant-current-led-driver-using-opamp



The current through the LED is "the pot output voltage" /  R2.   You will want to increase R2.

You want to limit the maximum voltage to the opamp to something in the range of 2V to 4V.   The regulator ensures the behaviour is consistent with battery voltage.  You could toss it and just connect a resistor form the pot cw terminal to +9V to limit the voltage range.  A cap from the opamp + input to ground will help prevent glitches.

Now you can simplify this circuit by feeding the pot into the base of the transistor and ditch the opamp but then then behaviour depends on the transistor gain - it still works though.   You could even use a JFET instead.

Plenty of options.

Thanks so much! Will try this tomorrow.