LED CLR + Trimpot + min value protection

[Toy Sun]

With many schematics, I see CLR values like 4.7k but when I calculate values I come up with minimums like 190, 200 ohms, etc... Now, my LEDs are firkin' bright, so maybe the 4.7k is the right value for those who like to be able to see... :

I want to start putting a trimpot in for my CLR, 'cause it would be cool to dial that in. I do think that I should have a resistor in series with the trimpot so I can't go down to 0 ohms, right? My ideas for that value would be something like 220 or 330 ohms, which doesn't blow up any of my stock of LEDs (all colors).

Any thoughts on this?

Thanks,
John

[antonis]

Are we talking about old fashion 5mm 20mA LEDs..??

[Toy Sun]

Yep

[antonis]

Use a 5k or so trimpot..

[davent]

My led's are just sorted by colour so various sizes and manufactures for every colour, required CLR all over the place so i test each before installing to find the optimum value.

Took a 12 position switch to get 12 value steps between 500R and 10K , another switch adds 10K in series for the super brights. I've had super brights needing 47k with a 9v source and old led's where the 500R was too much, the range is wide.

[PRR]

No, don't let the resistance go to zero. Something will smoke.

With modern LEDs, a 10k is probably fine. Nobody but you can even see it.

The real problem is the logarimic response of the eye. If 1k is still too bright, 2k is not much different. You may need 10k or 100k. Simple trim pots do not cover wide-wide ranges like 1k to 100k well. There's no point in an "exact adjustment". You should have the 1-2-5 series of all resistors in your favorite size and all decades. So tack in a resistor, peer, then a different resistor if desired.

[bluebunny]

. . . but when I calculate values . . .

Those helpful online calculators spit out - as you say - minimum resistance values, i.e. just enough to save the LED from sudden, horrible death.  But not your retina.  So not terribly useful.

Do what Paul suggests: test with a small collection of resistors.  A breadboard is great for this.  You could use one of those tiny breadboards that are good for absolutely nothing (except a Fuzz Face): keep it permanently built.

___
Aside: what a great idea!  I'm staring at one on my desk right now.  Tayda sent it by mistake ages ago and it's sat unused for years...

[Ben N]

I use a 25k pot in series with a small resistor (100R to 1k) and two pairs of small alligator clips (one for power, one for the diode) to test. Dial it in, measure, find your nearest value fixed resistor, and there you are.

[Toy Sun]

Thanks for the insights - particularly the fact that because of the scaling factors of the eye that a trimpot may not have the range. I'll keep doing the process I do now, which is to test on the breadboard and commit to a value. I was getting tempted by fancy 3PDT boards with trimpots, but I'll get over it!

[bluebunny]

I was getting tempted by fancy 3PDT boards . . .

This is strictly IMHO, but don't.  Seems like a good idea, but with two drawbacks:

1. If ever you need to remove the switch, it's toast.  And the board too, probably.

2. You don't learn how to wire a 3PDT switch.

[Toy Sun]

Totally get it - I have definitely worked w/o 3PDT boards and it's not hard at all. I've also had to toss a switch or two because there is no way to unsolder them from boards, so I think that it's maybe time to say goodbye to the boards.

[Phend]

I have another question about the general use of the LED in the beginning of a circuit.
That is where I find them connected most to of the time.
Question:
Other than being an "indicator" light what other purpose might the "LED / resistor" combination do to the "quality / sound" of the effect ?

[antonis]

I have another question about the general use of the LED in the beginning of a circuit.
That is where I find them connected most to of the time.

Could you plz post a relevant circuit schematic..??
('cause I think you're confused a bit anout "in the beginning of a circuit"..) 

[FiveseveN]

Yeah, it's not in the beginning, an indicator LED + resistor would be in parallel with the audio circuit. The most it can influence the sound is pop when you turn it on/off.

[Phend]

Here are 3 images, note the LED at what I think is the beginning of the circuit. (I maybe wrong, help me understand)
Thank you for any comment...

[idy]

That is a funny artifact of the way GGG draws the schematic. Better to think of it as attached to the power supply...and the bypass switch.

The power supply can be drawn in the upper left hand of the diagram (like GGG does) or at the upper right...or at the lower left....or.....

[duck_arse]

that solid black line across the top of each diagram is supply. any point on that line is the same as any other point, all other points, on that line. move the CLR connection to the far right hand end of the solid black line - same difference, no difference. there is effectively no start or end to that supply line.

[Phend]

Ok, I see that, it is just the way it is drawn.
Any comment on wither the LED affects the circuit in anyway, good or bad (other than using battery) ?

[anotherjim]

If there is an RC filter in the 9v input, there can be little point in placing the LED+CLR after it - it doesn't need a filtered supply. However, I would caution that a reverse polarity protection diode for the pedal should be fitted somewhere before the LED connection since although being a type of diode and should not conduct with reverse polarity power, the reverse voltage rating of an LED is quite low.

[Phend]

^ Thanks,  I have installed a diode for incorrect battery connection.
Actually I have upgraded all those "wooden box" effects I have made by adding Led'S and a push button switch.
Plan on stacking them in a yet still to be designed shelf.
Finding once a project is complete,  it probably isn't !