# what green LEDs for General Tso?



## lcipher3 (Apr 8, 2020)

Have a few boards coming - what GREEN LEDs to go with the GL5516 LDR?  

Standard ?








						LED 5mm Green
					

Get It Fast - Same Day Shipping




					www.taydaelectronics.com
				




Super Bright?








						LED 5mm Green Water Clear Super Bright
					

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					www.taydaelectronics.com


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## Cybercow (Apr 8, 2020)

A point to note about the difference between green LEDs that enveloped in green plastic vs the water clear green LEDs is that green plastic LEDs are actually cheap 'white" LEDs wrapped in green plastic, burn less bright and have a lower Fvd (~2.3v - 2.5v) - AND - the water clear green LEDs are doped in such a way that they actually radiate green light at ~ 560–520 nm, burn brighter, and and have a higher Fvd (~3.2v - 3.4v). Just an FYI for your consideration.


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## lcipher3 (Apr 8, 2020)

Cybercow said:


> water clear green LEDs is that green plastic LEDs are actually cheap 'white" LEDs wrapped in green plastic, burn less bright



So what would be the best for a LED/LDR vactrol type app like the compressor?


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## manfesto (Apr 8, 2020)

I preferred the clear green, I think @PedalPCB was gonna try them out?






						General Tso’s green LED
					

(Dunno if this belongs under “Troubleshooting” or “Mods” since I’m not sure what the original circuit had, but)  Built the General Tso’s yesterday and I thought it was way too subtle a compressor, even for an optical comp; I know the original Fat General is known for transparency and for not...



					forum.pedalpcb.com


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## lcipher3 (Apr 9, 2020)

manfesto said:


> I preferred the clear green, I think @PedalPCB was gonna try them out?
> 
> 
> 
> ...



Thanks!  that's great info - I'll add some of the clear.greens.

I bought 2 Gen Tso (not here yet) - I can experiment.

I used VTL5C4s in my AionElectronics version and have a set left.
I'm going to use those to compare with the LED/LDR setup.
Probably experiment with shrink sleeving / tubing.
Maybe socket them etc.

Anyone know (or if it's worth it) a way to compare the two?  I have a scope, waveform generator etc.
LED voltage to LDR resistance?
Square wave in and measure output for response?


BTW - the VTL5C4 in the Aion sounds awesome - but I like the pedalpcb design with additional pot for the treble.


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## music6000 (Apr 9, 2020)

I'm thinking of mounting it like this :


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## Cybercow (Apr 9, 2020)

lcipher3 said:


> So what would be the best for a LED/LDR vactrol type app like the compressor?


The answer is almost always found in the datasheet for any given part. And in this specific case, I would go with the clear green to go with the GL5516 LDR.

The GL5516 LDR has a spectrum peak value of 540nm and the clean green LEDs emit at ~525nm (closer to the GL55xx series than the yellow at 580nm).

And as Icipher3 suggests - experiment.


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## Chuck D. Bones (Apr 9, 2020)

Vf is a dead giveaway which wavelength the actual diode in an LED emits.  If you compare Vf, do it at the same If and not at the max rated If where the diode's internal resistance can skew the results.  The compressor pedal circuits run the LEDs below 10mA.  The Delegate and General Tso both peak out around 3mA.  At 3mA, green & yellow LEDs have a Vf of 1.9V, blue & white are just over 2.6V.  White LEDs are actually blue LEDs with phosphors added.  The phosphors absorb most of the blue light and re-radiate over a range of wavelengths.  The spectral response of CDS LDRs varies depending on the recipe.  The ones we buy are like Cybercow says, peak response is around 540nm.  Their response is at least 80% from 500nm to 650nm, so white, green or yellow will all work well. We want to direct most of the light onto the LDR.  Water clear LEDs emit most of the light straight out of the tip, so pointing them as Music6000 suggests gets the most light onto the LDR.  With diffuse LEDs, you can get away with mounting the LDRs facing the side of the LED.  

One more thing: circuits that use LDRs are (or should be!) designed to function over a specific range of LDR resistances.  Once you have enough light on the LDR to meet the circuit's design requirements, putting more light on the LDR is of little use.  Example: the LDRs in the General Tso have the most leverage above 10K.  From 1K to 10K, the gain changes by less than 3dB.


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## Bobbyd67 (Apr 9, 2020)

Chuck I just wanna say thanks !! I am learning more and more from your posts and it's awesome


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## lcipher3 (Apr 10, 2020)

I made a few measurements - just a power supply to the LED and resistance of the output LDR.
It's hard to be exact on my mA but it's a decent comparison.
The GREEN seems a lot higher resistance - I happened to have a YELLOW and it seems pretty darn close.
The LDR is the GL5516

*VTL5C4*
1mA = 784 ohms
5mA = 354 ohms
11.5mA = 220 ohms
20mA = 168 ohms
41mA = 120 ohms

*TAYDA GREEN A-1553 - LED top pointing at LDR inside a 5mm shinksleeve*
1mA = 7.7k ohms
5mA = 2.1k ohms
10mA = 1.2k ohms
20mA = 835 ohms
43mA = 577 ohms

*TAYDA YELLOW A-1583 - LED top pointing at LDR inside a 5mm shinksleeve*
1mA = 846 ohms
5mA = 320 ohms
11.5mA = 204 ohms
20mA = 155ohms
40mA = 126 ohms


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## Chuck D. Bones (Apr 10, 2020)

The VTL5C4 measurements are in spec.  That part is designed for very low ON resistance.   The General Tso does not require a low ON resistance opto.

Did you measure the dark resistance when you had the LDR and LED together inside the heatshrink?  That yellow LED is clear lens, high brightness; the green LED is diffuse lens, not high brightness.  You had three variables in your experiment.  I suspect the LDR resistance difference has a lot more to do with the LED brightness and lens type than it does with the color.

Like most optical compressors, the LED & LDR are inside the volume control feedback loop.  The circuit will adjust the LED current to achieve the desired volume, within the available control range.  Either of those LEDs will work in the General Tso. The yellow LED will just operate at a lower current and _may _have a perceptibly faster response.


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## lcipher3 (Apr 10, 2020)

>>I suspect the LDR resistance difference has a lot more to do with the LED brightness and lens type than it does with the color. 
Agree - the yellow was the only clear that I had to try.

The resistance of the VTL5C4  "off" is over-limit of my meters.  Shows open.
I have the LED/LDR under the shrink sleeve and black electrical tape to cut out all light - the off ranges from 4Meg to 7Meg - it sort of floats around.


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## Chuck D. Bones (Apr 10, 2020)

That's plenty high.  I just wanted to verify that the LDR had high dark resistance.


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## lcipher3 (Apr 10, 2020)

Chuck D. Bones said:


> That's plenty high.  I just wanted to verify that the LDR had high dark resistance.



It's plenty sensitive to any stray light.  Just the shrink sleeve isn't really enough to get it low (20-30K or so, light coming in from the "ends") - that's why I covered it all with black electrical tape.  I may try black silicone potting in the "ends" of the shrink sleeve to seal it up.

Of course inside a pedal box it should be pretty dark - but there's possibility cross-talk with between the two LED/LDR (?)


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## Chuck D. Bones (Apr 10, 2020)

Inside the box it should be plenty dark.  Depending on how the front panel LED is mounted, there could be some light from that inside the pedal box.  Not much, but not zero either.  

Should not be any crosstalk, the whole point of an opto-isolator is isolation between the two sides.


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## jubal81 (Jun 1, 2020)

Got mine dialed in to taste. I used a green diffused (Here's a link) pointed face-to-face with GL5516. I swapped the 1K5 resistors (R23 & R25) with 120K. Those are the CLRs. Yes, 120K. No typo. I was shocked, but the DMM don't lie.

With 120K, the GL5516 will bottom out around 10K using that LED. Still get a tremendous amount of squish, but now the decay is smooth and the compression and tone controls work perfectly. Really nice compressor, but I don't like the 'blend' setting at all. Better without it, IMHO.


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## Chuck D. Bones (Jun 1, 2020)

Are those LED's "super-bright" or high-efficiency?  It is amazing how low you can run the current in some LEDs and still get a visible amount of light.  Some LDRs are very sensitive, appears yours are.  Not over-dosing the LDR with light makes for a smoother release.  Increasing those resistors slows down the release, but sounds like it's all good.  The other way to reduce the amount of light on the LDR is to point the LDR at the side of LED, increase distance between LED & LDR, or both.


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## jubal81 (Jun 2, 2020)

Chuck D. Bones said:


> Are those LED's "super-bright" or high-efficiency?  It is amazing how low you can run the current in some LEDs and still get a visible amount of light.  Some LDRs are very sensitive, appears yours are.  Not over-dosing the LDR with light makes for a smoother release.  Increasing those resistors slows down the release, but sounds like it's all good.  The other way to reduce the amount of light on the LDR is to point the LDR at the side of LED, increase distance between LED & LDR, or both.



Yes, it's a superbright (2200 mcd). Had these around for indicators. Got some different LEDs coming this week for coupler duty (100 mcd, clear green, flat top).

Batches of GL5516 from two suppliers getting all the way down to 330-390R. Surprised how consistent they are.


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## Chuck D. Bones (Jun 2, 2020)

If you have a way of measuring the LDR's transient response, check it out.  It was a revelation for me when I did it.  I'll end up swapping out the LED in my Delegate because the one in there is too damned bright for the LDR.


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## cooder (Jun 2, 2020)

Super interesting stuff here again, thanks for all the insights!
So would similar combinations and considerations be applicable for the Delegate / EQD Warden?
What would be a recommended LED/LDR combo there? Cheers!


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## Chuck D. Bones (Jun 2, 2020)

My LDR experience is limited.  What I have figured out is that putting more light on an LDR than is needed by the circuit is counter-productive.  In both circuits, the gain is proportional to LDR resistance.  As the guitar signal decays, the compressor reduces the LED current so that the LDR resistance will rise and increase the gain.  If the LDR was over-dosed with light, the LDR resistance will not rise quickly enough to maintain a constant, or near constant signal level at the output.  The result is that the guitar signal fades and then recovers.  

The short answer is you don't want a bright LED and as long as the LDR resistance gets low enough during the attack, the color of the LED doesn't matter either.  Use what's in the BOM and don't obsess about getting every photon onto the LDR.  If you get enough compression and sustain and it all works smoothly, then you did it right.  If the compressor over-reacts and cuts the guitar signal too much immediately after the attack, then too much light is landing on the LDR.


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## cooder (Jun 2, 2020)

Great advice Sir Chuck! I'll give it a whirl...


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## jubal81 (Jun 2, 2020)

Chuck's right. You're going to want a low-brightness LED there. With that super-bright LED and GL5516, the sustain and tone controls might as well have been switches. It wasn't working properly, but I gotta say I had a lot of fun with it like that. 

Found another LED I'm going to grab on my next Mouser order - a 30 MCD clear, green, flat top (link). I also ordered some GL5528s from eBay to try.


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## lcipher3 (Jun 2, 2020)

Aside from cost - any reason not to just use a VTL54C?

*VTL5C4*
1mA = 784 ohms
5mA = 354 ohms
11.5mA = 220 ohms
20mA = 168 ohms
41mA = 120 ohms

*TAYDA GREEN A-1553 - LED top pointing at LDR inside a 5mm shinksleeve*
1mA = 7.7k ohms
5mA = 2.1k ohms
10mA = 1.2k ohms
20mA = 835 ohms
43mA = 577 ohms

*TAYDA YELLOW A-1583 - LED top pointing at LDR inside a 5mm shinksleeve*
1mA = 846 ohms
5mA = 320 ohms
11.5mA = 204 ohms
20mA = 155ohms
40mA = 126 ohms


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## jubal81 (Jun 2, 2020)

So after some digging in my closet, I actually found some diffused 5mm Tayda LEDs - green (A1-1553) and yellow (A-1555). These must be 4-5 years old, so I can't be sure it's what they're still selling.

With the stock values (1K5), the current for the LED drivers is limited to about 5ma. At which, I got:
Green: 3.2K
Yellow: 860R

So I made some new couplers with the Tayda green, swapped back in the standard 1K5 CLR and voila, sounds great and controls work just as you'd expect.

So, 5516 and Tayda green and I'd say you're set. And I'd wager you're getting a better response with that combo than the VTL5C4.


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## jubal81 (Jun 2, 2020)

@Icipher: I think to best answer your question, all 3 of those will work and it's going to be a matter of taste, so I'd recommend experimenting and comparing to find out which you prefer. 

The Tayda diffused green works really well for me and I think if people want to just build it with that and 5516, they'll get good results. On their website, they characterize it as 2,000 to 3,000 MCD, which is off my a factor of 100 I suspect.

I do recommend this build, though. It can't do everything my Cali76 can do, but with the tone control, and how I actually use compression, I prefer the General Tso - don't need that switch, though.


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## cooder (Jun 2, 2020)

Super, thanks for sharing your findings, jubal81!


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## lcipher3 (Jun 2, 2020)

jubal81 said:


> @Icipher: I think to best answer your question, all 3 of those will work and it's going to be a matter of taste, so I'd recommend experimenting and comparing to find out which you prefer.



Yeah I have it all built up except for the LDR socket - just been lazy lately!  I have the AION version (w/o the treble pot) and it sounds amazing with VTL54C .
I figure either the *TAYDA GREEN A-1553* or Yellow (or VTL) will all work great.

Did you couple LED side to cell,  or LED top to cell?


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## jubal81 (Jun 2, 2020)

lcipher3 said:


> Yeah I have it all built up except for the LDR socket - just been lazy lately!  I have the AION version (w/o the treble pot) and it sounds amazing with VTL54C .
> I figure either the *TAYDA GREEN A-1553* or Yellow (or VTL) will all work great.
> 
> Did you couple LED side to cell,  or LED top to cell?



I have them touching face-to-face. I got some little round PCBs to solder the LED and LDR to fit perfectly in a milkshake straw, which I then cover with heat shrink. But if I do my own layout, I think I've got an even easier solution worked out. Stay tuned.


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## cooder (Jun 2, 2020)

jubal81 said:


> I have them touching face-to-face. I got some little round PCBs to solder the LED and LDR to fit perfectly in a milkshake straw, which I then cover with heat shrink. But if I do my own layout, I think I've got an even easier solution worked out. Stay tuned.


That's next level sh#t....


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## lcipher3 (Jun 2, 2020)

very nice work


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