what green LEDs for General Tso?

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.
 
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
 
manfesto said:
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
Click to expand...

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.
 
lcipher3 said:
So what would be the best for a LED/LDR vactrol type app like the compressor?
Click to expand...
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.
 
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.
 
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
 
Last edited:
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.
 
>>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.
 
Chuck D. Bones said:
That's plenty high. I just wanted to verify that the LDR had high dark resistance.
Click to expand...

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 (?)
 
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.
 
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.
 
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.
 
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.
Click to expand...

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.
 
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.
 
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!
 
You must log in or register to reply here.
Back
Top