LDR feedback mod Big Muff

[Sophia2001]

Hi all,

I've tried to made this mod to my Big Muff on a breadboard:

Connect the emitter of Q3 with the emitter of Q2 through a switch (so you can turn the loop off). The sustain will be on full when this mod is engaged. Great addition: add a 1M potmeter in the loop (wired as a variable resistor) or, even better!!, a LDR (light dependent resistor). With the latter, you can control the amount of feedback with the light on stage / with your foot shading the LDR from the light.

Source: Coda Effects

Wishes:
- dark/low light = effect is on
- bright/normal light = effect is off
- potentiometer to control the sensitivity
- optional: LED to help set the sensitivity

I've used this schematic as a starting point:


I do not understand how to implement this into the big muff schematic. Anyone?

Thanks for your help 
Sofia  

[ElectricDruid]

Presumably Coda are using the schematic they reference on this page:

https://www.coda-effects.com/p/big-muff-circuit-analysis.html

T2 and T3 are the two clipping stages in the centre, so you just connect their emitters together with the LDR (and a switch to turn it off). Unfortunately, it won't work the way around you wanted: LDRs have lower resistance with more light, so you'll get feedback when it's uncovered, and less when it's shaded.

[Sophia2001]

Thanks ElectricDruid!

Would it be possible to implement this into the Big Muff schematic? By changing the resistor/LDR order, the schematic becomes a dark sensor. However, I don't know how to blend this schematic between the two emitter outputs.

Any ideas?






Sofia 

[antonis]

I think that ONLY a single LDR is needed between Q2 & Q3 stages..
(LDR's apparent value plays the role of feedback resistance..)

[ElectricDruid]

I think that ONLY a single LDR is needed between Q2 & Q3 stages..
(LDR's apparent value plays the role of feedback resistance..)

Yes, but what Sofia is asking is whether it would be possible to do a similar trick in the Big Muff schematic; use a LDR to control a transistor to control the feedback, thereby allowing you to get the LDR the "other way up".
While such a thing might be possible, I'd say it intrduces a lot of extra complications. That transistor is going to have to pass all the required frequencies without distortion, which requires the correct DC bias. And at the same time, we want to vary the amount of feedback using an LDR...very tricky to get right, if it's really possible at all.
I'd probably go for something more "complicated" but easier to use, like a VCA design.

If anyone's got a simple one-transistor way of doing it though, I'm all ears. I'd love to see that done sucessfully.

[Matthew Sanford]

Wishes:
- dark/low light = effect is on
- bright/normal light = effect is off
- potentiometer to control the sensitivity
- optional: LED to help set the sensitivity

I've used this schematic as a starting point:


I do not understand how to implement this into the big muff schematic. Anyone?

Thanks for your help 
Sofia  

Spitballing, but would connecting the emitters with the 150R or a bit bigger (and a switch) with the LDR emitters to ground achieve the wishes? Possibly in series with a pot, or pot between the emitters?

[diffeq]

Wishes:
- dark/low light = effect is on
- bright/normal light = effect is off
- potentiometer to control the sensitivity
- optional: LED to help set the sensitivity



I do not understand how to implement this into the big muff schematic. Anyone?

It is possible to do, if you use one LDR for light sensing and another one coupled to an LED for feedback control itself.


Some notes: R1 will need to be tweaked to suit the response you need, very likely increased to few kiloohms. R2 is sensitivity pot but its value depends on sensor LDR resistance in bright light.

[Sophia2001]

Thanks all for the help!

The schematic made by diffeq might work for me. Two additional questions:
- Can I use the +9V and the ground from the Big Muff schematic to feed the LED? Or must this be an separate power circuit? Will this introduces noise?
- For the LDR on the right side. The Big Muff feedback loop works at ~0 ohm and stops working around ~2K ohm. Most LDRs have a value of 2K - 5K in bright light conditions. So when the LED is full bright, the resistance of the LDR is still to high. I could buy a more sensitive, expensive LDR but there might be other solutions?

Greetings,
Sofia 

[antonis]

Although technically correct should be to implement shunt feedback (instead of series one), you can place a resistor in parallel with LDR..
Of course, you'll have to experiment with its value, which should be higher than 2k..
(actually, equivalent resistance should be lower than 2k for bright light conditions but higher than 2k for dark/low conditions..)
So first of all, verify your particular LDR bright light resistance..

[diffeq]

- Can I use the +9V and the ground from the Big Muff schematic to feed the LED? Or must this be an separate power circuit? Will this introduces noise?

It should be fine, the switching is slow and shouldn't introduce noise.

- For the LDR on the right side. The Big Muff feedback loop works at ~0 ohm and stops working around ~2K ohm. Most LDRs have a value of 2K - 5K in bright light conditions. So when the LED is full bright, the resistance of the LDR is still to high. I could buy a more sensitive, expensive LDR but there might be other solutions?

a resistor or a second LDR in parallel. Another thing to try is different LED colors, like ultra bright Green or Red. They might bring the resistance lower. Lowest most LDRs will go is couple of hundred ohms.

[ElectricDruid]

It is possible to do, if you use one LDR for light sensing and another one coupled to an LED for feedback control itself.

Now, *that's* a good idea!! 

[antonis]

For what LEDs you're talking about..??

OP wishes to use stage lights for varying LDR's resistance, no..??
 

[diffeq]

For what LEDs you're talking about..??

OP wishes to use stage lights for varying LDR's resistance, no..??
 

Yes, but OP needs inverted relation to light. Less light is lower resistance, which is what the transistor and LED+Ldr in scheme above achieve.

[Sophia2001]

Thanks again all! I appreciate your thoughts and ideas. I will go for the 2x LDR + LED solution suggested by diffeq. There is only one problem left. The LDR inside the feedback circuit has to much resistance.

The Big Muff feedback loop works at ~0 ohm and stops working around ~2K ohm. Most LDRs have a value of 2K - 5K in bright light conditions. So when the LED is full bright, the resistance of the LDR is still to high. I could buy a more sensitive, expensive LDR but there might be other solutions?

Sofia 

[diffeq]

I suggest buying other LDRs, maybe several variants to choose from

[Matthew Sanford]

You might parallel a resistor with the LDR to reduce it's range, though not as down to 0 Ohms