*best* LDR for Mutron III / Neutron circuit  [found/sold]

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digi2t
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Post by digi2t »

Here is my tutorial for rolling your own Vactrol for the Mutron. I used a clear bright white LED, and two Waitrony KE-10720 LDR's.

https://www.diystompboxes.com/smfforum/ ... ic=95269.0

The only thing I recommend is that you match the LDR's. I took a piece of wood, drilled two holes at the lead spacing of the LDR, and set it in a vice, underneath a light source that I could control the intensity. Pop an LDR into the jig, ensuring that it's sitting flat against the wood, and pointing directly at your light source. Connect you DMM to the LDR leads. Set your light source to an intensity where you get somewhere in the neighbourhood of a 10K reading from the LDR, it doesn't have to be exact. Write the reading down. Now, pull that LDR out, and insert another one in it's place. Connect the DMM, and take a reading. Compare the reading to the first one.

Keep going, until you get two LDR's that are within 100, to 200 ohms of each other. The Waitrony's are dirt cheap, so buying 10 or 20 of them won't break your bank. Definitely cheaper than VTL's. You'll be able to match up several sets.

No matter how many times I cut it, it`s STILL too short!

https://www.deadendfx.com/

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rrroo
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Post by rrroo »

I built the madbean version of this pedal. made my own LED/LDR. everything seemed to work in UP mode but not in DOWN mode. I found out that the bias voltage at op amp, defined by R18 and R19 is not optimal. if you replace these with a trimmer working as a voltage divider, you can dial in just the right filter sounds and the choise of LED/LDR is not that crucial.

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吉田ほにお
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Post by 吉田ほにお »

Hello rrroo.
I also made Mutron, but I have a problem. down mode works, but up mode doesn't. There seems to be no change even when listening to the sound. You said you adjusted the bias, do you need to adjust it to a value suitable for up and down each time?
Did you make the capacitor of the filter part with 1n8 2n2?

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bmxguitarsbmx
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Post by bmxguitarsbmx »

Up/Down requires a SPDT switch to operate properly in up and down mode.

The first section of the SPDT switch increases gain to the envelope circuit. The second section of the switch switches the LED driver from Non-inverting in "Up" mode to Inverting in "down" mode. Notice that when in Inverting mode, a separate bias is also switched in via voltage divider resistors.

If a schematic was posted, further clarity could come from using part numbers.

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吉田ほにお
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Post by 吉田ほにお »

dear bmxguitarsbmx

thank you for your reply. I'm flipping the switch in two places, but I'm having problems. It works with down, but up doesn't. Does the capacitor in the filter section operate at 1n8f and 2n2f?
It seems that there is a slight change in the sound when listening carefully in the up mode, but it is not as clear as in the down mode. The time for change is very long.
I recently tested another wah, but the up mode didn't work at that time either. The optocoupler wah is very difficult to adjust.
I would appreciate any hints.
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mutron3.jpg

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吉田ほにお
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Post by 吉田ほにお »

bmxguitarsbmx

The second section is to switch the LED driver from non-inversion in "up" mode to inversion in "down" mode, but since down is a + input, isn't it non-inversion?
Is it necessary to switch the range switch when switching down and up?
I tested it again and it feels like the up doesn't work. There is no change in sound quality.

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bmxguitarsbmx
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Post by bmxguitarsbmx »

吉田ほにお,

The schematic I have has "Down" with the + input to the LED drive connected to the resistor divider that sets the static frequency. The envelope is connected to the LED drive through standard inverting setup.

You do have to change the range when switching between down and up. Let's take and example of setting the static frequency at 40Hz. 40Hz is a great starting point for UP mode and you are sweeping the filter up into the audible range. 40Hz would not work in DOWN mode because you are essentially below all frequencies you can hear, and sweeping, down, below 40Hz is also inaudible.

I think what I might do is measure the voltages at the output of the LED drive and determine what the useful range of voltages are. Then monitor those voltages while playing to get an idea of where you are.

Also, what modes do you have hooked up? If the filter is way above hearing range in low pass mode, it will appear to do nothing. Same with the filter operating below hearing range in high-pass mode. Likely, very low Q settings will also seem to not be working.

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bumblebee
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Post by bumblebee »

I’ll just upload this here for those who don’t have it.
MuTron III neutron Project - R.G. Keen.pdf
(1.28 MiB) Downloaded 62 times

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吉田ほにお
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Post by 吉田ほにお »

Bumblebee
Thank you for the article.
bmxguitarsbmx
The current of the output of the LED drive was measured.
In the up mode, the silence state is 0.27ma at the maximum when playing at 0.04ma. When you stop playing, it will drop from 0.27ma, but it will take a long time to change the sound quality due to the long release or decay. Nothing has changed for a while. If you can adjust this changing time, it will be easier to feel the change in sound quality. When I tried the condenser of the filter part and set it to 4.7n, the change became noticeable a little.

In down mode, the silence state is 0.00ma at maximum when played at 0.27ma. When you stop playing, it will increase from 0.00ma. You can clearly see the change in sound quality here. There is a filter effect even if the current change does not change. The condenser seems to be fine with 1n8.
The down mode is effective because it can be heard with 3 types of filters.
In the up mode, you can hear the low pass, but it is difficult to hear the band pass and high pass.
Is it possible to improve the way the sound quality changes are heard by adjusting the release or decay in up mode?

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Post by bmxguitarsbmx »

Thank your the numbers. I think that gets us somewhere. 0mA - 0.27mA is barely turning on a LED. Let's increase the current through the LED by decreasing the value of the current sensing resistor (Rx in the scheme posted by Bumblebee). I'm just guessing but, I think you may want to shoot for current values between 5mA and 20mA for your static value in the DOWN mode.

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Post by 吉田ほにお »

bmxguitarsbmx

I adjusted the LED current, but I can't adjust it to the range of 5mA to 20mA.
Currently, I am referring to Mad Bean's Naughty Fish.
Adjusted with a combination of 5kpot and 330Ω. The photocoupler used is lcr-0202. When down, 5kpot is the minimum (330Ω only) and 1.74ma in silence. You said that the current changes in the range of 5ma to 20ma when down.
Is that a thing?
In Mad Bean's Naughty Fish, the bias voltage due to voltage division is around 3.61v when down, but if you adjust to this voltage, the wah effect will disappear.
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voltage
voltage
Mad Bean's Naughty Fish
Mad Bean's Naughty Fish

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bmxguitarsbmx
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Post by bmxguitarsbmx »

吉田ほにお wrote: 04 Mar 2022, 12:20 bmxguitarsbmx

...You said that the current changes in the range of 5ma to 20ma when down.
Is that a thing?
...
In down mode, current through he LED will be high current when not playing and low current when playing. The higher the current, the brighter the LED, the lower the resistance value of. the LDR, and the higher the frequency of the filter. I dont know the sensitivity of your LDR's, but LED's are not very bright at less than 1mA. It seems to me that you will want to adjust it to a higher current value 5mA to 20mA with no input signal present. Then when you hit a note, the current will drop, giving yo the sweep. The current values you listed are so low, I don't see much of a sweep happening. Hopefully that makes sense.

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Post by 吉田ほにお »

Thank you for your reply. Thank you for your advice.
For the time being, I made it according to the circuit diagram and layout, but it does not work well. I am thinking of reviewing the specifications of the parts again. First of all, I would like to improve the accuracy by changing the resistance to a metal film resistance.
Should I also replace the optocoupler lcr-0202?
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Photocoupler specifications
Photocoupler specifications

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Post by Manfred »

I have a Mutron on the workbench and the first thing I did was check to see if the O805 optocoupler was still working.
Yes, it works so I plotted its P-N junction curve of the LED with my Peak Atlas DCA Pro, as well as the LDR curve as a function of LED current.
The dark resistance of LDRs are 221 Kiloohms. Both LDRs have identical values and seem to have been selected.
This Kingbright WP2523SURC/E T-1 3/4 (5mm) Round LED could be a good candidate for a DIY O805 replacement.
WP2523SURC-E.pdf
(303.69 KiB) Downloaded 13 times
PN Junction I versus V LED Mutron III.jpg
Resistance LDR Mutron III versus LDR current.jpg

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