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Topic: photocell volume pedal question from Steve Black |
Steven Black
From:
Gahanna, Ohio, USA
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Posted 22 Apr 2016 1:08 pm |
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| Has anyone of you electronic buffs out there ever made a home built photocell volume pedal and do you have a schematic for one? I am trying to find one to no avail and would like a schematic, thanks Steve Black. |
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Jon Light
From:
Saugerties, NY
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John Bresler R.I.P.
From:
Thornton, Colorado
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Posted 22 Apr 2016 1:49 pm
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There's an Edwards light beam VP on sale on ebay - might be a good one for your project - the light beam Edwards VP's are driven with a photo-cell.
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Craig Baker
From:
Eatonton, Georgia, USA - R.I.P.
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Posted 22 Apr 2016 2:01 pm
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Steven,
Here's an article that should give you an idea of what's involved. . . not very much it seems.
http://www.neufeld.newton.ks.us/electronics/?p=1137
You can probably find other similar circuits using Google.
Let us know of your success.
Craig
_________________
"Make America Great Again". . . The Only Country With Dream After Its Name. |
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Georg Sørtun
From:
Mandal, Agder, Norway
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Posted 22 Apr 2016 2:03 pm
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Looks like my old (-90s) opto-Morley, but they have released many versions. Mine works fine, once I adjusted the taper to my liking.
Have never built a LED/LDR volume pedal, but I have built circuits using LED/LDR couplings and flags for lots of other uses, including for audio. The electronic circuit is simplicity itself - one or two LDRs; one or two LEDs; and a few resistors.
It's the mechanical side of it that is critical, as the pedal moves a flag that distribute light/shade between LEDs and LDRs. |
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Glenn Demichele
From:
(20mi N of) Chicago Illinois, USA
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Posted 23 Apr 2016 9:27 am
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I have a Goodrich pedal, and bad luck with replacement pot quality. Worried about pot failure on a gig, I built this "redundant" photocell circuit into my pedal rather than carry an extra volume pedal. If the pot gets scratchy, I just plug in the power supply and throw the little switch on the pedal. The CDS photocell is something I got in an electronics grab-bag when I was a kid in 1967. If it hasn't failed by now, it never will. The LED is mounted opposite the photocell, and the piece of cardboard blocks the light as the pedal moves. The pedal is totally passive, but I need the power supply to light the LED.
_________________
Franklin D10 8&5, Excel D10 8&5. Both amazing guitars! Homemade buffer/overdrive with adjustable 700Hz "Fender" scoop., Moyo pedal, GT-001 effects, 2x TDA7294 80W class AB amps, or 2x BAM200 for stereo. TT12 and BW1501 each in its own closed back wedge. Also NV400 etc. etc... |
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Georg Sørtun
From:
Mandal, Agder, Norway
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Posted 23 Apr 2016 11:20 am
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| Hi Glenn, you have a buffer before that low-ohm VP, right? |
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Glenn Demichele
From:
(20mi N of) Chicago Illinois, USA
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Posted 23 Apr 2016 1:08 pm
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Yes, I do use a buffer, and if the buffer should die, I can limp along. In that case, the pedal still works and the tone is ok but the level is lower.
_________________
Franklin D10 8&5, Excel D10 8&5. Both amazing guitars! Homemade buffer/overdrive with adjustable 700Hz "Fender" scoop., Moyo pedal, GT-001 effects, 2x TDA7294 80W class AB amps, or 2x BAM200 for stereo. TT12 and BW1501 each in its own closed back wedge. Also NV400 etc. etc... |
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Steven Black
From:
Gahanna, Ohio, USA
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Posted 23 Apr 2016 5:38 pm Photocell volume pedal reply from Steve Black
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| Hey Glenn that looks about what I am looking for to rig up in my Goodrich volume pedal, thanks for the schematic, are resistors in k ohms or are they mostly ohms? |
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Glenn Demichele
From:
(20mi N of) Chicago Illinois, USA
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Posted 24 Apr 2016 7:23 am
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The circuit gets simpler if you want to give up on the pot altogether (see pic below). The earlier schematic is what I actually have in my pedal. For the resistor in series with the input, I started with a 33k, and put a 15k in parallel because it worked better - these two can be replaced with a single 10k resistor. The best value to use depends on the resistance of the photocell. If your resistor is too high, the pedal action will be too fast. If it's too low (or if your light's too dim), you won't be able to cut off your signal with pedal up. This is why I replaced that resistor with a trimpot i the schematic below.
The three 330 ohm resistors limit the current in the LED to the correct value. There are three in parallel because of power dissipation. You can use anything for the light source, as long as it doesn't have 60Hz ripple on it. Even a battery powered LED bike light would work.
_________________
Franklin D10 8&5, Excel D10 8&5. Both amazing guitars! Homemade buffer/overdrive with adjustable 700Hz "Fender" scoop., Moyo pedal, GT-001 effects, 2x TDA7294 80W class AB amps, or 2x BAM200 for stereo. TT12 and BW1501 each in its own closed back wedge. Also NV400 etc. etc... |
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Steven Black
From:
Gahanna, Ohio, USA
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Posted 26 Apr 2016 9:56 am Photocell volume pedal
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| Thanks Glenn, this helps a lot, I have a goodrich volume pedal L10k that uses a 9 volt battery, but pots go bad in it really quickly don't know why they do, but I want to convert it to a photocell volume pedal and hope it works better than the pot type, I have an L120 goodrich volume pedal too but the pots go bad in it too, I wonder why Goodrich has bad pots? does not make sense to me. |
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Georg Sørtun
From:
Mandal, Agder, Norway
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Posted 26 Apr 2016 3:00 pm Re: Photocell volume pedal
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| Steven Black wrote: |
| I wonder why Goodrich has bad pots? does not make sense to me. |
Allow me to comment on this, as "bad VP pots" is such a often occurring problem.
If you experience "sparkle" and/or "scratch" noises in your pot-pedals, it is most likely not because Goodrich have inherently bad pots. FWIW: my old Goodrich - bought used years ago and not upgraded - never produce noise, and I expect it to behave "silently" until the contact-point is worn out.
There are mainly two reasons why VP pots start becoming noisy...
1: humidity and dirt in the air create an uneven layer on the resistive surface, resulting in unreliable electrical contact between rotor and stator. Pots with carbon contact-point on the rotor are much better at self-cleaning than those that have metals there, but no pot can clean itself perfectly for very long under real-world conditions.
Dirty pots quickly leads to the following (point 2)...
2: the series-capacitor (in amp or other unit) following the pot can charge up with DC - a few milliVolts of DC is a lot - that the pot will have to discharge over its mechanical contact as it travels from high to low resistance relative to common/GND. No types of pots are good at discharging DC over that contact point, and will sooner or later produce discharge-sparkles at points on the track where even the most minutes layers of humidity and dirt has built. Once this "sparkling" starts the track gets damaged at that point - burnt ever so slightly - and the "sparkling" cannot be totally remedied through cleaning and/or lubricating it.
- Lubrication of the track performed on new pots - before any "sparkling" occur, will make the pot last much longer, but making sure DC-charging in the coupling capacitor following the pot does not happen and discharging via the pot therefore cannot take place, is the correct solution if one wants pots to last.
All this "techno-babel" leads to the fact that LDR-based resistive dividers (obviously) don't have mehanical rotors w/contact points and track-surfaces that can fail, and LDRs are therefore neither sensitive to humidity/dirt nor DC-discharging.
So, yes, you are 100% correct in expecting that replacing the pot with an LDR circuit will permanently solve your problems with "noisy pots".
Note that Glenn's diagram shows a low-ohm "choke" - an unballanced divider that variably "chokes off" the signal to GND, and that it will only work satisfactory - keep the levels up and load optimized - if it is preceded by a buffer that can drive such a low-ohm load while also presenting the PU (or whatever else you have before your VP) with a more suitable - that is "much higher" - impedance. Once this is taken into account it is a simple circuit that should work well for decades.
More complete LDR-based circuits that are designed to emulate passive pots, have two LDRs, and usually also two light-sources and dual flags, to create a complete resistive signal-divider that can be tailored to emulate any taper. That's how my Morley is designed, and such balanced circuits can be made to work equally well with or without buffers - they are just a little more work to tune in for perfect operation. |
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Glenn Demichele
From:
(20mi N of) Chicago Illinois, USA
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Posted 26 Apr 2016 3:06 pm
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Nice Georg: A disclaimer: my "wonderful" circuit was spawned by anger and frustration, and includes copious amounts of hot-glue to stick it into my existing pedal. It was intended as "plan B" on a gig, and it works remarkably well, especially with my buffer preceding it. I still like an unpowered pedal, but running a cord is better than no pedal at all.
_________________
Franklin D10 8&5, Excel D10 8&5. Both amazing guitars! Homemade buffer/overdrive with adjustable 700Hz "Fender" scoop., Moyo pedal, GT-001 effects, 2x TDA7294 80W class AB amps, or 2x BAM200 for stereo. TT12 and BW1501 each in its own closed back wedge. Also NV400 etc. etc... |
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