Photocell/Ldr Quick question

[hendrix2489]

hey guys, yet another 'vibe question sorry, but i was looking on smallbearelec and trying toestimate how much it would cost to make an easyvibe.  My question is does it matter in the easyvibe what kind of ldr i use.  i know the light to dark range is important, but i was wondering  if the CL7P5HL   photocell would work good, its got a pretty good range.  thanks in advance

[scottso]

After building mine the thing you really need to worry about is the swing between resistances.  You want a huge swing from low to high.  Like 1k or 2k to 500k or even 1M on the resistances.  I had built mine originally with the VTL5C2 and it didn't work.  But I found out later after replacing them with an LED/LDR handmade combo it was because the LED was wired backwards.  Which means on the circuit board layouts you'll find on the web, its laid out backwards for most drop in photocell parts like the VTL5C2.  Basically if the positive lead for the LED is on the left it won't work unless you cross the leads or change the PCB layout or wire the photocell to the bottom of the PCB.

Edit: Fixed typo.

[hendrix2489]

thanks for the quick response but i was just wondering if the CL7P5H would sound good in an easyvibe.  it has Dark Resistance 14 Meg, Light Resistance 28K.

[markm]

Which means on the circuit board layouts you'll find on the web, its laid out backwards for most drop in photocell parts like the VTL5C2.  Basically if the positive lead for the LED is on the left it won't work unless you cross the leads or change the PCB layout or wire the photocell to the bottom of the PCB.

Which layout did you build from?

[scottso]

I used the one from here, which is basically the same as the one on Geo (It just adds some jumpers on the top to make wiring the pots and switch neater):

http://www.storm-software.co.yu/diy/layouts/easyvibe.diy

[scottso]

thanks for the quick response but i was just wondering if the CL7P5H would sound good in an easyvibe.  it has Dark Resistance 14 Meg, Light Resistance 28K.

That would probably work. I read somewhere to use one that goes down to 10k or 20k but I don't know how important that is.  Mine swing from 2k to about 2M and works just fine if that helps.

[hendrix2489]

i just finished building it over the weekend.  It sounds awesome.  I like both the vibe and chorus settings, and i will post pictures when done decaling. 
P.S. i used heat shrink and the ldrs are facing the side of the Leds.  I do not know if that makes a difference but it sounds great. The effect was plenty deep enough.   

[Mark Hammer]

You can always stick a fixed resistor in parallel with the LDR if it's max resistance is too high, and you can stick a fixed resistor in series if it's min resistance is too low.

In theory, you can tie the ground side of all the LDRs together and route them through a common pot (variable resistor) to ground (or Vref) to "tune" the unit.  Come to think of it,  I imagine you could do the same thing with the ground resistors in the Phase 90 or any comparable design.  The FETs go to ground/Vref as always but the parallel 22k resistors are adjusted, en masse, by a common additional resistance to the ground/reference point.  As near as I can tell, the phaser itself doesn't particularly care if the path to ground is via 1, 2, or 3 resistances or if the last portion of that path is via a common resistance.  What matters to the phaser is where the phase-shift is applied as a function of the cap and the combined total resistance to ground.

The nice thing about this is that while you can change where the phase shift is applied via the FET bias control on the Phase 90, adjusting bias also tampers with sweep width, whereas adjusting the resistance to ground has no impact on the sweep width, merely where the sweep is situated in the spectrum.

make sense?

if you want to get real sick, you make that additional shared resistance a 5th LDR and drive that with its own LFO to shift where the shift is.

[scottso]

if you want to get real sick, you make that additional shared resistance a 5th LDR and drive that with its own LFO to shift where the shift is.

I think my head just exploded! 

[~arph]

But I found out later after replacing them with an LED/LDR handmade combo it was because the LED was wired backwards.  Which means on the circuit board layouts you'll find on the web, its laid out backwards for most drop in photocell parts like the VTL5C2.  Basically if the positive lead for the LED is on the left it won't work unless you cross the leads or change the PCB layout or wire the photocell to the bottom of the PCB.

But surely you can just turn the vtl5c2 on it's back? the LDR leads position can be swapped and this way the LED leads will be the other way around   

[Paul Perry (Frostwave)]

In theory, you can tie the ground side of all the LDRs together and route them through a common pot (variable resistor) to ground (or Vref) to "tune" the unit.  Come to think of it,  I imagine you could do the same thing with the ground resistors in the Phase 90 or any comparable design. 

Hmmmmmmmmmm.........  think there might be a problem........

[Mark Hammer]

And that would be?

I'm looking at the schem, and as long as the shared resistance to ground is noticeably smaller than each of the parallel resistors (which is, I suppose, an important qualifier I should have added), I don't see why the noninverting input should care how it reaches ground.  I'm not saying there couldn't be a reason, just that I don't see it.

Always happy to learn something new, though.

[slacker]

The only possible problem I can see is that any signal or voltage fluctuations at the junction of the parallel resistors and the shared resistor might cause oscillation by getting back into the phase stages via the parallel resistors, if that makes sense     I guess that's why you said the shared resistor should be smaller so that any fluctuations would not affect the phase stages.
I'm trying to figure out if I can do a quick hack on my easyvibe to test this out, if I manage it I'll let you know what happens.

[Paul Perry (Frostwave)]

I'm looking at the schem, and as long as the shared resistance to ground is noticeably smaller than each of the parallel resistors (which is, I suppose, an important qualifier I should have added), I don't see why the noninverting input should care how it reaches ground.  I'm not saying there couldn't be a reason, just that I don't see it.

Ah, I thought you meant, tying the ends of the LDRs together and running them through a common resistor to the point that is grounded by the cap, so that the shared resistor actually carried signal, rather than providing an adjustable DC offset only.