LDR what to use Help please.

[Brymus]

Hi I am getting ready to place a small order for parts and was wondering what to order fro use as LDR.
I dont mind making my own with heat shrink.
What should I order,can I get the parts at Mouser?
I know this is probly posted elsewhere I just dont remember where.

[GibsonGM]

Well, it depends on what circuit you're building, Brymus...LDR's come in various types, meaning how they 'resist' in light and dark.  Typically, you'll have a choice as to what the "Light" resistance is, and what the "Dark" resistance is.  So, if you have a generic LDR and the circuit needs it to act a certain way, you'd have to use resistors to customize it (if possible). 

Try to find out what the thing you're building needs, and get close if you can....

[Brymus]

Thanx Mike
I was gonna try a couple of phaser circuits ,And I noticed alot of circuits use them so I thought I should get the stuff to make a few.
Im just not sure what to get.

[big bustle]

it seems like most ldrs you find in circuits 'round these parts is the vtl5c3. this could be cuz its great for audio or one guy used it and it just stuck in every one s minds. i don't know

allied has them for very cheap right now

futurlec has a bunch of cheap ones in dip  format

i recall seeing one with similar specs to the vtl5c3. don't remember which one it is

[Heemis]

Just elaborating a bit on what Mike said.

If you're looking to replace a variable resistance in your circuit, like a potentiometer, with an LDR, you want to look for one that has a similar resistance range.  IE if you have a 500k pot, you're going to want an LDR with a low light resistance 1-10k, and a dark resistance around 500k.  Sometimes you cannot accurately replace a pot, because the range of the LDR's resistance is too large, in which case you need to tailor the light level of the LED to only hit within those resistances.

In other circuits which call for LDRs, you want to get as close to the same specs as possible, because the circuit was likely designed around that LDR's characteristics.

I haven't had any luck finding CdS cells at mouser, but smallbear has a bunch: http://www.smallbearelec.com/Detail.bok?no=711

And so does Futurlec: http://futurlec.com/Photocells.shtml

As you can see, the light and dark resistances for these photocells are listed.

[petemoore]

  Different ways to research it.
  Finding the low resistance of the LDR used in 'x' phaser, or recommendations...takes some searching, look at some ''phasers w/LDR's'' schematics and check out the parts lists.
  Sometimes you need very fast, other times may require very low resistance [for an LDR], it only matters when it matters...that's where the searching and/or testing comes in.
  There's not always a work-around for the characteristics of an LDR that you don't have.

[Brymus]

Thank You for the replies and links.
It appears I need to decide which circuits I really want to build and make sure I get the right parts.
So this wont make my pending order,but will have to wait while I do some more reading and research.
thanx again everyone.

[Mark Hammer]

While they should certainly be in the same ballpark, in order to make it easier to use a single capacitor value that will be able to place the notches in the audible range, there is no requirement to have LDRs of the same value. 

It is easy to think that each stage matters, but in reality it is the cumulative phase shift over stages that matters.  Each stage contributes up to a maximum of 90 degrees of phase shift above (or below, depending on whether the R or C goes to ground) some frequency, with lesser amount below (or above) that frequency where you are currently at the 90-degree mark.  If, at some single point during the sweep cycle, stage A is yielding 85 degrees shift at 1khz, stage B produces 90, stage C produces 87 degrees, and stage D produces 81 degrees, you have a total of 343 degrees of phase shift at 1khz...at that moment in time.

Since the most phase shift that a single stage of one-pole allpass can produce is 90 degrees, frequencies higher than 1khz may have 360 degrees of cumulative phase shift, but no more than that, while frequencies lower will have less cumulative phase shift.  Obviously, at that particular moment in time, there will be some frequencies where the total across all 4 stages will be below 180 or even 90 degrees total, and no cancellation will occur.

What you really want is for the total phase shift to keep changing at the same rate as the LFO moves up and down.  The reason why JFETs need to be matched is because they can simply stop changing resistance at a certain point, and the window between where they will and won't change resistance is relatively small.  That's the price you pay for using them. 

If only three of four JFETs are presently changing resistance (the fourth having reached the point of no further change), then the sweep generator may well be producing a particular waveform output, but the rate of actual phase-shift change will be of a different form.  The extreme form of this might be one in which (if you were measuring the actual overall resistance change across the four JFETs) the sweep ascends a bit, then simply stops and stays fixed for a while until it starts to descend a few moments later.

In general, the "musicality" of a phaser sweep is a function of how it makes the turnarounds.  It is, after all, the turnarounds that define whether the sweep is triangular, sinusoidal, etc.  One of the advantages of LDRs is that their window of changeability is very wide.  Obviously, at a certain point, you cannot make the light source any darker and the LDR will not change resistance, and conversely a 300W halogen bulb at 3 inches would not be able to elicit more resistance change either.  But the degree of lightness and darkness that a small bulb or LED might provide is well within the critical window, such that all LDRs used in phasers will produce usable resistance change throughout the sweep cycle, with none of them "choking".  And as long as they are doing that, and your RC value doesn't result in notches outside the human hearing range, you'll be in great shape.

That is but one of the reasons why LDR-based phasers and Uni-vibes can sound great.

[Brymus]

WOW
Thanx Mark ,I had to read that several times to take it all in.
That was quite helpful.