led/ldr

[slackhammer]

forgive me if this has been covered before (tried searching), but could someone explain exactly what an led/ldr is and what it does...

[Samuel]

The LED of course is a light-emitting-diode....The LDR is a piece of photosensitive material with two leads attached. When light strikes it, a resistance is created between the two leads which corresponds to the amount of light striking the surface. So an LED/LDR combination is basically a four legged device that has an LED pressed agains the surface of the LDR. When voltage is applied to the LED side, it lights against the LDR, creating a resistance across that side.

[ExpAnonColin]

Also commonly called a "vactrol" (talk about brand-name-take-over-of-product-name), an "optocoupler" (although optocouplers apply to a wider spectrum of devices) and you'll also see things referred to just as LDR-which can be called a photocell, a photoresistor, a CdS cell, etc.

-Colin

[lightningfingers]

my LDRs get less resistant when you apply light to them

[David]

Lightning is right.  As light intensity increases up to some maximum value, the resistance of an LDR drops.  Reduce the light intensity and the LDR's resistance increases to some maximum value.

[Mark Hammer]

Anywhere that having a variable resistance would be useful, a vactrol/optoisolator/LED-LDR (same thing) can be useful.  The company that makes Vactrols (a particular brand of LED/LDR combo units) has some very nice application/theory notes posted in PDF form, but I've quite forgotten the link.  If you or someone else can find them, it will explain just about all you want to know about them in quite clear terms with practical examples.

The one thing the documents do not cover off is the relationship between LDR rise/fall time and usability in specific contexts.  The photochemical reaction in the LDR takes time to occur and produce the change in resistance.  Much the way your eyes take a certain amount of time to "see" again, if the lights are suddenly turned off, it also takes the LDR time to recover when the light source is turned off.  

Typically, the time it takes for the resistance of the LDR to drop when the light is applied is much much faster than the time it takes for the resistance to go back up again when the light is reduced/cancelled.  If all you want the LDR to do is provide a static/fixed resistance level that can be changed electronically, that's not a problem, but if you need the LDR to change resistance values quickly over time, that can be problematic.

For example if the LDR takes a half second to go from minimum ON resistance to maximum OFF resistance, then expecting it to cover that ground while responding to the 4hz signal from a modulation source in a phaser or tremolo will be a bit of a stretch.  It may be able to cover *part* of the sweep but not all of it.  In some instances, that speed/width tradeoff can be helpful, since in many modulation devices people like the sweep to be less broad when it is going at a faster rate.  In other cases, however, it can be a severe hindrance if the drop in sweep range is so substantial as to make faster sweeps essentially inaudible.

When the LDR has to respond to some types of instantaneous events, as in the case of noise gates, compressor/limiters or envelope-controlled filters, some sluggishness is actually helpful since it reduces the sputtering or "envelope ripple"  as the note decays.  On the other hand, too sluggish a recovery from sudden impulses and you impair the capacity of the LDR to respond to the next  note/event if that event occurs too quickly.

This is all the long way of saying that while the LED aspect of an LED/LDR combo is not all THAT critical, the LDR half needs to be optimally matched to what you need it to do in that application, not only in terms of resistance value but in terms of dynamic changes in resistance value.  Fortunately, companies that make them recognize this and produce a variety of models that cover different resistance ranges with different sorts of rise/fall characteristics, and these are listed as part of its specs.  So, if you want something that responds nicely to an LFO, for example, you look for one with fairly quick rise/fall times.

[Samuel]

Yeah had that backwards, my b.

[ExpAnonColin]

The company that makes Vactrols (a particular brand of LED/LDR combo units) has some very nice application/theory notes posted in PDF form, but I've quite forgotten the link.  If you or someone else can find them, it will explain just about all you want to know about them in quite clear terms with practical examples.

I have quite a few vactrol datasheets in the datasheet archives which could be of assistance.

-Colin