center tapped ldr

[candidate]

so what's the deal with center tapped ldr's?  can they be fabricated with normal ldr's?  these are used in photocouplers in effects like the MXR phase 100 and the DOD 440 envelope follower

[armdnrdy]

Here's some info for you:

http://www.diystompboxes.com/smfforum/index.php?topic=85398.0

http://www.thetonegod.com/tech/rockncontrol/rock.html

[Mark Hammer]

1) The LDRs "track" in the same direction.  That is, they do not behave like a pot with one resistance getting higher as the other gets lower.  They both get lower or higher at the same time.

2) The motivation behind having a pair packed into one package is that the circuit fragment driving the LED may pack insufficient current to drive more than one LED (or some other number) at a time, so LDRs are doubled up to "share" light sources in an efficient manner.

3) The need to have "matched" LDRs varies with application.  For something like a tunable filter (as in the state-variable configuration used for countless auto-wahs), matching is more important.  For something like a phaser, matching is much less important.  Where filters are tuned by means of the specific value of the resistances provided by each LDR, phasers produce notches based on the sum total of all phase shift across all the stages in the circuit.  Discrepant LDRs may be producing XX degrees of shift in stage 1, YY in stage 2, etc., and it will still sound fine.  What is crucial about LDRs in phasers is that, unlike FETs, they have a very broad range of distortionless adjustment, such that you can't normally hit any "dead spots" in the sweep, like you might with JFETs.

4) As mere resistances, one can always improve on the matching of different LDRs by means of parallel and series resistances.  Let's say that LDR-A maxes out at 2M3 dark resistance and goes down to 170k when lit.  LDR-B  maxes out at 580K and goes down to 60k.  Okay, let's say we stick a 220k in parallel with the higher value LDR, its sweep range will now go from about 96k to 200k.  If I stick a 330k resistor in parallel with LDR-B, it will max out at around 210k, with a min resistance of 54k.  Stick a 39k in series with LDR-b and you have a reasonably-matched pair; at least better matched than they were before.