LED + LED / LDR Mod

[petemoore]

  Whole new range of sounds for the Neutron !
 Comp liked it too [DOD280].
  I can't be the first to try this, too simple and cool, otherwise I'm glad to take credit as the first to discover it !
 I took a Red LED [could be other colors] and put it right behind the High Brightness Green LED so the light shines through the Green LED's body.
 [I'd chosen a Green High Brightness to light the two LDR's in the Neutron Filter], each LDR super-glued to the side of the Green LED, aluminum foil as partial reflector-cover].
 The 'backlighting' Red LED's brightness controlled using a 500k pot [+1k current limiting] resisdance.
 This mod affords much greater control over the sweep range, ramp rates, allows %^&*ed wah settings, I'm welcoming the wide range of responses, very useful they are, this knob and the pregain knob do interact.
 The "Filter-Floor" can be elevated from 'mud' to 'peizo' frequencies, with gain knob down, a balancing act between the gain knob and "RED's knob''...worth checking to be certain, very cool I sezz, very cool !
 It mellowed the DOD280's Env. Ripple nicely, hafta say the new responses available plainly improved it's usability, mine needed help right there !
 ''This'' RED LED is very dim @9vdc with a 500k pot...use whatcha got...I like the range/spread of the 500kpot+1k just perfect, 100k didn't dim the RED near enough.

[Tonemonger]

 :icon_biggrin:Genius !

[petemoore]

  Cool, glad you caught it, excellent mileage for the Neutron, it's recommended as a must-have surface mount knob.
  Much greater control of the LDR's became possible with the ability to set the level of the 'bottom of the ramp', and combining that with the peak levels using the 'bottom' and 'gain' knobs.
  Those two knobs have been getting a work out  very results.
  Easy to do if you're rollin' your own, setting a square LED to behind the Green LED base would be even easier, I used a round one, offset, the 500kpot +1k offered great range and spread, very easy to adjust the upper and lower ramp levels...just play notes or not listen as they move around, or turn the gain knob down and hear the "Backlighter LED" move the sweep to ''extreme'' up or down !

[Mark Hammer]

Very interesting idea.  Is it the one being filtered by the other, or would summing their light in some fashion be useful?  For example, say one had a piece of triangular clear lucite that you could glue over the LDR, such that you could mount one led flush with one face, and the other with the remaining face, like so:
     LDR
LED\    /LED
      \ /

[petemoore]

   Following the Neutron schematic, using 1 Green LED:
                    LDR
  lG R E E N led
                    LDR
  With an LDR on each side of the head of it.
 The Green LED is rectangular, but has a notch part way up, then a flat tip portion toward the tip of it, the LDRs are glued to this portion, but extend a bit past the LDR tip aluminum foil was rolled around and down each side to help direct foreward pointing light into the LDR's.
 Behind this Green Led, a RED Led was also placed:
                             LDR
  REDLED:Green LED
                             LDR
 The RED Led provides plenty of brightness to the LDR's when it's light is travelling through the Green LDR's clear body, it's a symmetrical orientation.
 The 500k pot [and 1k current limiting stop resistor] set the brightness of the RED led, backlighting through the clear body of the green LDR.
 

[cpm]

so the point is having a bottom limit for the "off" state on the envelope, so you set a minimum brightness into the LDR instead of going completely dark, am i righ?

i would've tried other methods though (not verified)
1) move the Vref point on the opamp that drives the LED, so when inactive still there is some difference to the LED's sinking voltage, and it still lights
2) put a pot in parallel to the LDR (its seems there already is a resisitor), to limt the maximum resistance when off

any of those two ideas might result in a different "taper" on the LED/LDR transitions, but still worth considering

[petemoore]

1) move the Vref point on the opamp that drives the LED, so when inactive still there is some difference to the LED's sinking voltage, and it still lights
  Seems like that could be done with simple resistor alteration, 1 variable resistor [+stop R] even, I'd have to take a look, then I'd have to actually do it to see how the two different approaches compare.
2) put a pot in parallel to the LDR (its seems there already is a resisitor), to limt the maximum resistance when off.
  R8 and R9, I thought of reducing these 220k's, with a dual-ganged pot near the photocell area, doubling as improved photo-cell mounts, these resistors are somewhat buried on my board.
  I can cite advantages to these mods would have for those with closed photocomponents...
  I "ask and suspect" that there might be some smoothing to the way the LED's and/or LDR's respond when the LDR start from a 'middlin' state and the LED's are sweeping from low to medium light, the altered sweep rates and ramp angles make it easy to get a 'compressed' ramp.
  I have mild curiousity as to whether the light mixes from Backlit LED and Envelope LED ramp angles and rates of climb/fall, mod can be closely mimic'd with electronic tweeks to the original values and topology.