sunlight / LDR difficulties, questions, workarounds?

[caress]

i'm working on an art/sound installation and am coming across some difficulties, so i figured i'd see if anyone had any bright ideas...
here's the breakdown:

a public art installation which is a rather large mobile attached to a steel frame.  the pieces of the mobile are cast aluminum bowl-shapes which will be facing different directions.  the whole thing *sort of* resembles an ascending (or descending) passage of 8th notes (an arpeggio) on musical staff paper - if you can sort of imagine it...  they are attached using braided steel wire and will be able to freely (though gently) move with the wind, etc.

on the top of the frame, where the mobile hangs from, will be a box containing electronics.  8 ipod shuffles, all loaded with particular loops and compositional snippets will be fed to 2 SSM2164 quad VCAs, which will go to 2 quad opamps then on to individual push-pull transistor amplifiers driving 1watt speakers.  the VCAs will be controlled via LDRs driving a simple LM324 circuit that i asked about a long time ago... http://www.diystompboxes.com/smfforum/index.php?topic=85121.msg711004#msg711004
the speakers and associated LDR will be nestled within the various bowl-shapes and 3 spotlights will be around the piece, illuminating it on darker days and evenings.

the idea behind the whole thing is that people can interact with the piece - as people move closer to inspect the piece, they create shadows on it and change the volume of individual loops - dark = loud, light = quiet.  the 8 loops together form a composition, but can work equally well as duos, trios, etc. - changing the perspective of the piece/music by viewing it from different places, having more/less people interacting together to create new sounds and challenging the idea of what composition is in and of itself.  i'm looking at a number of loop-based composers, chance/aleatoric music, etc. for inspiration.

THE PROBLEM:
this is outdoors and in full sunlight.  obviously this creates a problem with creating shadows, etc.  i'm trying to find a way to make it so that the LDRs will still be somewhat responsive in the light - i certainly understand that there are limitations, but if i can get halfway there i will be happy.

a link to the ssm2164 datasheet circuit - http://img12.imageshack.us/img12/6859/vca.png  the LDR CV is replacing the 100k pots on the voltage control inputs.  there are a few other small tweaks, but nothing that will affect my questions/concerns.

thanks for reading this far!  any help?   

[frequencycentral]

Paper will increase LDR's minimum resistance.

[Gurner]

How about a differential amplifier with two similar ldrs...one on the +ve input, one on the -ve input.

One of the ldrs is simply there to pick up the prevailing 'ambient' light ...the other ldr is more directed at the people who are in close proximity. Use the differential voltage as your control voltage.

[Johan]

what if you use another LDR for reference to cancell the differance between dayligh and night..something like a LDR across R2 in your schematic in the other thread( the second LDR picks up ambient light). that way the small change in the LDR at daylight is referenced to a small value at R2. at night the changes across the LDR is referenced across a much larger value at R2 since the "ambient" LDR gets little light...
J

EDIT:  pretty much what Gurner wrote while I was typing...

[caress]

hmmm similar ideas yet implemented differently?

what if you use another LDR for reference to cancell the differance between dayligh and night..something like a LDR across R2 in your schematic in the other thread( the second LDR picks up ambient light). that way the small change in the LDR at daylight is referenced to a small value at R2. at night the changes across the LDR is referenced across a much larger value at R2 since the "ambient" LDR gets little light...
J

you are talking about R2 in this schematic?  or would it be R3?

[Gurner]

Just a throwaway idea (& it sounds like you're too far along now anyway), you could make a simpler/cheaper/compact installation if you use an audio amplifier IC with built in DC volume control. Something like a TDA7052A.

Therefore that would mean just a simple differential amplier with your LDRs ...use the differential amplifier DC output to drive the TDA7052A's DC volume control pin - job done.

[artifus]

like this?

from: http://bondoukousoftware.com/InfraredTheremin.html but offers little explanation.

[Gurner]

That was the circuit I initially had in the back of my mind when I read the caress's request (the internet is a small world eh?). I'd seen it a while ago and contacted the originator (James Brooks) about the ambient light cancelling part of his circuit, here's what he said....

"I'll try to describe the functioning of that section better. I'll refer to the left side of the schematic. You see the two photodiodes in series. The top one is to capture reflected IR light from the finger. The lower one captures only ambient light. If they're seeing the same amount of light, the current flows thru each, so no voltage builds up between them. If the top one is seeing more light because a finger is reflecting in front of it, it drives more current than the lower one pushes to ground. Where can that current go? Only out the 54k ohm resistor. It does this, but following Ohm's Law, the voltage needs to rise to push that current out the resistor. The op amp just magnifies the voltage. I think by 33 times, the ratio of the two resistors wired to it. Maybe a few millivolts to a few volts"

....I took another look at it prior to posting, but didn't post it up here because it's not the differential solution I was thinking of - it might be worth a pop though cos it's surely simple.

[artifus]

thanks for posting that gurner. i had guessed something like that was going on but hadn't got round to experimenting yet, nice to hear it from the horses mouth so to speak - i hope he doesn't mind it being linked to here. it's an interesting principle that could surely be applied to other sensors, no?

[caress]

though i understand the concept behind the differential amp setup, i'm having a difficult time figuring out how i would implement this in my project.  i tried a few different setups on the breadboard to no avail...
the voltage range i'm working with is quite small as well - 0v for max volume, more or less 2v for totally muted - with the majority of the useable range being between 0-.5v

[Gurner]

I've certainly never implemeted one for an LDR based input (in fact I've not dabbled with LDRs much at all so don't know what tolerance they're manafactured to....but a differential/instrumentation amplifer works best for  components with close tolerances)

The idea would actually be far easier to implement using an instrumentation amplifier IC (one gain setting resistor job done!), but my thinking goes along the lines of a potential divider to each input of the instrumentation amplifier...

Voltage supply
       |
fixed resistor
       |
    LDR
       |
    Gnd

the junction of the LDR & fixed resistor feeds into the +ve pin

Same arrangement feeding the -Ve pin (for this to work well, both divider 'chains' have to be well matched)

The idea being, when both are receiving the ambient light (sunlight, cloudy, dusk...whatever) ...the voltages at the junction will be the same - there's no difference to amplify.

Ok, now someone approaches the ldr that you've setup to 'target' people....the LDR's resistance will change (vs its partner LDR in the other leg of the opamp), which will result in a slight change of voltage at its junction with the fixed resistor - this difference in voltage (vs the partner ldr) gets amplified.

[caress]

i still can't get the differential amp to do anything useful, though i still need to try it tomorrow in full sun.  one setup that worked ok so far was v+ through a resistor to LDR leg 1, LDR leg 2 through a resistor to ground, connecting VC input to LDR leg 2.  too simple though.., there's got to be a drawback?

[caress]

bump for any other thoughts?

[deadastronaut]

no idea if this  helps, but its using ldr's in a sort of reverse mode...when the ldr gets darker an led lights up...

it works great in bright sunlight conditions too..and adjustable sensitivity..........just a thought!...

http://www.aronnelson.com/gallery/main.php/v/chickpea/darklight2.jpg.html

[Gurner]

bump for any other thoughts?

If you mean about the differential arrangement you've not had success wuith, without a little more detail...it's difficult to troubleshoot.

Which IC did you use ....can you knock up a schem of what you experimented with ...what were the DC voltages etc.