LDR ideas

[steve.ridley at bbc.co.uk]

The LDR-based "mini-theremin" project discussed recently got
me thinking of other creative uses of LDRs, especially as 
Maplin will sell 100 LDRs for œ14 plus TAX.  These are just 
a few ideas "off the top of my head", so not yet tried.  

1.   A light controlled filter - several cascaded RC filters 
     using LDRs for the resistors, with overall feedback 
     via another LDR.  A dual op-amp and a handful of passive
     components should complete it.  Buffering each stage 
     should improve the performance.   Move your hand between
     the LDRs and the light source to vary the frequency and
     resonance.  Angling your hand will cover some LDRs more
     than others may alter the slope.  Switchable filters to
     adjust the range.

2.   Light controlled all-pass filters - similar to above. 
     Set it up as a phaser.  Hold it out of the window.
     Cloud-controlled phasing!!!

3.   Light-controlled mixing.  Taken to an extreme, it may
     offer a solution to the Synthi A matrix problem.  Make
     a matric of LDRs, cover over the ones you want to
     attenuate.  Punch holes in a piece of thick card to
     make presets...

There are also possibilities of LDR based touch keyboards etc 
too, but I'm sure someone must have tried some of these ideas 
before.  I know of ancient tone generators based on LDRs and
spinning glass disks with patterns painted on - went noisy due
to dust, insects etc.  Daphne Oram did similar things with film.

There are a few serious problems with all of these, and the 
worst one will be the light source.  Under artificial AC lighting,
there will be 50/60Hz modulation, especially with flourescent
lights.  Under stage lighting conditions things will fairly wild!


Any comments?


STEVE RIDLEY