Light dependent resistors

[Signal:Noise]

I fancy adding a couple to my SA-1 for a bit of theramin style action but having zero knowledge of electronics they've got me confused. can you buy them in different values at all, because I can't seem to make any sense of the tech specs on various componenet websites.

[papaspank]

Hope this little bit of knowledge is of some use...

Light dependant resistors (LDR's) are pretty much the same thing as variable resistors, the only fundamental difference is.. the resitance they put up is affected directly by the light hitting them, rahter than you physcially turning a rotary to lower/increase the resistance.

Most of the info isn't going to mean much to you - unless you've really studied electronics.

However - the bits that are of most interest are these...

How much light they take to alter the resistance.
usually measured in LUX bigger number-more light needed. Sometimes you get a little lux to ohms chart to reffer to, but this is rare 

How fast they react.
Usually measured in milleseconds (ms), again bigger number, more time it will take.

The restance range of the LDR.
Measured in ohms, the symbol of which looks like an unverted 'U' with longer tails on the top...
   

like this,
    _
   / \
_\  /_


in this case a bigger number means that less current will get through, which means it will (usually) have less overall effect on the circuit (a smaller pitch change for example).

The RANGE of the LDR
if you have one with little difference between the lowest and highest stated ohm numbers...
Then the LDR will have a more limited effect.

And, just like computer 'speak' anything above 1000, gets refered to as 'k' (e.g. 1200 = 1.2k)

SO....Say you have an LDR rated at 500-20k ohms, with a speed of 90ms,

and one that's the same speed, but goes from 500-1.5K ohm.

The second one would problably not give you the kind of scope that you'd be looking for.

If in doubt, get a few variants of them to try which gives you the best overall control.

I usually find that any form of variable voltage device, is a little bit of a hit and miss affair...
as you're getting the circuit to do something the designers didn't expect it do, or even design it to do! 

It may also be an idea to co-join it to a small ranged (measured in ohms-just the same as the LDR) variable resistor, because you can then 'tune' the LDR - so it changes pitch in the type of lighting conditions that you're happy with.

Hope this is a useful hint/tip...

And sheds some light ( ) on the subject.

[Signal:Noise]

Cheers man, just what I was looking for.

[Circuitbenders]

For proper theremin styleeee action you want something that reacts fast. Its pretty easy to make an optical theremin from one of those horrible plastic keyboards that consist of nothing but an analogue oscillator but you'll probably find that most LDR's react very quickly to extra light but a lot slower to less light, which can be annoying when you put the sensor totally in the dark and it still takes about 3-4 seconds for the pitch to drop all the way.

Anyone know where to get some fast reacting LDR's?

[papaspank]

Well, yeah... I've had a look too.

Came across these - but the drop off time specified seems to be l-o o o n g!!!!

http://www.maplin.co.uk/Module.aspx?ModuleNo=35963&criteria=LDRs&doy=1m6

not sure if they'd be any use to you.

Going out on a limb here - could a capacitor be used in conjunction with it to act like a 'buffer' to negate the 'sloping' effect?

Just a thought - not sure if it's a good one really, as you'd still need somewhere for the cap to get it's voltage from. 

[catweazle]

You can try a photodiode (which has a reaction time in micro seconds)
http://en.wikipedia.org/wiki/Photo_diode
but you'll need a op as an amplifier ...

LDR are most sensitive to yellow light,  maybe a yellow LED mounted near the LDR (reflected by the hand) helps a little

[papaspank]

Came across this, it seems to be a bit faster that yer average LDR...


However - you may still need to get an 'op as amp' - as mentioned in previous reply.

The reaction time does seem to be a bit better though...

click here for link