LDR experiments & questions

[Martin Czech]

Ok, funny enough, I seem to have more time in the summer than in
winter, cycling season IS summer, but the training is much  harder and thus
shorter. I invest this extra time on little investigations on very primitive
circuits that are basic for many electronic music applications.

The first circuit I tried is a ldr/led pair.  Here is the data for led
current versus ldr conductivity (the measurement equipment was not top
notch, the led was glued to the ldr with adhesive tape, not perfectly
dark, I used my pullover to get more darkness on the ldr ;->).


Idiode [A] Gldr [S or mho]
1.94e-6    1.898e-7
4.13e-6    7.633e-7
5.78e-6    1.397e-6
8.04e-6    2.63e-6
13.8e-6    6.58e-6
20.8e-6    1.35e-5
42.7e-6    4.12e-5
61.9e-6    7.09e-5
102e-6     1.39e-4
200e-6     3.37e-4
399e-6     7.63e-4
805e-6     1.61e-3
1.0e-3     1.98e-3
1.59e-3    3.03e-3
2.02e-3    3.68e-3
4.15e-3    6.25e-3
6.08e-3    8.20e-3
8.08e-3    9.80e-3
16.1e-3    1.47e-2
20.6e-3    1.67e-2
23.0e-3    1.73e-2
26.0e-3    1.86e-2

This isn't quite linear as I -naive as I am- expected.  I thought that
the photon rate of the led is linear dependend to the led current and
that in turn the conductivity of the ldr is linear dependend on the
photon rate. This seems to be not the case, there is a rapid change at
low led currents and then the curve gets flat. I suspect that at higher
light intensity all possible carriers (holes and electrons ?) are
allready activated, there are no carriers left. Also the led might be
not so effective at higher currents. Is this explanation correct ?

What is the theoretical 1st order formula for led intensity and what is
the formula for ldr conductivity ?

Most circuits I have seen so far use a parallel resistor of 100k or so,
Here's the data with 100k parallel:

Idiode [A] Gldr [S or mho]
1.94e-6  1.02e-5
4.13e-6  1.08e-5
5.78e-6  1.14e-5
8.04e-6  1.26e-5
13.8e-6  1.66e-5
20.8e-6  2.35e-5
42.7e-6  5.12e-5
61.9e-6  8.09e-5
102e-6   1.49e-4
200e-6   3.47e-4
399e-6   7.73e-4
805e-6   1.62e-3
1.0e-3   1.99e-3
1.59e-3  3.04e-3
2.02e-3  3.69e-3
4.15e-3  6.26e-3
6.08e-3  8.21e-3
8.08e-3  9.81e-3
16.1e-3  1.47e-2
20.6e-3  1.67e-2
23.0e-3  1.73e-2
26.0e-3  1.86e-2

If you take a look at the two graphs (gnuplot etc.), you'll see, that
adding the parallel resistor makes almost no difference, only at the
very beginning at very low currents. I think the parallel resistor
might be a kind of safety device, to avoid an open circuit should the
light turn totally of.

Do you know what the operating range of ldrs in commercial design
is, in terms of min - max resistance ? Do they really use the full scale
from darkest darkness to all what the led can give?
A range of 10:1 is no problem, but 1000:1 ?

By the way: I want to use ldrs for a phaser project, I think that all
allpass stages should be tuned to the same frequency (ie. ldr
variations can be compensated with C variations). Or is this feature
not so important?  How large should the sweep range be for a good
phaser ?

The next thing I will have to check is if the circuit is more effective
with red, yellow or green leds. It should be the green ones, since for
all ldrs I know the maximum light dependence is at 500nm wavelenght.
On the other hand: red leds tend to be much brighter at the same
current...

The next thing to look at was the time behaviour.  I set up a simple
voltage divider (500 Ohm leg to gnd and ldr leg to source), the led was
pulsed with 0mA and 10mA. The voltage divider was fed with a 1 kHz
square wave (because it is easier to see to envelope on the scope). The
resulting envelope is of exponential shape (like a simple AD envelope),
it takes ~2ms to raise to 50% of the final value (measured after a very
long time) but it takes 4 times longer ~8ms to fall from 100% to 50%.
This is wellknown, but it was interesting to see this "live".
This is however fast enough for phaser applications, companders,  maybe
also for filters.

By the way: ldrs come at least in two flavours: sealed epoxy package
and naked disc. The naked are smaller, it would be more convenient to use
them, but do they have some passivation against environmental influences
(finger tip sweat etc.)?

m.c.

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m.c. (alias mad scientist) has made it finally: 3 CDs à 72 minimum ready.
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