AW: LDR experiments & questions

[Haible Juergen]

	>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 ;->).

Glue them together, and then dip the whole thing into this paint
that is used for plastic model aircrafts etc. (Humbrol Enamel,
less than DM 2,- for a little can)

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

The maximum resistance is very undefined, and may be in the 
100 megohm range. To make it worse, this resistance depends
on the history of the LDR, i.e. whether it has seen some light 
during the last *days* (!) or not. Unit to unit variation is also 
very high. So setting a upper limit with a parallel resistance is
a good choice in many applications.

	>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 ?

For a phaser, 500 Ohm to 200 kOhm gives some direction.
But if you build an "infinite" compressor, you might use whatever
the device gives.
For phasers be careful with low impedance: The driving opamp
may see a high capacitive load when the LDR is at low resistance,
in a typical phaser circuit. Use opamps that can handle that, like
the TL06x (not TL07x or TL08x), or limit your maximum LED current.

One of the good things of LDR based phasers (besides the sound,
of course) is that you can build very simple driver circuits.

I have a unit with the 6 LEDs of the Vactrols in series, plus one LED
for the frontpanel (a low current LED is perfect here), and the whole
string is driven by a *voltage* (i.e. no series resistor), so the LEDs
will perform a pleasant expo conversion themselves. Make sure that
your driving voltage source has current limit. I used a norton diode
(JFET with resistor), but you might as well put resistors in your opamp's
supply connections.

	>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?  

I wouldn't care too much for that. I have a phaser (ota-based) that
allowes its poles to be spread by an extra CV (my "6-pole variable
slope filter / phaser), and there is a subtle difference, but not that
dramatic. My suggestion is just take a handfull of LDRs and go ahead.

	>How large should the sweep range be for a good
	>phaser ?

Never measured it, but surely much smaller than the LDRs would
allow. No problem here.

	>The next thing I will have to check is if the circuit is more
effective
	>with red, yellow or green leds.

I think this depends on the material that is used for the LDR.

JH.