AW: light phaser

[Haible Juergen]

> >While we are on the phasing subject...  Are there any machines that
> use
	>LDR for voltage controlled phasing.  

*Many* phasers work with LDRs.

	>It is said that LDRs offer low noise, so this would be
interesting.

Right.

	>The LED for the light controll could be connected in series,
one
	>current generator (linear?) should be sufficient for ~8 stages
(15V).

Would be possible to use a current generator, but you'd need
an exponential one. Better solution: Connect the LEDs in
series and directly drive them with a *voltage* from an opamp.
The U/I courve of the LEDs will make the exponential conversion.
Just make sure that you have a current limit. You can either choose
an opamp with a known current limit, or connect a FET in series
with the LED string. I used a BF245C, Gate and Source connected
together, so I get a passive, current limiting "Norton" diode.

	>It is said that LDR circuits have a bit slow response towards
the "off"
	>state, well it takes some time to get rid of all carriers.  Is
this a
	>problem for moderate modulation , say 1:1000 ?  It needn't be
so fast
	>for a phaser anyway.

This is not a problem, but a feature! It rounds the edges of a
triangle modulation signal, so you don't need a sine shaper.
And it attenuates the modulation depth at higher modulation rates,
which is very "natural" feeling. So just changing the Rate, and
leaving the Depth alone, you go from slow, deep sweeps to
fast, slight vibrato. With an "ideal" (fast) LDR you would go
to siren howling instead.

	>There must be a kind of saturation effect, if the light is
increased to
	>some point the resistance won't drop any more. Has anybody any
	>experienced with this?

I just happen to build one at the moment.
The "saturation" on the low impedance side is a good thing.
You'll also want a limit at the low impedance side, so I connected
a 100k in parallel to each LDR. But that's a matter of taste,
of course.

Careful if you use opamp circuits that drive a capacitor->LDR
path to GND. When the LDR becomes low impedance, the
opamp sees a capacitive load and might oscillate. There are
very few opamps that will work here. The ones I know are
TL061 / TL062, LF356, TL022.
Things like TL082, TL072, LF353 or RC4556 did *not* work
in my circuit.

JH.