Making LDRs "behave" better by tweaking LFO wavefo

[rx5]

ok...thanks

how do you wire a normal JFET to make it function like a CdS?

preferably positively triggered, more voltage -> the lower the resistance of JFET becomes.... thanks.... pls post links...need to know more
===============================================
been experimenting lately with LEDs and CdS...

tried using high-brightness green: attack was fast and decay also fast....

tried using high-brightness red:  fast attack and SLOW decay.... how come???

setup was both parts encased in plastic tube(sealed from light) then LED was voltage driven with a 9V battery(seriesed with 100ohms) , then for the CdS, connected it to the analog multimeter(resistance x10K)....

any reason why using RED led resulted in slow "dropping" of resistance???

[Mark Hammer]

It's not the LED, its the match between LED spectral content and the spectral sensitivity of the photocell.  

Photocells have bandwidth like anything else, although we're not talking the equivalent of a 6-pole filter at each end of that frequency/wavelength distribution.  So with a "preferential" wavelength range, just about ANY visible colour light will produce a resistance change, though not all wavelengths will produce the same amount of resistance change at the same luminance or intennsity.

As Lorren notes below, LDR peak sensitivity is indicated in nanometers - the light "wavelength" - with longer wavelengths being more towards the red end of the visible spectrum and shorter wavelenths being more towards the purple end - passing through green, then blue on the way.  Since yellow is in between red and green in the spectrum, any photocell which has peak sensitivity for yellow light will also be robustly affected by red and green LEDs shining on it, though yellow light will get your biggest "kick" at the LDR.

This is the long way of saying that the difference in decay time is likely the difference between a non-peak colour of LED poking the LDR in the shoulder and the LDR going "Uh-huh" and quickly diverting attention, and a peak colour slapping the LDR silly, sending it reeling for a few moments.  The decay difference you hear is the result of the differential recovery time for peak vs non-peak wavelength.

Make sense?

[Nasse]

Not only LDRs but LEDs too are different from their make and material. I was fooling some month ago with some LEDs. I made simple triangle wave generator working at 9V and tried some LEDs at its output. I had some "superbright" and "ultrasuperbright" ones and some "regular" ones. Some seemed to give more "linear" variation from darkness to full intensity, some were quite quirky for their response. If I remember superbright green-blueish ones were nice ones. So I think you should test your leds too, before putting them in shrink or something

[blackbox]

Hey Peter-
The Audiohm series (like the NSL32SR2, etc.) have a really fast response time and are a great choice for certain applications like high end compressors, phasers, etc.  The venerable NSL-32 is much slower but a better choice for things that require a slower response time or envelope ripple filtering.  There's pretty much a part for every need.

Lorren
Blackbox Music Electronics
http://www.blackboxmusicfx.com

Has anybody played with units from Silonex?



Most photocells are made of either a Cadmium Sulphide (CdS) or Cadmium Selenide (CdSe) photoconductive material. Either material is suited for specific applications, but only a compound of the 2 material will optimize audio performance and remove significant audio performance issues.

See:

http://www.silonex.com/audiohm/constants.html

They have effects for application circuits too!

http://www.silonex.com/audiohm/compressor.html

-Peter

[blackbox]

You wire up the gate through a resistor (depends on what range you need) to the voltage control source and the drain & source act as the variable resistance.  This isn't a direct replacement for an LDR which will work in any application where variable resistance can be used.  Using a JFET will only work well in some applications but it is very fast and cheap.   It won't for instance make a good VCA.  You can however use it for a phaser stage like the MXR but it requires matching and is prone to distortion.  It's best in non-audio applications like VCOs, etc.  My advice is try it.

Lorren
Blackbox Music Electronics
http://www.blackboxmusicfx.com

ok...thanks

how do you wire a normal JFET to make it function like a CdS?

preferably positively triggered, more voltage -> the lower the resistance of JFET becomes.... thanks.... pls post links...need to know more
===============================================
been experimenting lately with LEDs and CdS...

tried using high-brightness green: attack was fast and decay also fast....

tried using high-brightness red:  fast attack and SLOW decay.... how come???

setup was both parts encased in plastic tube(sealed from light) then LED was voltage driven with a 9V battery(seriesed with 100ohms) , then for the CdS, connected it to the analog multimeter(resistance x10K)....

any reason why using RED led resulted in slow "dropping" of resistance???

[Mike I.]

More points to consider:
a) Don't lose sleep about spectral match between LDR and LED... if not an exact spectral match then simply increase or decrease the drive current of the LED to suit. Or adjust the LED-LDR spacing.

b) The emission angle (beam angle, related to LED lens shape) of LED also affects the  total amount of optical energy incident on the LDR.
As an example, you are wasting battery power with a diffused LED where the LDR only intercepts 10 percent of the emitted light. Narrow beam-angle can be used for more efficient coupling.

c) I have not seen any fx units where the LDR is brilliantly illuminated by a lamp or LED - for lamp units, a dull-red to yellow filament is sufficient. Most LDR's seem to be fairly sensitive over their useful resistance range.

d) Note that LED's are current-operated devices - the light output is directly proportional to drive current (within limits - see f below, but certainly has a wide linear range). A current source driver for LED's makes sense when you want to accurately translate an LFO waveform to variable light output.

e)LED's vary considerably in efficiency between different types.
Certainly the newer ones are more efficient than the early 70's units. But...

f) The more efficient units seem to have  a threshold effect where the unit becomes more efficient above a  minimum current value, the older less-efficient GaP or GaAsP are more linear with respect to light output vs. drive current. So perhaps the older units would be more suitable in terms of "minimum light to LFO waveshape" distortion.

g) For some applications it may be desirable to use voltage-source drive for the  LED - this will make the light output vary exponentially with drive voltage - giving a simple type of volts/oct converter. Can give an excellent sounding sweep with an LFO. A second matched LED can be used for bias stabilization as this is a more finicky circuit than a simple current-source driver.

h) Experimenting with different LED/LDR combos becomes a lot easier when you make an adjustable drive circuit with non-interacting Manual and LFO Amount controls (aka offset and  gain controls). This implies that the LFO waveform must be centered about ground (or the mid-supply pseudo-ground) so that varying the LFO Amount control does not affect the offset.
Regards, Mike

[Mark Hammer]

As always, Bill (in joke, don't ask), you bring a more resonant ring of authority to these debates than I ever could.

One thing I will add to your comments is that LDRs are pleasingly maleable in the extent to which you can tinker with series and parallel resistances to compensate for whatever else you have to do in driving them to make them behave as you want.  Does the baseline brightness of the LED have to be too bright/dim in order to get the response time characteristics you want?  No problem, just add a series/parallel resistor to increase the resistance to where you want it.  If you use an LDR as one half of a voltage divider, you can get all sorts of division ratios and tapers just by playing with the other resistance leg of the divider and clever use of a resistor in parallel wih the LDR.

BTW:  Since Gilles is in Mtl for a little while, and I have some holiday time owed me, feel like swinging into Ottawa and coming with me for a drive to Mtl?  I'd obviously have to coordinate with "Mr. Curls" Caron.