Re: Inverting LDR response for downsweep?

[WaveshapeIllusions]

Okay, that was weird. I thought this failed to post.

Anyways, I've been poking around with envelope filters and wanted to use LDRs for upsweep and downsweep in the same pedal. Would I just connect the LED to V+ instead and invert the control signal? Is there an easier way to do this?

[drolo]

You may have a look at the mutron or lovetone meatball schematics for an idea on how it works

[drolo]

I had made a modest attempt at something like that too with a different twist :

http://www.diystompboxes.com/smfforum/index.php?topic=100936.0

;-)

[R.G.]

Is there an easier way to do this?

An LDR is driven by some light source, generally an LED. The light source is, in turn, driven by -something else-.

The easier way to do all this is to electronically invert the direction the electronics drives the light source. Have a look at the Mutron circuit for one way to do this.

Simply changing the power supply of the LED from ground to power or vice versa may not do anything, depending on the circuit that drives it. Many LED drivers can only pull one direction, up or down. Changing the termination of the LED may make it not work at all.

[Mark Hammer]

...And note that simply making "the opposite" is not enough.  Downward sweeps need to start from somewhere below the maximum upwards sweep points to be musical.

[drolo]

...And note that simply making "the opposite" is not enough.  Downward sweeps need to start from somewhere below the maximum upwards sweep points to be musical.

I was wondering when you would pop in to say that :-)

[Mark Hammer]

Hey, I always live up to my contractual obligations! 

[WaveshapeIllusions]

Thanks for all the advice. The idea to connect it to V+ was because I figured it would normally be on and a negative going signal would turn it off.

Looking at the Mutron and others, all I do is invert the signal? That would make the positive going pulse into a negative going pulse, right? It looks like the initial voltage fed to the LED is different too, so that would bias it on until the signal pulls the voltage down.

I've also been thinking of running the LED in series with the collector load of a bjt. The EA trem does that for a rate LED. Do you think I could make the LED go from on to off by using a different bias on the base?

On the other hand, I'm not sure if this is even the best method for control. I'm trying to make a gyrator-based peaking EQ. It looks like just one resistor modifies the frequency, so I figured an LDR would be the best bet. Could JFETs do a better job of this? I kind of like the LED/LDR method though since they seem to inherently do a lot of the smoothing on their own.