I want to make Infrared LED/LDR combo into expression pedal

[ryanscissorhands]

Explanation:

I'm fairly new to electronics theory and whatnot, but I've been trolling this forum for over a year now, so I can at least be pretentious.

I want to use an infrared LED/LDR combo outside of a pedal to act as an expression pedal. the idea is that when your foot is close, more IR light gets reflected into the LDR. (I eventually want this to vary the speed or depth of a tremolo pedal). My problem is that, as far as I know, most LDR's have a high resistance in little light, and low resistance in bright light.

What I need is to be able to put this varying resistor in series with either the speed or depth pot of a tremolo. However, i want the opposite response from the LDR--I want nearly zero resistance when my foot is away from it, and for the resistance to increase with more reflected IR light.

One of the reasons why I want the pedal to control depth is so I can use it with an Ebow. I can get a decent sharp violin sound with my Ebow, but my sustained sounds don't sound very good. If I could manually add a little depth to a tremolo, I could slowly add trem for sustained notes.

Now, the questions:

1) Do some LDR's have this opposite reaction to light? If so, how can I tell which ones do, and which ones don't?

2) How else can I get the opposite effect from the LDR?

3) SHhuld I give up, but a cheap Behringer volume pedal ($30 US) and figure out a way to implement it?

4) Any other suggestions? Anything is welcome. Except give up. I'm too picky/stubborn.

[filterazonatie]

I won't be much help at all, but I just wanted to say that that's a REALLY COOL idea, and my hat goes off to you!

Actually, maybe instead of the IR route which seems really difficult to me (do you re-calibrate it for different temp rooms, can you really apply heat with your shoe, etc?), you could try the RF thing that ZVEX has on his "Probe" pedals.

"The copper antenna projects a field of RF energy that is affected by your foot (or other appendage), altering the circuit that generates it."

Definitely more difficult, but worth a shot. \

Check it out at
http://www.zvex.com/trem.html

Barring that, I would go with a simple expression pedal modification, It would be soooo much easier...[/url]

[ryanscissorhands]

Well, my idea spawned from the Probe(TM) series, because I need a very gentle expression. But I figured it would be expensive to implement what the probes use.

Good call on the IR/room temperature thing. Hmmmm. I was thinking IR because. . . well, that's cool, and I wasn't sure if I could do it with normal LED's.

The other thing I could consider (although it's WAY beyond my scope) is the robotics proximity sensor that was referred to in a theremin question of late (I believe by Paul Perry).

[NaBo]

Hey ryan,

AFAIK, there's no such thing as an LDR that's low resistance in dark and high resistance in light.  It's just how cadmium sulfide/cadmium selenide is.  You'll need to use a CdSe, and keep in mind that IR light is wavelengths upwards of 715 nm.. so the response of the CdSe is tapering off quite sharply.  Check the graph here http://www.geocities.com/thetonegod/rockncontrol/rock.html

But take a look at Tim Escobedo's Idiot Wah and Photon Filter.  With the Idiot Wah, if you use an LDR as the variable resistor to ground, you end up with th "backwards" sweep as you describe, foot away = low resistance, foot close = high.

The Photon Filter is a cunning and simple way that this is reversed.  Instead of the LDR being the resistive element to ground, it's an LED.  The LDR controls how much current is going to the LED, and the more current it lets through (foot away), the more resistance at the LED.  This solution will take some experimentation to get a good range and taper for your purposes.  But it's relatively low parts count, so it's really not too daunting.

I'm sure there's other ways, using an inverting op-amp or maybe just JFETs... just none I'm knowledgeable enough to implement myself... yet.  :wink:

As for the proximity sensor, I trust you've seen Transmogrifox's ProxyOssi page?  I can more or less get what he has there, but the critical stuff that comes after, the "phase locked loop" stage to convert the signal to resistance, is no longer active on that site... the links are dead.  I've done some research on PLL's online and I can kind of absorb some of the info, but again, I'm not at the point where I could figure out how to actually come up with the circuit snippet and implement it myself.

P.S.  I just found this http://www.datasheetarchive.com/datasheet/pdf/88/885868.html

It seems this could potentially make things a lot easier on you, too.  The little suggested implementation circuits would be useful either way.