What is this circuit section doing??

[armdnrdy]

This is the pedal control section from the Mutron Flanger.



Dino, (digi2t) yours truly, and an assorted cast of characters have been communicating back and forth, throwing ideas around, and gathering information for a Mutron Flanger build.

The above circuit looks to me to be a voltage comparator with an adjustable voltage divider on the non inverting input of IC10A and an adjustable (controlled by the reflection of the silver pedal shutter) voltage divider on the inverting input.

Can anyone give a description of what this circuit is doing? I'm a little lost!  

[slacker]

Where is LDR1 placed in relation to the LED, what controls whether it's in the dark or in light?

On the face of it it's some odd feedback loop that makes no sense to me, the resistance of LDR1 determines whether the voltage on the inverting input is higher or lower than the non-inverting input, which in turn determines whether the LED is on or off, which sets the resistance of LDR1................................

[gritz]

I don't suppose you guys actually have one of these hen's teeth and can supply a sketch / photo showing the orientation of the LED, LDRs and shutter? That would be cool, simply from a "wow, look at that!" point of view.

It looks to me like the LED, LDR1, IC10A and that mysterious shutter might form a servo (negative feedback) loop that causes the resistance of LDR2 to accurately (or repeatably) follow the position of the shutter (and thus the pedal). Or something. It's possible that Mutron chose this method to generate a quiet, smooth control voltage, rather than resort to noisy, don't-last-for-ever pots. That's my guess anyway.

Edit: slacker beat me to it on the "how does it all fit together?" front.

[armdnrdy]

I don't suppose you guys actually have one of these hen's teeth and can supply a sketch / photo showing the orientation of the LED, LDRs and shutter?

Courtesy of Dino:

[slacker]

Interesting, which one of the LDRs is LDR1? From the schematic it's presumably the one next to the LED.

[armdnrdy]

Interesting, which one of the LDRs is LDR1? From the schematic it's presumably the one next to the LED.

Correct!

[slacker]

I'm totally stumped then, it appears that a high resistance on the LDR is likely to turn the LED on, which will lower the resistance of the LDR which will likely turn it off. Is it supposed to cause the LED to flash? The expected light and dark values for the LDR might help.

[armdnrdy]

The thing that I don't understand is what the comparator section is doing if the LDR (controlled by the pedal) connected to IC10B is controlling the output voltage going to the VCO input of the 4046. (not shown in the above image)

Maybe the comparator section "softens" the abrupt attack of the pedal shutter moving up and down, turning on and off the light to LDR2.

The capacitor C51 might add a slight delay while it charges and discharges through R67 before the voltage makes it's way to the base of Q3. (just a theory!)

[digi2t]

From what I know, T4 is the trimmer that sets the sweep range of the pedal. That is the two ends of the sweep move up/down together. The pedal is independant of the START/STOP pots. Could it be that it`s just set up to do this as linearly (is that a word?) as possible?

(Notice the «OMG the guy`s got one, but is a total noob» question at the end. )

Actually, Larry brought up a good point in our PM exchange. Since that LED/LDR1 combo is inside the same tube, could the shutter also affect the intensity of the LED? As the shutter moves out of the way, light bleeds out to LDR2, and LDR1 on the LED side reads this variation, adjusting the LED intensity slightly? Keep things really super linear?

Please tell me if my cheese has gone off it`s cracker again...

[armdnrdy]

(Notice the «OMG the guy`s got one, but is a total noob» question at the end. )

If I had a Ferrari, that wouldn't mean that I would know everything about the motor!  

Actually, Larry brought up a good point in our PM exchange. Since that LED/LDR1 combo is inside the same tube, could the shutter also affect the intensity of the LED? As the shutter moves out of the way, light bleeds out to LDR2, and LDR1 on the LED side reads this variation, adjusting the LED intensity slightly? Keep things really super linear?

Please tell me if my cheese has gone off it`s cracker again...

As in our communication, I think that the LED/LDR1 combo will react to the silver reflective shutter. Remember that there are bi-convex lenses that will amplify the light as well!

[slacker]

The comparator can only do two things, it can either turn the LED on or turn it off, if it just did either of those it doesn't seem to serve any purpose, it has no controls so it's not being used as a fancy switch. Thinking it through the, the only thing that makes sense to me is that it causes the LED to flash on an off. Maybe it's like a PWM light dimmer circuit and flashes the LED on and off faster than you can see, creating a constant brightness that doesn't vary with supply voltage, this would in turn keep the range of control voltages to the VCO constant.
That seems far too complicated though.

EDIT: wrote that before I read Dino's post, sort of fits with what he said, twiddling the trimmer sets the brighness and therefore the sweeps. Still sounds too complicated though. Maybe there is something in Larry's idea.

[armdnrdy]

Dino.....we need measurements of the base of Q3 with the pedal in the toe down position and the heel down.

I'll breadboard this section tonight.

[slacker]

Good call, or even better if you've got a scope get that on there.

[armdnrdy]

Very similar circuit:

http://electronut.in/ambient-light-sensor-using-an-op-amp-comparator/

[Digital Larry]

I'll tell you what I THINK is going on.   This often results in much hilarity when I'm proven wrong.  Let's see what happens.

If the left side of the circuit is just a range adjustment, then I would not expect the comparator to be switching after the trimmer settles down.  I think the LDR in the comparator circuit attempts to linearize the response of the right hand LDR to the trimmer's range adjustment.  Without that it probably would do nothing for most of its range and then all of a sudden go over the entire range.

I didn't do any math and I'm not at all familiar with this circuit.  However, way back when, I did design an LDR which had a split (center tapped) element into a voltage controlled midrange notch (think of the bottom resistor in a Fender tone stack).  The center tap of the LDR went to ground.  One end went to the notch circuit and the other end went to the op-amp circuit.  This way, regardless of the non linearities of the LDR itself, the resistance presented to the notch circuit would be linearly related to the voltage coming into the op-amp.  There are enough differences between that and this circuit that my confidence is about 50%.

[armdnrdy]

So Larry,

Let me get this straight. What you are saying is that you are 100% sure that you're 50% confident? 

Come on guys....this circuit only has a few components! Surely we can figure this out. (he mumbles as his brain begins to swell)

[Digital Larry]

Let me get this straight. What you are saying is that you are 100% sure that you're 50% confident? 

It might be the other way around.

[Gurner]

Dunno...I'm as perplexed as the next guy ....my punt (& it's an outside one!) is that LDR1 might some form of ambient light cancellation? (cos I reckon when the shutter is open a degree of ambient light will reach both LDRs .....if it's daytime, then that light spill-in could be significantly more than on a dark stage .... therefore perhaps such an arrangement is to cater (help cancel out) for extremities?

[Digital Larry]

It may be for temperature compensation - but probably not.

See:

http://www.silonex.com/audiohm/levelcontrol.html figure 14.

Figure 15 shows what is pretty close to the Mutron circuit used for linearization.  I stick by my original guess.

[Lurco]

linearization: optical feedback from output to inverting input.