LDRs as pots?

[Gregg]

You're right - they use 4 terminal LDRs /I guess custom made/for their tone stack:

[tbryanh]

Excuse me, may be I'm dumb or blind /or both/ but I can't figure out how Mesa's device is supposed to work if two of the "pot" terminals are connected to ground.

I don't think the "pot" terminals are connected to ground. Only the cathodes of the LEDs inside the LDRs are connected to ground.

Look at the attached Vactrol. There is no internal connection between the resistor elements and the LED.

As a side note, the Vactrol attachment is a little confusing. The arrow in the center of the resistor element makes the resistor look like a potentiometer. It is not a potenetiometer. The arrow only indicates a connection point, not a wiper that is controlled by the LED.

[Gregg]

I think you should observe the schematic more carefully.
There's no internal connection between the LED and the center tapped photocell - they are connected together externally.
The Vactrol is not a pot by itself it's just a LDR with a center tapped photocell. To make a taper pot out of it you'll still need a 4 terminal /2 independent photocells/ LDR which obviously is not available except in 1000+ quantities but can be made at home out of readily available parts online.
Here's my test version of it:

[tbryanh]

Does anybody have the schematic for the Mesa TX4 D board pictured above?

It is the board with the LDR pots and op amp drivers.

http://www.tubefreak.com/triaxis.htm seems to have all the diagrams except that one.

Thanks

[tbryanh]

I think you should observe the schematic more carefully.

To make a taper pot out of it you'll still need a 4 terminal /2 independent photocells/ LDR which obviously is not available except in 1000+ quantities but can be made at home out of readily available parts online.

Yeah, your right, I needed to look at the schematic a little closer. Yes, it does appear that the cells need to be isolated from each other. A centertapped cell such as the Vactrol or the one in Mesa's patent will not work.

Attached is the one from Mesa's patent. I touched it up a little bit to show the connections as I think they should be.

For the upper LDR, the bottom of cell 1 is connected to ground, but the bottom of cell 2 is not.

Likewise, for the lower LDR, the bottom of cell 3 is connected to ground, but the bottom of cell 4 is not.

[tbryanh]

Here's my test version of it

If you get a chance to post a schematic of your test unit with a parts list, that would be cool.

[loudthud]

It looks to me like item 25 is connected backwards.

[Gregg]

My test unit is based on the hand drawn schematic posted by Don Symes back in the thread /page 3, answer #29/. The changes I made:

1/ I'm using LM324 because it's capable of supplying large currents.
2/ Because of 1/ the LDR's LED is connected to the opamp output only through a 100 Ohm resistor.
3/ The whole schematic is doubled /as in Mesa's patent/ and the 10k pot doesn't connect to ground but its wiper is connected to ground instead /as suggested by another forum member/. The other terminal goes to the "mirror" side of the schematic.

The trimpots on my picture are the scaling resitors which let you set the LDR pot value.

I don't have the "damping" filter because I don't have any idea how to determine those values.

My LDR pot has been only "dry" tested so far. It worked but I didn't "listen" to it in a real circuit.

[tbryanh]

It looks to me like item 25 is connected backwards.

Yeah, it does appear that it is shown backwards.

Here is a touched up version.

[tbryanh]

It looks to me like item 25 is connected backwards.

Yeah, it does appear that it is shown backwards.

Here is a URL to a touched up version to indicate that.

http://subdomain.freeservers.com/schematics/LDR_Pot_Touchup2.jpg

The Manage Attachments function doesn't seem to work right now for AMPAGE.

[tbryanh]

I don't have the "damping" filter because I don't have any idea how to determine those values.

The patent explains that the damping filter is to compensate for the delay in the resistor part of the LDR responding to a change in light intensity from the LED part of the LDR.

Without the damping filter, the LED will oscillate between full on and full off.

I am guessing that all you need to do is view the signal to the LED with an O'scope and use a capacitor substitution box to dial in the smallest value capacitor that stops the oscillation.

You might want to increase the value a little past that point, but probably not much.

The patent explains that you want the delay from the capacitor to equal the delay from the LDR.

Using too large of a capacitor would probably cause an oscillation too.

In summary, If the op-amp responds too quickly or too slowly, an oscillation will probably occur.

[tbryanh]

On second thought, too large of a capacitor probably will not cause an oscillation.

Probably what will happen is the response time will be slower than what it could be; i.e. slower than the response time of the LDR.

So, to summarize again.

Using an oscilloscope and capacitor substitution box, dial in the smallest value capacitor that stops the oscillation.

Then increase the value a little to insure the oscillation does not happen when temperatures change and devices age.

If you prefere a slower response time, increase the value of the capacitor until the slower response time is reached.

[tbryanh]

To clear things up a little more.

The issue of finding the smallest value capacitor refers to the LDR Variable Resistor http://subdomain.freeservers.com/sch...e_Resistor.JPG

For the LDR Pot http://subdomain.freeservers.com/sch...t_Touchup2.jpg an RC network is used. This might be a little more comlex to find correct values of resistor and capacitor.

It might be that a simple capacitor can be used for the LDR Pot instead of the RC network.

So now the question is, why was a simple capacitor used for the LDR Variable Resistor and an RC network used for the LDR Pot?

The reason the resistor is there might be to set the gain of the op amp to scale the range of the LDR Pot.

[tbryanh]

Sorry, the URLs got parsed. Here they are again.

http://subdomain.freeservers.com/schematics/LDR_Variable_Resistor.JPG

http://subdomain.freeservers.com/schematics/LDR_Pot_Touchup2.jpg

[tbryanh]

I was able to upload the LDR Variable Resistor and the LDR Pot today.