How Do Morley Wahs/Photoresistors Work?

[Sir Edward V]

What I already know:

Morley wahs are not controlled by potentiometers, they are controlled by a photoresistor that responds to the amount of light shown to it by a LED. When the wah is tilted back and forth, a thin metal sheet with a shape cut in it moves in respect to the photoresistor. So on one side of the sheet there is a LED that is always on, then on the other is this photoresistor. By moving the treadle, the photoresistor is shown more light from the LED changing the resistence.

So what I want to know is how I can tell what the min and max resistance is other than using a multimeter. Also it seams like with this design it isn't really like a potentiometer which have the three lugs, its more like a variable resistor that would just have two lugs... Am I missing something or is that exactly what it is? If it is I don't know if I will be able to use this morley volume pedal for that delay mod I want to do...

Thanks all.

[bongomania]

That is exactly what it is, a variable resistor. Note that in most applications you only use two of the three lugs on a pot anyway, as they are used as... a variable resistor. You can use a multimeter, just reading those two lugs while adjusting the light exposure.

[niftydog]

You can just measure the resistance across the photoresistor with a multimeter, but for accurate results you need to disconnect it from the rest of the circuit. To work it out any other way would require you to measure voltages and apply ohms law - either way you need a multimeter!

That is exactly what it is, it's a variable resistor. A potentiometer can be made into a variable resistor (or rheostat) by shorting the wiper to one end of the resistance track - thus turning it into a two terminal device.

To implement a volume control with a photoresistor would require you to design a variable gain amplifier with the photoresistor as the gain controlling element. I would not recommend feeding a signal directly through the photoresistor as it's frequency response is likely to be woefully inadequate and the resistance range way out of the ballpark.

[Sir Edward V]

nonono, I don't want to use a multimeter, because I don't own one and don't want to buy one.

so basically I just need to pray that the delay I do this mod to uses a pot as a variable resistor?

[Sir Edward V]

Oh BTW, NEVER NEVER NEVER NEVER take Morley wah treadles apart!!!!!

it took me at least an hour to put back together... !!!!!!

also, the morley circuit board is HUGE and takes up about 3 times more space than it needs, it basically fills the whole enclosure when they could fit it in half a boss enclosure

also btw, I intend on housing a delay in this morley to get switchable control of delay time or feedback

I still need the delay though, I'm having trouble finding one for cheap



ha, found the schem for the "wah" I have (it is actually a volume pedal... ha, tricked you guys!)

http://www.morleypedals.com/psves.pdf

[niftydog]

Well, it's not so much that... what you have to do is emulate the operation of the pot with the photoresistor, which might be dead easy, but it might be really difficult. For instance, what happens if the delay works like this: high value resistance = long delays, but pushing the treadle forward on the old Morley decreases the resistance? Now you might be happy with toe down = shorter delays, but for my money I'd want it the other way around!

I dunno man, this could be a big bite for a beginner.

[niftydog]

oh, and if you plan on tinkering, you NEED a multimeter, no ifs buts or maybes. You can buy cheap ones.

[Sir Edward V]

Originally Posted by niftydog

oh, and if you plan on tinkering, you NEED a multimeter, no ifs buts or maybes. You can buy cheap ones.

::sigh::

but I don't wanna!

Yeah, I know I need one, I just didn't want to spring for one...

[bongomania]

Dude, I got a totally deluxe 2Mohm model for like $28 shipped off Ebay. It's not that much of a spring.

[Sir Edward V]

Originally Posted by bongomania

Dude, I got a totally deluxe 2Mohm model for like $28 shipped off Ebay. It's not that much of a spring.

$28 more than I want to pay...

[Sir Edward V]

well i am looking at a schematic for the dd2, one of the delays I was thinking about getting was the dd3 and supposedly the two are very similar, but the dd2 needs a whole pot and not just a variable resistor...

are there no ways to convert this variable resistor into a full fledged pot?

[Sir Edward V]

Originally Posted by niftydog

Well, it's not so much that... what you have to do is emulate the operation of the pot with the photoresistor, which might be dead easy, but it might be really difficult. For instance, what happens if the delay works like this: high value resistance = long delays, but pushing the treadle forward on the old Morley decreases the resistance? Now you might be happy with toe down = shorter delays, but for my money I'd want it the other way around!

I dunno man, this could be a big bite for a beginner.

actually, toe down = short delays is optimal for what I am doing

edit: i was just thinking while looking at the schematic for the dd2, and I wasn't going to have the treadle replace the pot, I was going to have it add on to what resistor values were there already. So I was thinking that by thinking about it this way I could add the resistence from the photores. to either one side of the pot or the other to manipulate the values. I think this would work, but make it sound different than just turning the pots manually.

[niftydog]

There's no good (read: easy) way of turning a variable resistor into a pot.

Adding resistance to the existing circuit might work, but you need to find out more about the photoresistor before you go much further. It could be way out of the ballpark.

eg; one I have here measures 10ohms in bright light and 600,000 ohms in the darkness of my cupped hands. (In proper darkness it could easily be a million ohms or more.) If the pot you want to alter is like, 20kohms, then the photoresistor value is going to quickly swamp the pot value.

Yes, you can add resistors in parallel to bring it down, but then the linearity (or otherwise) goes haywire and you limit the range of values you can achieve.

Worth experimenting for sure, but I'm just trying to raise some potential (ha ha) issues before you start punching walls and screaming bloody murder!

[Sir Edward V]

Originally Posted by niftydog

but I'm just trying to raise some potential (ha ha)

I'll have to get a multimeter and try things out... I would much rather use this morley casing than a crybaby... much more room and I hate the sweep of the crybabys

[bongomania]

Originally Posted by Sir Edward V

I'll have to get a multimeter

Thank you, I was worried that I'd have to slap some sense into you.

[niftydog]

keep us posted, I'm willing to help if you decide to tackle this.

[Sir Edward V]

thanks as always

I think I'm going to try, but I need the delay first

I bought a multimeter at radioshack today for $30 I want to see what the actual resistence is on this photoresistor as it would be used, I think to test it properly I will have to desolder it and have it rest where it would be and test it in the dark with the power plugged in

i might have to have someone help me...

I tried it with leaving it in with the power off, with just the light in the room it measured around 10kohms, then with me shading it best I could it was 20kohms

[jsbass]

Nothing beats an old school Morley PWB.

[JanusZarate]

Subscribed, for curiosity's sake

[niftydog]

The photoresistor doesn't need power to work but your reading will be affected if you leave it in circuit. Just disconnect one of it's two legs and measure it. Ideally you'll want to simulate the light from the LED with a flashlight or something.