Understanding LDR's/Mu-tron

[lego4040]

I dont understanding LDR terminology at all, only know they do what they do Now I just finished the Krypton Phaser(MU-TRON ii) I used the Tayda's https://www.digchip.com/datasheets/parts/datasheet/518/KE10720S-pdf.php Someone is building this and has informed me they are going to try both a set of  smallbears 5mm (8002)8 - 24K/500K and (9200)   10 - 50K/5 Meg. Before I button this up Id like some input, thank you

[somnif]

The first number is the resistance of the LDR when light is shone on it. The second number is the resistance in the dark. LFO pulsing the light makes the resistance vary between the two.

[somnif]

And for the record, the diffuse yellow LEDs and Tayda LDRs worked fine in my Krypton. Admittedly, I've not compared them to other combinations, but I'm quite happy with what I've got.

Looking at the schematic, the upper limit shouldn't matter TOO much, as its in parallel with a 220k. The lower limit on the Tayda LDRs I measured all came in significantly lower than 50k in my tests too (often less than 10k as well when arranged like it would be in the pedal).

[alanp]

More simply, a LDR is only a resistor.

The trick is, that THIS resistor changes value depending on how bright the light around it is.

[midwayfair]

I'll try to give some insight into what's going on without getting too technical (not least of which because I don't know enough of the chemistry involved to give a deeper explanation.)

Light dependent resistors are constructed from a material that, when it's exposed to light, will allow more current flow through the material. Lots of materials have this property, we just don't see it very often because the effect is small.

But LDRs made from stuff like Cadmium Sulfide exhibit a LOT of this effect. When you shine light on them, if they can absorb the light, the absorbed photons will alter the electron orbits. You can alter the chemical makeup to alter what frequencies of light are absorbed. This gives photocells one of their important characteristics: there is a peak light frequency to which they react most strongly.

So when you make the LDR, you're not just connecting the two wires together; they get separated by some material. When the electrons get excited by absorbing a photon, they jump up an orbit. If you excite an electron enough, it can become from and it'll end up in the conductor material that's connected to your circuit. In other words, you'll get some current flow. If excite lots of electrons a whole lot, you'll get more conduction. Put them in the dark again and the electrons re-emit photons to release energy, go dark, and there's very little conduction.

So you get a couple things going on: first, none of this is instantaneous. In fact, it's measurable in milliseconds for the electrons to be given off, and dozens of milliseconds, maybe even hundreds, for everyone to calm the hell down again and go up to the highest intrinsic resistance through the LDR. So you get two more important properties: Turn on time, and turn off time. Turn on is how fast they do their thing when exposed to light of the proper frequency. Turn off is how long they take to increase the resistance once the light is out.

Finally, depending on how the LDR is constructed, there's only so much resistance you'll get when you're in the dark, and you can only drive it so low by exposing it to light, because it requires more and more energy to excite the electrons from a high orbit to an even higher orbit. Eventually you start bumping up against some physical limitations, plus you can burn out the LED if you try to make it too bright. I bet you could drive an LDR to incredibly low resistances by sticking it in front of a stage light, though.

I mentioned the peak frequency before, and you might be thinking "oh, that means there's a best frequency for the LDR" or you might be thinking "oh, that means there's a *right* frequency for an LDR." The second isn't exactly true; any LED will be emitting light that makes the LDR work, maybe just not optimally. (White LEDs emit all colors of light, though, so they can cover all your bases.) But it might also be that any particular circuit isn't served best by using an LED of the optimal color, if for no other reason than the fact that we are sometimes lazy and would rather change the LED color than rework the circuit to react best to the LDR.

[lego4040]

Thanks guys, I showed the spec sheet for the Tayda LDRs to some else who is building this pedal. He said the Tayda's will get you in the right ballpark but maybe not enough sweepin phase. He was ordering the LDRS from SmallBear and One set was similar to the Taydas and the other should give a real good sweep. (8002 and (9200) were the models and the 9200's were the sweepy ones

[Bret608]

For what it's worth, I built mine with the 8002 from Small Bear and a diffused yellow LED and it sounds great. I used to own a vintage Bad Stone phaser and this has just as much depth/sweep. It's just maybe a little more vocal sounding and less airy or metallic, I guess you could say.

I learned a lot from Jon's explanation as always!

[lego4040]

Yeah, the tayda's are close to the 8002's but not as much sweep. I have them coming now and will swap out to see the difference