Determining The Specs On A Photocell - How Do You Do It?

[Paul Marossy]

How do you determine the specs on a photocell? I have a looper pedal circuit that I need to basically clone and add a switch to, and it uses a photocell. BTW, isn't a photocell basically just an LDR?

Is it just as simple as measuring the resistance of it when it has no light shining on it? It looks like it measures about 50K with light blocked from it.

Any help appreciated!

[frank_p]

The different ones I have have a resistor value for when the photocell are in complete darkness and an other R value for when they are in bright light (no Lux values specified).

But if you have the code there are some datasheets for these too.  Ex:
http://pdf1.alldatasheet.com/datasheet-pdf/view/236591/ETC/N5AC-501085.html

[Paul Marossy]

Right, I figured that out by looking at an arbitrary datasheet. But I have no idea what the "model number" is. It's one of those things that looks like the one on page 14 here: http://www.datasheetcatalog.org/datasheet/perkinelmer/VT500.pdf

[R.G.]

No problem.

You measure
- dark resistance X seconds after removal of an accurately measured light source
- Light resistance Y seconds after application of an accurately measured light source
- Speed of change from dark to light resistance
- dark adaptation, measured by measuring the immediate dark resistance, and then the increase in resistance as the cell becomes dark adapted over a few seconds to minutes.

When you get those down, you just look up the nearest thing in a catalog. Simple, ain't it?

[Paul Marossy]

Not exactly a simple process, which is what I thought. I guess I would have to just kind of make an educated guess based on what measurements I can do?

The photocell is just being used to add and then vary the total resistance between the signal at the input jack and the effect send. You control it by how you place your foot over it to control the amount of light hitting the photocell.

[frank_p]

Right, I figured that out by looking at an arbitrary datasheet. But I have no idea what the "model number" is. It's one of those things that looks like the one on page 14 here: http://www.datasheetcatalog.org/datasheet/perkinelmer/VT500.pdf

That is a nice little document, thanks.

Not exactly a simple process, which is what I thought. I guess I would have to just kind of make an educated guess based on what measurements I can do?

Not too complicated neither.  Fun to fool around with if you're not in a hurry.
What the LDR is used for in the looper ?

[R.G.]

Not exactly a simple process, which is what I thought. I guess I would have to just kind of make an educated guess based on what measurements I can do?

Probably. Of course, LDRs are DESIGNED to vary hugely, so my experience has been that if you get enough range and speed for the application, the small stuff about ratio, linearity, and history can be lived with. Depends on the application. That's that "educated guess" thing.

The photocell is just being used to add and then vary the total resistance between the signal at the input jack and the effect send. You control it by how you place your foot over it to control the amount of light hitting the photocell.

Oh. OK. In that case, the requirements are pretty lax. Our friend Ohm's Law helps.

I once saw exactly this integrated into the guitar end of a signal cord. The idea was that you could fade the guitar by moving your hand over the hole in the plug. The patent office had awarded these guys a patent on this for some reason I can't fathom.  Anyway, I put this in the "too tricky to get to work" category, not because of the difficulty in getting it to work, but because of the reliance on ambient light, and the huge variation of lighting conditions on stage. It's kind of a bedroom-only device, at least as I saw it.

In any case, you can use about any photocell that gets over a megohm or so. The LDR is probably in series with the signal, and reacts with the input resistance to cut down signal when it's covered. Wide variation in LDR is acceptable, you just need to get one faster than the several seconds that outdoor light sensors use.

[Paul Marossy]

I once saw exactly this integrated into the guitar end of a signal cord. The idea was that you could fade the guitar by moving your hand over the hole in the plug. The patent office had awarded these guys a patent on this for some reason I can't fathom.  Anyway, I put this in the "too tricky to get to work" category, not because of the difficulty in getting it to work, but because of the reliance on ambient light, and the huge variation of lighting conditions on stage. It's kind of a bedroom-only device, at least as I saw it.

In any case, you can use about any photocell that gets over a megohm or so. The LDR is probably in series with the signal, and reacts with the input resistance to cut down signal when it's covered. Wide variation in LDR is acceptable, you just need to get one faster than the several seconds that outdoor light sensors use.

Thanks RG! I appreciate the help. I think that will get me in the ballpark.

Yes, the LDR is in series with the signal coming from the input jack and can be switched on or off. When the guy was demonstrating this "Eye Of God" pedal for me, it seemed like it took a while for the LDR to respond when he put his foot over it to block the light. But the light inside the "PRS" room wasn't that bright, either...

Oh, and yeah, that patent thing seems kind of silly. That would be difficult to make work consistently and be somewhat unpredictable because of all the variables with lighting conditions. I guess that IS the appeal for some people, though.

[R.G.]

Yes, the LDR is in series with the signal coming from the input jack and can be switched on or off. When the guy was demonstrating this "Eye Of God" pedal for me, it seemed like it took a while for the LDR to respond when he put his foot over it to block the light. But the light inside the "PRS" room wasn't that bright, either...

Oh, and yeah, that patent thing seems kind of silly. That would be difficult to make work consistently and be somewhat unpredictable because of all the variables with lighting conditions. I guess that IS the appeal for some people, though.

Yeah, probably so.

I think a good volume pedal is probably more usable.

We have a guy at the office that can do "whale songs" with a distortion pedal and a volume pedal. Kinda neat if you like that kind of thing.

[Paul Marossy]

We have a guy at the office that can do "whale songs" with a distortion pedal and a volume pedal. Kinda neat if you like that kind of thing.

I know how to get all sorts of sounds out of my guitar, but I'm not sure how to get those sorts of noises.