Question about a funky CDS cell...

[Earthscum]

I got one of those photocell grab bags from Electronic Goldmine. All the cells are standard... some plain, some big, some small, some in metal cases...

I have 1 in particular that I can't really figure out. More like, wtf would the application of this be?

It is in a metal TO-18 case with the clear epoxy window, but it has 3 leads. I was stoked... I got one with a tap. Nope.

I noticed tonight that it has an "Emitter" tab on it. I'll refer to that as leg 3, since on BC10x NPN's, that goes to ground. So, between leg 1 and 2 measures ~220k dark. 1-3, open. 2-3, open. When I put a steady low light to it I got ~65k between 1-2, 270k between 1-3, and 270k between 2-3.

So, anybody come across one like this? If so, what's the application? Obviously it's not a center tap, so a t-filter is out of the question... although, with some careful planning, I could make use of all the legs, using 1-2 as the total value and leg 3 go cap to ground/feedback, I suppose.

[artifus]

could it be a Pyroelectric element?

[Earthscum]

Ironically, I was just looking at the datasheet for THIS and wondering the same thing...

[Earthscum]

Here's a graphic of the cell in question, since I couldn't get a picture of it...



The dots are just where the leads attach to the material.

Dark:
1-2 270k
1-3 >10M
2-3 >10M

Tested Light:
1-2 72k
1-3 205k
2-3 205k

Funky, right? Look at the graphic, yes it's correct. I wonder if this isn't just  a reject that was supposed to be a 3-way photoresistor network.