Hi Lama, Moose and all.
>>Does anyone have links to ideas about upgrading from a 70s era
>>cadmium-sulfide
>>cell to a silicon-blue one? If I'm adjusting a pot anyway...
>>
>>
>CDS cell generates its own current to drive the meter. Si-blue is a
>light variable resistor, so it needs a power source.
I think you're speaking about Selenium cells vs. CdS.
AFAIK, Selenium cells generate enough power to drive a galvanometer, thus
no need for batteries. However, they're useless in low light levels (below
EV 7 or so -- that's 1/30s @ F2 with ASA 100, or equivalent). They do go
bad, depending on the total amount of light received thru their lifes.
Sometimes they can be recalibrated -- with further loss of sensitivity.
On the other hand, CdS cells (like those on the OM-1 and XA) are light
variable resistors. Connecting them to a battery generates a current
proportional to light level (not sure about linearity!) -- in total
darkness there's almost no current at all. Much more sensitive than 'Se'
cells, they have a somewhat slow response to changes of light level.
Finally, there are the SBC (aka SPD, aka PhotoDiodes). They do generate
their own current, but so *small* that it wouldn't be able to drive a meter
needle, so it must be amplified by active electronic components (like the
delicate MOSFET on the OM-2).
Hope this helps,
...
Carlos J. Santisteban
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