on 10/24/03 11:36 AM, Moose at olymoose@xxxxxxxxxxxxx wrote:
> CDS cell generates its own current to drive the meter. Si-blue is a
> light variable resistor, so it needs a power source. You would have to
> determine full characteristics of the meter movement and design a
> circuit to generate the correct voltage and current range across the
> coil resistance. Then, of course, you have to figure out how to mount
> the cell and where to put the rest of the circuit. I used to be up for
> this kind of project years ago, but no more.
>
> Moose
Moose, you're off a bit on this... the -selenium- meter cell generates
electricity from light. The generated electric current varys with light
intensity, so a simple galvanometer movement becomes a 'light meter'.
The CDS cell is a light-variable resistor. The light intensity increases or
decreases the resistance, altering the current flowing -through- the CDS
cell, but that current must come from somewhere else, hence the need for a
battery of some sort. Depending on the design of the whole circuit, the
meter can be dependent or independent of the actual battery voltage/current
(within some minimum range).
A silicon-blue cell functions similar to a selenium cell, but its generated
current for a given amount of light is -much- smaller, so in order to use a
silicon-blue cell an amplifier is incorporated into the circuit, along with
the meter movement. Hence, the need still for a battery, though by this time
most designs are independent of the actual battery voltage (so long as there
is enough to drive the amplifier and display used).
So in order to add the silicon-blue cell you would also need to add an
amplifier, although if your amplifier put out enough current the existing
display system might be retained... just 'insert' the silicon-blue cell +
amplifier in place of the CDS cell in the circuit. Packaging concerns are
probably the primary consideration here...
--
Jim Brokaw
OM-'s of all sorts, and no OM-oney...
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