Re: fun things to try

[P3D Gabriel Jacob <jacob@xxxxxxxxxxxxxx>]

Ed Romney writes:

>> 1.You can buy 1/100 sec. wristwatch digital stopwatches very cheaply, a few
>> dollars. If you gated one with a CDS cell it should read slower shutter
>> speeds very well. How would you do this, would you turn the power on and
>> off? I'm not a solid state designer by any means. That is how a digital
>> shutter tester actually  works; a CDS cell turns on and off a digital clock
>> running at 1/10,000 sec.
John Bercovitz writes

>Are CDS cells that quick?  Are there quick versions of CDS cells available?

Photocells are not that quick that they would work with 1/10,000 sec
accuracy. Typical speed of response times for CdS photocells would be
in the order of 70ms and 10ms rise and fall times respectively. So if
a shutter of 1/250 is being tested or 4ms, a photocell would not be very
accurate. The responce times for CdS cells, are quoted for dim light. If
brighter light is used, the responce time is faster and it is possible to
get acceptable response times and this could work. Also there are different
types of CdS photocells that peak at different wavelengths. The photocells
that peak farther into the infra-red generally are faster but are also less
stable. IMO if one was to set something up with a scope I would use a 
photodiode since this has much faster response time and wouldn't
necessarily need to be biased as a CdS cell would.

Gabriel  
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