Re: Calculation help, light level measuring with CdS photo cell


Chris wrote:

Hi. Everything Ban says about CdS photoresistors is true. They're
crude and sloppy, and they have a lot of hysteresis. And yet, they're
still used in many outdoor light sensors. First, they're responsive to
a wide range of light frequencies all across the visible spectrum,
where photodiodes and phototransistors are usually very
frequency-specific. Also, your eye's perception of light is also
logarithmic. Compared with your perception of light intensity, your
output voltage will seem more linear.

Possibly you could explain more about where that voltage output goes,
and what you're trying to accomplish? Are you trying to make a light
intensity meter, or are you trying to do something else?

Good luck
Chris

Hi Chris,

It turns out silicon photodiodes are also responsive across the visible
spectrum, plus the near IR and near UV. (If some are not this
responsive, it is because an optical filter has been added to the
sensor).

Photoresistors are okay for a simple "on/off" application, but it
sounded like the OP wants more than that, since he was asking about
getting a linear response.

Regards,

Mark

.

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