Photocell Theory Question

[davidbear]

Hi All,

I am wondering what is the relationship between light intensity (photons/sec/mm2) and the resistance of a CdS photocell?

I want to make a light meter from a CdS photocell. The units can be arbitrary, but I would like to get the form of the function correct. I have failed to find that information by Google and I am hoping that someone here might know the answer. My current guess is that the resistance is logarithmically related to light intensity much in the same way that a thermistor's resistance is related to temperature. However, I have no real basis for this belief and realize that the photocell has a finite resistance in the dark (I have not measured the resistance of a thermistor at 0K , but the form of the equation is well documented).

Thanks for the help.

 

[colin55]

Photocells and similar devices are not an "instrument." They have no definite relationship between brightness and change in resistance or production of energy.
All you will need is a lightmeter from a camera (or as a separate unit), and calibrate the meter you are using.
I use solar cells to determine the brightness of LEDs as your eye cannot detect slight changes in brightness and cannot remember the output of a LED from one day to next.

 

[davidbear]

Thanks for the reply, although I really don't understand this statement:

Photocells and similar devices are not an "instrument." They have no definite relationship between brightness and change in resistance or production of energy.

Under a given set of conditions (temperature, pressure, phase of moon, etc.) a particular CdS cell must have a resistance that is in some way proportional (linear, quadratic, exponential, logarithmic, etc.) to the number of photons that strike the cell at some energy. Otherwise the resistance would not depend on how bright it is (and it does, and it is quite consistent). My question is what is the nature of that relationship.

This is for my kid's science fair where the experiment is to determine the brightness and temperature from a light as a function of distance. I know the relationship should be roughly inverse square (with deviations due to the shape of the bulb, the presence of ambient light, etc.), but feel compelled not to predetermine the measurement by doing the experiment as a means of calibrating my arduino based light meter.

 

[kchriste]

You need to find the data sheet for the Cds cell that you intend to use. A good data sheet will tell you what you need to know:

https://www.datasheets.org.uk/pdf/1889949.pdf

 

[colin55]

Devices that are instruments have a definite relationship between output and detection.
Devices such as temperature sensors, voltage detectors, etc can be relied upon to produce a known result.
Most of the devices you are talking about have a wide variation between one batch and the next. They are not even sorted into groups – like transistors or LEDs.

You may be able to find a graph of the relationships you are talking about but they are generally not used as an “instrument.”

Most of the time they are used to detect either brightness or lack of illumination and the circuitry is adjusted to operate at a particular level of brightness, for example.
You just need a single transistor to amplify the output of the photocell. I simply connect a solar cell to a digital multimeter and it produces a perfect result.
The result is always the same, with the same LEDs, same current and same distance.
When the solar cell is moved closer, the reading is higher, but in my case I require the distance to be same in all cases, as I am altering the illuminating medium.
Light meters are fairly expensive, but some digital cameras have a built-in lux meter with a readout on the screen.
This is the best approach.

 

[davidbear]

You need to find the data sheet for the Cds cell that you intend to use. A good data sheet will tell you what you need to know:

https://www.datasheets.org.uk/pdf/1889949.pdf

Thanks kchriste, that was perfect: it is a log-log relationship.

collin55, thanks for the detailed response, I understand where you are coming from now.

DavidBear

 

[Willbe]

overview

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