Photoconductive Photocells

Cad sulphide and cad selenide cells are the pits. You can get a 5:1 resistance hysteresis based only on previous history.

You're far better off with a silicon photodiode or a small solar cell.

Cheers,

Phil Hobbs

How about a bridge circuit to help stabilize.

greg

Photoresistors are no "cells". Very temperature sensitive so if you hold it in the hand, the resistance changes. This only happens at lowish illumination (at 0.1lx the resistance doubles from 20° to 40°). They are also dependent on the past illumination, the resistance rises. They might need a whole day in the dark to recover. Another disadvantage is the long time constant for higher resistance. BUT they are almost as sensitive as a photomultiplier tube, and can drive loads like a relais directly. Don't use it as a measuring sensor or use at least

100 lx for 100ms and then keep a longer pause(30s) until the next measurement.

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ciao Ban
Apricale, Italy

The CdS cell has different time constants for attack (light increasing) and decay (light decreasing), so it is extremely difficult to calibrate. In addition, the decay has two different time constants acting at the same time, one of a few seconds, another of many minutes.

It would be better if you used a photodiode.

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~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

When you say "almost as sensitive as a PMT", I don't think you're taking stability into account adequately. It's true that photoconductors exhibit a gain of (carrier lifetime)/(transit time), which is large in CdS, but (a) photoconductors have twice the shot noise of photodiodes, because the recombination contributes as much noise as generation, and (b) CdS is so variable with time and history that it's really hard to make use of all that sensitivity.

PMTs are also a bit noisier than photodiodes, but are much, much more stable than CdS cells. Cad sulphide and phototransistors are the sorts of devices that look less and less wonderful the closer you get.

Cheers,

Phil Hobbs

Just try checking the dark resistance if you want to see something interesting !

A CdS or CdSe cell is totally unsuitable for this application. To be honest, if you're looking for low levels of contamination I think you're looking at the wrong method.

Graham

I think there are situations where CdS-Photoresistors are of advantage. I have an old reflex camera with a 22.5V battery and it works very well since

40yrs. The meter is so sensitive in low light situations, no amplifier needed. And the accuracy is sufficient for this purpose. Lets see: the current at 10mlx will be around 20 uA. When you compare that with a diode it will hardly put out more than 1nA, since you need also more filtering even less. The temp. stability is not much worse. And it seems if you avoid strong illumination the stability and repeatibility can be good as well, at least for this app. Well, maybe Hasselblad have researched a bit how to treat this resistor, but they did well, IMHO The noise of the diode will be much higher, since the resistance is about 30 times that of the photoconductor. And even the time constant of the diode is difficult to get lower with all its own + amplifier input capacity. I totally agree about precision measurements with you though.

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ciao Ban
Apricale, Italy