are glass bead thermistors sensitive to light?

Thread Starter

dazza111

Joined Jul 6, 2011
3
Hi,
I want to know if a glass bead thermistor would be light dependent.
I am directing a laser beam into a Light Dependent Resistor to measure turbidity in liquid samples, but am finding that there is heating of the LDR. This is causing the resistance reading of the LDR to change and give a false reading. I have glued a very small grass bead Thermistor to the back of the LDR and hope to measure the rise in temperature of the LDR when it is illuminated by the laser light. I am currently painting the Thermistor black and trying to see if it has any light sensitivity.
What do people think.
thanks
Dazza111
 

Alec_t

Joined Sep 17, 2013
13,149
What is the laser wavelength? If it's infra-red, or something close, then there will be significant heating. Also, how much current are you passing through the LDR? That will also cause heating.
 

DickCappels

Joined Aug 21, 2008
9,299
If it is only a "resistive" thermistor it will not be able to convert photons into amps, but as noted above it can affected by the heating effects of light. At time Texas instrument made "Stabistors" which were stacks of PN junctions -those could be used as photo detectors if the junctions were exposed to light.

This a little bit far out, but if you switch your laser off and on you can use the resistance of the thermistor as a zero reference. That can be done digitally if you have more resolution than you need, or you can make an analog "black clamp". The circuit below might to get the idea across.

The signal labeled "Back Porch Sample" is samples the voltage when the laser is off.

1662982821364.png

If you can run the thing slowly enough so that the time constant for the photodetector does not "filter out" the signal to any appreciable extent, this might give better results than trying to temperature compensate your photo detector.

Not: The example circuit above is a pretty fancy black level clamp used to establish the DC level for processing an AC coupled video signal. It is only to give you an idea of how to subtract an unknown and drifting offset from the signal.
 

MrSalts

Joined Apr 2, 2020
2,350
Hi,
I want to know if a glass bead thermistor would be light dependent.
I am directing a laser beam into a Light Dependent Resistor to measure turbidity in liquid samples, but am finding that there is heating of the LDR. This is causing the resistance reading of the LDR to change and give a false reading. I have glued a very small grass bead Thermistor to the back of the LDR and hope to measure the rise in temperature of the LDR when it is illuminated by the laser light. I am currently painting the Thermistor black and trying to see if it has any light sensitivity.
What do people think.
thanks
Dazza111
When measuring temp very carefully and something that doesn't change rapidly (like turbidity), it is best to take a measurement quickly, turn the laser off to prevent heating of the detector, the some time later, turn iThe laser on again (briefest) to make a measurement and repeat.
 

MisterBill2

Joined Jan 23, 2018
13,115
Certainly a glass bead thermistor will be heated by the energy delivered by the laser beam. That is the way energy absorbtion works. Painting the thermistor black should improve the absorbtion of energy and increase the heating a bit. How many milliwatts does the laser deliver? A photodiode would be a more effective way to measure light intensity, and the much faster response will allow operation with a pulsed laser beam, and that can greatly reduce the heating effect.
T
 

Reloadron

Joined Jan 15, 2015
6,958
Also, how much current are you passing through the LDR? That will also cause heating.
That should be a consideration. Is the light source heating the LDR or is the LDR self heating? I would start by checking if the LDR is heating less any light source. Rather than using a thermistor or similar, if you access to one I would just use a IR Thermometer. Not like you need an expansive one to just note an increase in temperature.

Ron
 
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