CdS Photoresistor

Simply put, a photoresistor (LDR) is a resistor that changes with the amount of light that falls on it. The more light hitting it, the less resistive it is. Here are a couple of sites I found talking about them. One talks about using it in a voltage divider.

Please give the type number and manufacturer of the LDR (the type number is required, knowing the manufacturer could help). Details of the microcontroller input would also be useful.

Welcome to CR4. The more information you can give (when asking a question), the better your answers will be.

Hi,

OK here it comes:

LDR CdS Photoconductive Photocell model PDV-P9203 manufactured by API Advanced Photonix.

Dark resistance 20M, 10 seconds after being hold at 10 lux 2856ºK.

R at 10 lux 2856ºK Min 5K Max 20K.

Sensitivity 0.9

And here is what I do not understand, in order to do the math, I need to model the Photocell. I have tried to calculate the model as R=R0*(L0/L)^α where α I think is the sensitivity of 0.9 , R0 is the resistance at 10 lux 2856ºK, and R will be the resistance at L lux. If R at 10 lux varies between 5K and 20K I have two models, and R will be between this two models. Then when I will try to calculate the lux, I will also have two models. With the data from the analog input of the MCU I have calculated the resistance of the LDR. But I am not sure of how to do the math for finding the lux.

For this reason I was asking if somebody knows how to perform this calc.

And why I want this, because I have to make a sensor for turning on a light powered by a photovoltaic cell and a battery. Really I don't need to calculate the lux, but I always want to understand what's going on before doing something.

Regarding the microcontroller is just an arduino uno board, I use it just to make fast and simple measurements. I am no engineer and so I use easy stuff. Also budget is not big, so I have no fotometer, and no oscilloscope.

Regarding the measurement circuit is the CdS with a 1 K resistor, the analog port reading between them. The supply is 5 Volts from the USB port. Then I use the voltage divider rule with the LDR model on it, and I calculate the voltage anf the LDR R.

Thank you for your patience.

What type of a light are you trying to turn on, an LED? you can breadboard that in less than 5 minutes.

Dear sir,

It is a 20 Watts led light.

Sure you can do it in 5 minutes.

Mean that it is easiest just to connect a relay directly to the photovoltaic cell, and the cell will be the sensor.

Or just use the MCU to drive a transistor and the transistor to drive a relay.

And the LDR connected also to a trimmer so that the user can regulate the light level on which the light will be connected.

Or use a window comparator, to drive the light.

But I am also interested in understanding how to model the LDR, and how to calculate the lux received by the LDR.

Thank you very musch for your answer.

As you've stated, the device is specified with a range of resistance values for a given illuminance, so you can do as many sums as you like, but you can't calculate the lux from the resistance without first calibrating your device.

I suggest borrowing a decent quality light meter to calibrate against (if they can still be found outside a cal lab in this digital age). May pick one up cheap in a second-hand camera shop or on eBay.

Afraid it doesn't help much with your model.

Dear JohnDG,

Thank you very much for your answer.

I think I will try to use a digital photocamera that has a fotometer. I have found an article on how to do it.

And I agree with you, it is not possible to measure anything if you can not have a correct reference.

But I am very satisfied for having posted my question in CR4. Sometimes it's good to have second's opinions.

Best regards,

Many thanks for your answer.

Still do not understand.

I am trying to do the math to calculate the lux that the photoresistor is getting.

When I know how I will post it.

Kind regards,

When replying, please use the Reply button on the post to which you are replying. We don't know whether you are replying to me, or to dcpppf.

Hi, I am sorry.

OK here it comes:

LDR CdS Photoconductive Photocell model PDV-P9203 manufactured by API Advanced Photonix.

Dark resistance 20M, 10 seconds after being hold at 10 lux 2856ºK.

R at 10 lux 2856ºK Min 5K Max 20K.

Sensitivity 0.9

And here is what I do not understand, in order to do the math, I need to model the Photocell. I have tried to calculate the model as R=R0*(L0/L)^α where α I think is the sensitivity of 0.9 , R0 is the resistance at 10 lux 2856ºK, and R will be the resistance at L lux. If R at 10 lux varies between 5K and 20K I have two models, and R will be between this two models. Then when I will try to calculate the lux, I will also have two models. With the data from the analog input of the MCU I have calculated the resistance of the LDR. But I am not sure of how to do the math for finding the lux.

For this reason I was asking if somebody knows how to perform this calc.

And why I want this, because I have to make a sensor for turning on a light powered by a photovoltaic cell and a battery. Really I don't need to calculate the lux, but I always want to understand what's going on before doing something.

Regarding the microcontroller is just an arduino uno board, I use it just to make fast and simple measurements. I am no engineer and so I use easy stuff. Also budget is not big, so I have no fotometer, and no oscilloscope.

Regarding the measurement circuit is the CdS with a 1 K resistor, the analog port reading between them. The supply is 5 Volts from the USB port. Then I use the voltage divider rule with the LDR model on it, and I calculate the voltage anf the LDR R.

Thank you for your patience.

I can't understand this data sheet at all:-

But it looks as though your LDR will be 20M in the dark and about 10K at 1 Candela (~10lux) so at the very least you should increase the size of your series resistor to say 10K in order to get significant changes in the pot voltage at around 10 lux.

Dear Randall,

You are right, I have to cange the series resistance.

I will use a trimmer plus a fixed resistor, to improve the change in voltage.

Thank you very much for your answer.

Best regards,