Brightness measurement using ADC

lowpowermcu · ‎2012-04-10

Posted on April 10, 2012 at 17:48

Hi all,

I want to measure the brightness using a light dependent resistor ''LDR'' uisng the stm32l15x devices. I an thinking to use the ADC to measure the ldr output but the the curve of the resistance vs illumination flux shows a logarithmic function.

Is there somebody who did it?

Thnaks & Regards,

MCU LÃ¼fter

emalund · ‎2012-04-10

Posted on April 10, 2012 at 20:11

do you need fast (method1)

do you need precise Method2)

even better, have you considered a light to frequency IC like this $2 device

http://www.ti.com/lit/ds/symlink/tsl235.pdf

Erik

raptorhal2 · ‎2012-04-10

Posted on April 10, 2012 at 23:19

I have done it with a voltage divider feeding a reference through a programmable resister to the light sensor to ground. The programmable resister accommodates the wide range of the light sensor.

But I like Erik's solution better. It requires fewer components. Depending on the light intensity range, you may need a 32 bit timer.

Cheers, Hal

lowpowermcu · ‎2012-04-11

Posted on April 11, 2012 at 10:18

Hi baird and Eric,

Thank you for your answer but I would like to use the LDR (Light Dependent Resistor).

Do you think it is possible?

raptorhal2 · ‎2012-04-11

Posted on April 11, 2012 at 13:45

Please provide a part number for the device you wish to use.

Cheers, Hal

frankmeyer9 · ‎2012-04-11

Posted on April 11, 2012 at 14:37

The applicability does much depend on your application.

In general, the voltage/current can be measured and converted just like every other effect influencing a resistance.

Photoelectric resistors are tend to be relatively slow, at least those I know. This could be an advantage if you want to measure pulsed light, like LED lighting.

Additionally, you need to consider the spectral sensitivity of your sensor and your application.

emalund · ‎2012-04-11

Posted on April 11, 2012 at 15:00

Photoelectric resistors are tend to be relatively slow

which brings us back to my question above

do you need fast (method1)

do you need precise (method2)

Erik

lowpowermcu · ‎2012-04-11

Posted on April 11, 2012 at 15:30

Hi erik,

I want to use the ldr VT9ON1 and the stm32l152 device.

I want a low cost solution, with acceptable accuracy.

Thanks & Regards,

MCU Lüfter

emalund · ‎2012-04-11

Posted on April 11, 2012 at 15:30

never having used a photoresistor, I took a peek

http://search.digikey.com/us/en/cat/sensors-transducers/optical-photo-detectors-cds-photoconductive-cells-ambient-light/1967023?k=light sensor

I was surprised to find they are slow as snails

Erik

emalund · ‎2012-04-11

Posted on April 11, 2012 at 15:43

seeing how high the resistances were, I came to think: how do you connect the resistor. Just from Vdd to the pin (with a bleeder) will lead to HUGE inaccuracies.

Erik