Photoresistor Module Readings Plateauing

@jordanm23

Haven't used a light dependent resistor for decades. What is your instructional source?

https://docs.sunfounder.com/projects/umsk/en/latest/04_pi_pico/pico_lesson11_photoresistor.html

https://www.electroduino.com/ldr-sensor-module-how-ldr-sensor-works/

Hi Jordan, @jordanm23

I haven’t used that module but it looks like it’s using a CdS photo resistor which I have played with.

My application was a light seeking robot.  One that would cruise around in the sunlight streaming through a window.  It was a simple circuit and I remember trying a few pull down resistors and monitoring the analog values as I walking through the house.  I also remember pulling down an app on my phone that measured lux, but that was overkill for my application.

So tweaking the pot only gives you 0 to 600 reading?

I’ve played with reading the analog pins on a Pico and I never found an analogRead().  It’s more like:

// Voltage check
// 12-bit conversion, assume max value == ADC_VREF == 3.3 V
// shift the number 1 to the left 12 bits = 4096, conversion_factor=0.00080566
const float conversion_factor = 3.3f / (1 << 12);

***

adc_select_input(2); // Select ADC input 2 (GPIO28)
resultADC2 = adc_read();   // read the voltage divider on ADC2
converted = resultADC2 * conversion_factor * 2.0f;  // the voltage divider gives half the true battery voltage

Tom

Hi @jordanm23,

  The link you provided shows a circuit with an LM393, which is a comparator, not an amplifier. Unfortunately, you do not unambiguously say how you have connected it, as you mention Vout, which is not shown on the diagram,  so I can only offer some notes about what might be happening.

The only possible output voltages from the sensor module connector P1 pin 2 (labelled OUT) will be either around 0.4V or near to Vcc, depending on whether the voltage to pin 2 of U1 is greater than the voltage to pin 3, or vice versa.

Alternately, connector P1 Pin 1 (labelled AC) is shown to be connected to the output of R1/N1, which should show a variation with light level, but connecting this point the Pico means that the LM393 and the potentiometer R4 are effectively out of the circuit, and should have no effect on the signal sent to the Pico. Measuring this voltage (with respect to GND) with a multimeter, whilst varying the light level, will show the range of voltages that may be sent to the PICO. Try this test both with the signal connected to and disconnected from, to see if the PICO is changing the voltage, which it shouldn't.

In addition, note the PICO input range is only 0 to 3.3V ... connecting it to a 5V source may damage it, and is obviously well above the maximum voltage that it can measure. The Pico has 3.3V output, which can safely be used to power the board. However, the LM393 is specified to require 5V for normal operation, so do not expect it to work with only 3.3V. 

As for the value of R1 of 10k, the voltage variation will depend on the exact device fitted, which is an unknown. But as a first guess, I Googled the generic name LDR (Light Dependent Resistor), of which I think it is an example, and the first hit at

https //components101.com/sites/default/files/component_datasheet/ldr 20 datasheet.pdf

had the following table

suggesting possible values from 400 Ohms to 1 MegOhm.

At 1MegOhm (complete darkness), the voltage would be almost Vcc, whilst at 400 Ohms (bright light), it would be (approximately) (400/100000) * Vcc = (1/250) * Vcc, which will be close to zero Volts.

Of course, this is implying two different devices will be the same, which is a massive assumption that I can't check, and could prove to be embarassingly inaccurate, but it suggests R1=10k will be fine as a first try.

Sorry, this is a ramble of points to consider. Feel free to report your findings and ask another question if anything is unclear.

Best wishes, Dave