How to calculate value of voltage by light intensity?

Question

I want to write the code to turn the LED on when the light intensity that falls on the photoresistor decreases. So in daylight LED is off, and when it's getting dark LED goes on. I know that resistance depends on light intensity. When is bright resistance is low and it goes higher when it's getting darker. Also, I get that voltage changes with resistance. So I need to turn the LED on when the voltage coming from the photoresistor is low. The problem is I don't know how to calculate exact values. How to calculate voltage value when the Sun is going down and the light is low? Also, how can I compare this value with an integer when writing if loop.

Can I use voltage value in code or do I have to convert it?

Answer 1

Perhaps calibrating LDR light-versus-resistance is unnecessary...

Think of maintaining a constant light level, be it from daylight or LED source. Thus, the LDR will be maintained at a fixed value of resistance. LED will provide all the light in darkness, and no light during daylight.

LED provides some light during day/night transitions. This might be accomplished efficiently with PWM, fast enough that LDR resistance doesn't change much during PWM on/off periods.

^{simulate this circuit – Schematic created using CircuitLab}

The software adjusts PWM duty cycle to maintain LDR resistance at a constant value. The microcontroller's ADC would then measure (and maintain) a constant voltage.

Some adjustment of R1 and R2 may be necessary for your unspecified LDR - this circuit and light-adjusting algorithm serves as a circuit-concept rather than definitive, sure-to-work idea. The distance between LED (D1) and LDR1 must be carefully adjusted so that during daylight, LED light is off, or nearly off. R3 limits LED current through D1, limiting its light output...if the LED is too dim at night, move it further from LDR1: the amount of light-coupling from LED D1 -to- LDR1 affects circuit "gain"...some moving-about is required.

A green LED is suggested, since both eye and LDR have peak response in the green-yellow part of the spectrum, but any colour LED should work.

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A microcontroller may not be necessary - this is a feedback idea that can be done with a linear DC-coupled amplifier. In darkness, LDR1's increasing resistance will raise its DC voltage...which in turn should increase LED current to compensate:

^{simulate this circuit}

Answer 2

To expand on an engineered solution with specs as I hinted in comments.

As most know by now show me a good product and it will have great details in the datasheet. Likewise for DIY projects. The better you measure what you want, and tolerate, the better the outcome. Otherwise, it's trial and error (repeat).

Light levels

Here are some examples of the illuminance provided under various conditions:

Illuminance 
[lux]           Surfaces
========        =============================================
0.0001          Moonless, overcast night sky (starlight)
0.002    .....  Moonless clear night sky with airglow
0.05–0.3        Full moon on a clear night
3.4      .....  Dark limit of civil twilight under a clear sky 
20–50           Public areas with dark surroundings 
50 ...........  Family living room lights (Australia, 1998) 
80              Office building hallway/toilet lighting 
100 ..........  Very dark overcast day 
150             Train station platforms 
320–500 ......  Office lighting  
400             Sunrise or sunset on a clear day. 
1000 .........  Overcast day; typical TV studio lighting 
10,000–25,000   Full daylight (not direct sun) 
32,000–100,000+ Direct sunlight

Log Light Sensor

Just using a load resistor defines the voltage.
Then decide if you want to measure it with an ADC or use a precision comparator with a band-gap Vref or just use a 3.3V logic IC and rely on Vdd/2 or add % hysteresis. Add a cap to filter flashes of light or darkness etc.

You define how you want it to behave or not misbehave. ;)

Cheap, easy to use and accurate

and designed long ago by the best in the industry

Then Sharp licensed the designs to Vishay, now packaged on a PCB by Adafruit

Pick your target level lux, then voltage threshold then R.

Don't forget to not have the light near the sensor or it may toggle on and off.