Analog light sensor
This is a report on an assignment for a course in process measurement technology at Novia UAS. The task is to design, simulate, construct, test and calibrate an analog illuminance sensor based on an LDR.
- Report part 1: Design
- Report part 2: Construction, testing, and calibration (Download high quality version)
- Browse the project files on GitLab or download entire project as compressed tape archive
Sneak peek at the schematic
![(Picture of schematic)](schematic.png)
LDR characteristic curve
It's surprisingly difficult to find any information about resistance/illuminance characteristics for photoresistors/LDRs. At best you get a straight line on a log-log graph which looks too good to be true considering that datasheets specify at which illuminance the gamma value is measured.
R = R10*(10/E)gamma
R is the resistance, R10 is the resistance at 10 lux, E is the illuminance and gamma is a "constant"
Datasheets tend to define gamma = log10(R10/R100)
Adafruit is the only source I've found with more detailed information about typical characteristics of LDRs: slightly curvy traces on a log-log plot. I meant to add this as a reference in part 2, but unfortunately I forgot about and I don't feel like making any more late ammendments.
Own "research"
The sensor is designed to translate conductance (the inverse of resistance) directly into a voltage: U = k/R
Approximate model: log10(U) = a*log10(E)2 + b*log10(E) + c
A lot more details are in the second PDF
The inverse formula is a bit more funny:
I like to call this "unevenly engineered". If it was over-engineered it wouldn't use a sucky LDR and have a sucky 10% accuracy.
Copyright
I won't bother cluttering the reports any further with explicit permissions unless someone actually wants permission to use them. If you do please send me a message and I'll do something about it.
I also may have to do something about one of the images, I'd rather make an equivalent myself than trusting fair use if I were to open source this project.