# Relationship between light intensity and resistance

### Photoresistor [Robotic & Microcontroller Educational Knowledgepage - Network of Excellence]

We do have a good sense for the brightness of the light. However, the relationship between resistance and light intensity is usually non-linear and therefore. This lowers the resistance of the semiconductor. In this experiment the relationship between the resistance of an LDR and the light intensity falling on it is. The resistance of the LDR decreases as the intensity of the light falling on it the relationship between the percentage of light transmitted and the resistance of.

## Relationship between the light intensity and the resistance of an LDR ?

You can download the spreadsheet below which will create the plot and perform all of the necessary analysis from your entries. This graph indicates that resistance decreases exponentially as the brightness of the light increases.

Ultimately, what we are looking for is the equation for this graph into which we can plug the resistance to obtain the lux. The base of the logarithm could in theory be anything, but I will use a base log.

## How an LDR (Light Dependent Resistor) Works

Here is the resulting plot of log of the lux as a function of the log of the resistance. This plot is pretty close to a straight line, and using Excel although you could also do this by inspectionI obtained a best fit line with a slope of Fortunately, with a little bit of algebra, I can manipulate the equation into the form that I want.

Let's assume that I start with a general form of the equation above: In theory, you could use any resistance value, but I chose 5 kohm because the resistance of the LDR was in the order of a few kilo-ohms under typical room lighting conditions. I plugged the shield into the Arduino, and then built a simple voltage divider circuit with the LDR and resistor. This voltage divider circuit is the crux of the sensor circuit.

### How an LDR (Light Dependent Resistor) Works - Kitronik

The 5 volt supply is split between the LDR and the 5 kohm resistor. As the resistance of the LDR changes, the fraction of the voltage across the two resistors also changes. If the voltage across the 5 kohm resistor is measured using the Arduino, it is very easy to add some code that will determine what resistance the LDR is exhibiting. The LDR is connected to 5V, the resistor is connected to ground, and the point in between is connected to analog input 0.

It is worth to take into account that the photoresistor used in the HomeLab reacts most on orange and yellow light. The sensor VTG is not meant to be a specific measuring device.

It is meant to be more a device to specify overall lighting conditions — is there a lighted lamp in the room or not. In this case one has to just measure the resistance of the sensor in the half dark room, note it in the program and compare measured values — is it lighter or darker.

**Working of LDR Live (Light Dependent Resistor)**

The exercise here is a little bit more complex as the light intensity is measured also in lux. For doing this, exists an approximate formula and floating-point variables. In the C-language are floating-point variables float- and double-type variables, which can be used to present fractions. Their flaw is high demand of resources.

### How an LDR (Light Dependent Resistor) Works - Kitronik

Computers have special hardware to calculate floating-point variables, in the 8-bit AVR microcontroller calculations are executed in software which demands a lot of memory and time. If the flaws are not critical, the floating-point variables are worth using.

Relationship between resistance R of VTG and intensity of light E There is an approximate formula showing the relationship between the intensity of light and electrical resistance in the sensor datasheet. As seen on the graph on the rightwith using logarithm scale, the resistance and intensity of light are almost in linear relationship and form a in-line formula, because following conversion applies: We have also data on one of the points on that line: