Testing an LDR

Physics Coursework – Testing an LDR based sensor

I have devised a project to test the effect of a change in distance between the light source and LDR in a potential divider circuit. Ideally, it would be possible to determine the distance of any light source given it’s light output in Lux, however, practically, I only expect to be able to predict the distance of a particular bulb.

Background

Before performing my experiment, I needed to carry out some background work to decide how I should carry it out. The following documents my background research, and the conclusions it led me to. All background research was done independently, and I came to my decision without outside help.

Types of Sensor

The first, and most obvious, choice for my experiment is the LDR, or Light Dependent Resistor. Light dependent resistors work by having a small area of semi-conducting material whose resistance decreases as the light intensity increases. There is an inverse relationship between the two. The majority of LDRs use a cadmium sulphide cell as the semi-conducting material. Cadmium sulphide cells are cheap, have quite a large range of resistance, and are sensitive to a variety of wavelengths of light, from ultraviolet through to infrared. When cadmium sulphide (CdS) is subjected to light, the absorption of photons excites electrons to a higher energy state. When in this higher energy state, the electrons are able to flow as a current. The more electrons absorbed by the CdS, the more charge carriers available, and thus the less resistant it becomes

Alternative methods I discovered through my research for detecting light levels include photovoltaic cells, which convert photons into electricity. However, it would not be as easy to adjust the sensitivity or range as in an LDR, which can be placed in a potential divider circuit. One very accurate way of measuring light levels would be a cryogenic particle detector. These detectors are accurate enough to detect even a single photon coming into contact with the sensor. However, it would not be practically feasible to use such sensitive equipment, and the cost somewhat prohibitive. The detectors need to be kept at extremely low temperature (>70 Kelvin), and this alone would be impossible to maintain without very expensive equipment. Due to limitations of the two prior methods, I decided to use a light dependent resistor in my project.

When researching LDRs, I found that CdS can take as long as 15ms to reach a high resistance after being exposed to a bright light, and so I was careful to leave sufficient time between readings for the LDR to achieve it’s ‘true’ resistance.

It is possible to measure light intensity with an LDR. Light intensity is measured in Lux, and can be formulated from the two equations:

RL = 500 / Lux KΩ

Vo = Vs * (RL/(RL + R1)

Assuming the two equations to be simultaneous, we can rearrange as follows

Vo = ...