Light Dependent Resistor (LDR)

LIGHT DEPENDENT RESISTANCE (LDR)

A light-dependent resistor, alternatively called an LDR,  photoresistor, photoconductor,  or  photocell,  is a variable resistor whose value decreases with increasing incident light intensity.

An LDR is made of a high-resistance semiconductor. If light falling on the device is of high enough frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. The resulting free electron (and its hole partner) conduct electricity, thereby lowering resistance.

When in darkness its resistance is very large and as more and more light falls on it its resistance becomes smaller and smaller.

Resistance in dark:2 MΩ

Resistance in light:200 Ω

Here we must note that any radiation with wavelength greater than the value obtained in above equation CANNOT PRODUCE any change in the resistance of this device.

Construction of a Light Dependent Resistor: there two common types of materials used to manufacture the photoconductive cells. They are Cadmium Sulphide (CdS) and Cadmium Selenide (CdSe).

The band gap energy of Cadmium Sulphide is 2.42eV and for Cadmium Selenide it is 1.74eV. Due to such large energy gaps, both the materials have extremely high resistivity at room temperature. Hence, these materials are widely used in LDR for practical purpose.

A long, thin and narrow strip of CdS is fixed on the surface of ceramic substrate in the form of zigzag wire as shown in following figure. This construction gives minimum area and maximum length. Then the structure is enclosed in round metallic or plastic case and two terminals (made up of either tin or indium) are taken out for external connections. The structure is covered with glass sheet to protect it from moisture and dust and allows only light to fall on it.

Characteristics of photoconductive cells

Photoconductor	Time Constant	Spectral Band
Cadmium Sulphide CdS	100 milli sec	0.47 to 0.72 um
Cadmium Selenide CdSe	10 milli sec	0.6 to 0.77 um
Lead Sulphide PbS	410 micro sec	1 to 3.2 um
Lead Selenide PbSe	10.2 micro sec	1.52 to 4.2 um

Now when the device is dark, its resistance is called as dark resistance. This resistance is typically of the order of 10¹³ ohms. When light falls on it, its resistance decreases up to several kilo ohms or even hundreds of ohms, depending on the intensity of light, falling on it.

The spectral response characteristics of two commercial cells were compared in our laboratory. And we found that there is almost no response to the radiation of a wavelength which was shorter than 300nm. It was very interesting to note thatthe Cadmium Sulphide cell has a peak response nearer or within the green color of the spectrum within a range of 520nm. Thus it can be used nearer to the infra-red region up to 750nm. It was found that the maximum response of Cadmium Sulphoselenide is in the yellow-orange range at 615nm and also it can be used in the infra-red region up to about 970nm.

Mathematical analysis of photoconductive cell

The sensitivity of photoconductive transducer is defined as the ratio of change in resistance to the proportional change in the irradiation level. Thus, the spectral response of the sensor must match with the appropriate response from light source. Mathematically –

Since the photoconductive cell has relatively large sensitive area, a small change in light intensity will cause a large change in its resistance. Generally, all photoconductive cells show the property of change in resistance in the ratio of 1000:1 for dark to light irradiance change of 0.005W/m² to 50W/m².

But it is also interesting to note that the the relation between irradiance and its resistance is NOT LINEAR. It is actually exponential relationship, as follows –

Where, R_f = dark resistance in ohms, R_i = final resistance when light is incident on it and finally R_t = resistance at any time (t).

Though this all discussion sounds good for a photoconductive cell i.e. an LDR, it has a disadvantage that when its temperature changes, its resistance changes drastically for a particular light intensity. Hence, this device is NOT SUITABLE for precise measurements in analog applications.

Applications

·         It is used in burglar alarm to give alarming sound when a burglar invades sensitive premises.

·         It is used in street light control to switch on the lights during dusk(evening) and switch off during dawn (morning) automatically.

·         It is used in Lux meter to measure intensity of light in Lux.

·         It is used in photo sensitive relay circuit.