For the past 2 days we’ve been severely perplexed with the role of LDR (light-dependent resistor) in electrical circuits. Supposedly, LDR is made from semiconducting materials (eg. cadmium sulphide, CdS). Some of you had said that since resistance lowers as the light intensity (illumination) increases, this will raise the current, right? So the lamp will light up, right? Right? Well, not exactly. Check this fact:
Light-dependent Resistor
Component of electronic circuits whose resistance varies with the level of illumination on its surface. Usually resistance decreases as illumination increases, as the nature of the material is altered by the presence of light. LDRs are used in light-measuring or light-sensing instruments where light intensity is converted to a digital signal (for example, in the exposure-meter circuit of an automatic camera), and in switches (such as those that switch on street lights at dusk).
LDRs are made from semiconductors, such as cadmium sulphide.
Source: http://www.tiscali.co.uk/
Here’s another one..
A photoresistor or LDR is an electronic component whose resistance decreases with increasing incident light intensity. It can also be referred to as a light-dependent resistor (LDR), photoconductor, or photocell.
A photoresistor 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.
A photoelectric device can be either intrinsic or extrinsic. An intrinsic semiconductor has its own charge carriers and is not an efficient semiconductor, eg. silicon. In intrinsic devices, the only available electrons are in the valence band, and hence the photon must have enough energy to excite the electron across the entire bandgap. Extrinsic devices have impurities added, which have a ground state energy closer to the conduction band — since the electrons don’t have as far to jump, lower energy photons (i.e. longer wavelengths and lower frequencies) are sufficient to trigger the device. If a sample of silicon has some of its atoms replaced by phosphorus atoms(impurities), there will be extra electrons available for conduction. This is an example of an extrinsic semiconductor.
Cadmium sulphide cells
Cadmium sulfide (CdS) cells rely on the material’s ability to vary its resistance according to the amount of light striking the cell. The more light that strikes the cell, the lower the resistance. Although not accurate, even a simple CdS cell can have a wide range of resistance from less than 100 Ω in bright light to in excess of 10 MΩ in darkness. Many commercially available CdS cells have a peak sensitivity in the region of 500nm – 600nm (green light). The cells are also capable of reacting to a broad range of frequencies, including infrared (IR), visible light, and ultraviolet (UV). They are often found on street lights as automatic on/off switches. They were once even used in heat-seeking missiles to sense for targets.
Standard cadmium based LDRs have a frequency response that varies according to light level, but is routinely below 1Hz, so they are unsuitable for data links and picture scanning. Silicon based photodiodes and phototransistors are orders of magnitude faster.
Probably the best known LDR is the ORP12. Smaller cheaper devices are more popular today.
The other day before today, we’ve discussed how we can measure small EMF by using a potentiometer and s standard cell. Initially, we find the approximate value of current eg. by using the required voltage of thermocouple. After we have obtained the value for R1 and R2 can we accurately measure the current in the circuit when the circuit is balanced (ie. the galvanometers are zero.)
The diagram on the left is a Wheatstone Bridge circuit. Compared to potentiometer, a Wheatstone bridge is very capable of high precision measurement of resistance, capacitance and inductance. Two things stand out: there is no need to determine the standard cell’s EMF and there is no need to calibrate the galvanometer.
Posted by nazarin 
Posted by nazarin 
Posted by nazarin 
