SENSOR 2 – LDR
A LDR or photoresistor is an electronic component whose resistance decreases with the increasing incident light intensity. It can also be referred to as a photoconductor. It 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. In short as light falls on it the resistance of LDR will decrease from normal conditions. Can be used as Night switch alarm, in counter circuit with laser beam to calculate the person entering into the door etc. LDR's of various sizes and shapes is shown below.