A light sensitive switch is basically a device which is able to sense a particular level of light intensity and respond to the applied conditions.
The response may be by activating its output stage, which in turn may be used to switch ON an alarm or something which may alert the user.
Or simply the device can be used to activate some external variant which may proceed firther with the intended operations as desired by the user.
How to Make a Light Sensitive Switch Circuit - Refer the CIRCUIT DIAGRAM for proper understanding.
Here, the circuit employs a very simple concept and requiresbare minimum number of components for the required implementations.
A couple of cheap transistors some resistors, a relay and an LDR are all that is needed for initiating this interesting function.
If you see the diagram carefully,you will find that the the whole wiring is basically a voltage comparator. The LDR which is the main light detector is used fpr sensing the light levels.
A bright light makes its resistance fall drastically. This fall in the value of the LDR passes more current to the base of the first transistor sp that it starts conducting. As soon as it starts conducting, the second transistor is cut-off and the relay is also cut off. The relevant connections of the relay contacts quickly get busy with its configured functions of activating an external device set by the user.