Introduction
This project was inspired by a power outage we experience now a days. Since every day we experience black out, we decided to make our own DARK SENSOR EMERGENCY LIGHT. It is so very useful during power outage rather than using a candle that may cause fire incidents.
This is an automatic emergency lamp with day light sensing, means it senses darkness/night and turns ON automatically. Similarly it senses day light and turns OFF automatically. The good thing about this circuit is that it incorporates ordinary, cheap components which can be easily procured from the local market.

Description I-Schematic diagram

Conclusion
We have to make the light sensor Night switching, we used two transistor, one LDR, one potentiometer (100K ohms), a diode (1N4007), bulb, rechargeable battery (4.5V), and a resistors (2pcs 33 ohms and 2pcs 1K ohms ). We connected a resistance of 1K in series with base of transistor (T1) and also connected the other components of the circuit as shown in the circuit diagram. We used the common emitter configuration of the transistor in this circuit.
If we only used one transistor than the circuit will be daylight switching, we have to make the night switching so we inverted the output by connecting the base of the other transistor with the collector of the first transistor.
Transistor (T1) is ON when there is a light present on the LDR because the voltage drop across the variable resistor is greater than 0.6V so that’s why the emitter collector is shorted all the current passes through this and transistor (T2) is turned off. That’s why we have no output when there is light because we are getting the output across T2. And whenever the LDR detects no light, T1 is off and T2 is on so we got the output in the dark.

A bipolar junction transistor can be used in many circuit configurations; as an amplifier, oscillator, filter rectifier or just used as a switch. If the transistor is biased into a linear region, it will be used as an amplifier or another linear circuit, if biased into saturation or cut-off, then it will be used as a switch, controlling other circuits. This article describes the BJT when used as a switch.

Transistor as a switch

BJT used as an electronic switch, in grounded-emitter configuration.
Transistors are commonly used as electronic switches, both for high-power applications such as switched-mode power suppliesand for low-power applications such as logic gates.
In a grounded-emitter transistor circuit, such as the light-switch circuit shown, as the base voltage rises, the emitter and collector currents rise exponentially. The collector voltage drops because of the collector load resistance (in this example, the resistance of the light bulb). If the collector voltage were zero, the collector current would be limited only by the light bulb resistance and the supply voltage. The transistor is then said to be saturated - it will have a very small voltage from collector to emitter. Providing sufficient base drive current is a key problem in the use of bipolar transistors as switches. The transistor provides current gain, allowing a relatively large current in the collector to be switched by a much smaller current into the base terminal. The ratio of these currents varies depending on the type of transistor, and even for a particular type, varies depending on the collector current. In the example light-switch circuit shown, the resistor is chosen to provide enough base current to ensure the transistor will be saturated.
In any switching circuit, values of input voltage would be chosen such that the output is either completely off,[21] or completely on. The transistor is acting as a switch, and this type of operation is common in digital circuits where only "on" and "off" values are relevant.