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In the society, some times street lights glow in day time due to any reason. In mines area people face many difficulties due to absence of light in the nights. In frontier and hilly areas, people face many problems due to damaged street lights.

For solve above these problems, we create a device in which the lights glow in night and in day time, they off automatically and don't glow. Due to use of it, we can solve above problems and can also save electricity and men's power.


This project is based on Light Dependent Resistance (L.D.R.). Light Dependent Resistance [LDR] is a resistance, in which opposing power of current depends on the presence of quantity of light present, i.e. the resistance of LDR increases or decreases, according to quantity of light which falls on it.

If LDR places in darkness, the resistance of LDR increases and when light falls on it, the resistance of LDR decreases and act as a conductor. Any LDR in the presence of light and darkness changes its resistance is depends on the different types of LDR.


  • Collection of parts of the circuit are easily available.
  • Accuracy of this circuit is more than accuracy of other circuits.
  • By using laser, it can be used for security purposes.
  • It can be used to stop the wastage of electricity.
  • The cost of circuit is low.
  • This circuit saves the men's power.


  • It can be used in street lights.
  • It can be used in mines areas.
  • It can be used in hilly areas.
  • By using laser, it can be used for safety purposes.
  • It can be used in frontier areas.
  • It can be used in houses.
  • It can be used in jail lights.


    1. TRANSISTOR:-When a thin slice of p-type is sandwiched between two blocks of n-type, then n-p-n transistor is formed. It consists of emitter, base, and collector. In the project, common emitter n-p-n transistor ( BC-147 & SL-100) is used.

    2. DIODE:- When a p-type and n-type semiconductors are joined a diode is formed. It conducts when forward biased and does not conduct when reverse biased. In the project, IN-4007diode is used.

    3. RELAY:- It helps to contact and discontact. In the project, we use relay of 6 ohms.

    4. CARBON RESISTOR:- A carbon resistor has generally four rings or bands A,B,C and D of different colours corresponding to the value of resistance. In project, we use carbon resistance of 220 kilo-ohms, 1.5 kilo-ohms and 820 ohms.

    5. LDR:- LDR means light dependent resistance which is used to complete the circuit.

    6. TRANSFORMER:- Transformer is used to convert low alternating voltage to high alternating voltage by decreasing the current and vice-versa. We use a transformer of 6-0-6V for the circuit.

    7. CAPACITOR:- Capacitor is used to block DC. In the circuit, we use the capacitor of 220mfd and 1000mfd.

    8.BULB:- An electric bulb is connected to the circuit when LDR comes in the darkness.


    Let 6V. DC is flowed in the circuit. When light falls on LDR then the resistance of LDR will decreases and approximate equal to zero. 6V DC come from 220 kilo-ohms resistance and pass through LDR. At this time, it becomes earth, so that no voltage flows through the base of transistor T1. Due to absence of positive voltage at base, emitter hasn't negative voltage in the comparison of its base, so that transistor T1 don't conduct and no current flows through it. But resistance of 1.5 kilo-ohms is connected with 6V DC line, so that from 6V DC approximate 4V DC passes through 820 ohms and reach at the base of transistor T2. For conduction, emitter has some negative voltage in the comparison of its base. But at this time, 4V DC reach at base of transistor T2, so that emitter has highly negative voltage in comparison of its base, so that transistor T2 highly conducts. The collector of transistor T2 is connected with relay coil, so that magnetic induction around relay coil will disconnect with rest point and connect with active point. So, According to the circuit, the bulb will glow.

    Now, when light don't fall on LDR so that the resistance of LDR increases. Now LDR has not zero resistance due to darkness, so that 6V DC line and 220 kilo-ohms resistance, some positive voltage reach at the base of transistor T1.The collector of transistor T1 has positive voltage because 6V DC pass through 1.5 kilo-ohms resistance and reach at collector of transistor T1. In this way transistor T1 conduct and current start to flow in transistor T1. Transistor T1 and resistance of 1.5 kilo-ohms are connected in series so that the current also flows through 1.5 kilo-ohms resistance. Now, the above terminal of 1.5 kilo-ohms resistance has 6V DC, it means the lower terminal of 1.5 kilo-ohms resistance has lower voltage i.e. 3 or 3.5 V. From 3V approximate 1.5 V or 1 V. DC reaches at base of transistor T2. The collector of transistor T2 has positive voltage by relay coil. But at this time, emitter has some negative voltage in the comparison of its base.

    So that the quantity of current which flows through transistor T2 and relay coil will very low. Due to this, magnetic induction around relay coil will not more effective and pole point will disconnect with active point and connect with rest point. According to the circuit, the bulb will glow.


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