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
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.It can be used in jail lights.
BRIEF DISCRIPTION OF COM PONENTS
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
5. LDR:- LDR means light dependent resistance which is used to complete
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
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.