This circuit describes the principle of light dependent resistor (LDR) operation. LDR is actually a device whose sensitivity is based on the intensity of light that is falling on it. The LDR’s resistance decreases whenever intensity of light falling on it increases & vice versa (resistance increases when intensity of falling light decreases). In absence of light or in dark, resistance shown by LDR is in the range of mega ohms, which is decreased to a few hundred ohms in the presence of bright light.
This phenomenon can be measured with the help of a multi-meter. For the measurement, keep your multi-meter in ohms or resistors measurement region only. When you cover the LDR, its resistance ‘ll be high & when you place it in light it is decreased. We are making use of this property of LDR to make it act as sensor, since, a varying voltage drop can be gained with varying light.
Working of the circuit
Since we are aware of the fact that LDR offers very low resistance in daytime & high resistance in the darkness. Now in day time LDR offers low resistance that is why transistor T1 is in its saturation state & base of transistor T2 is at very low voltage & it ‘ll not conduct, means transistor T2 is in cut off mode. So, LED connected at the output does not glow.
And in night, LDR offers high resistance, So, transistor T1 ‘ll be in cut off mode which ‘ll make the base of transistor T2 high, it ‘ll start conducting & LEDs connected at the output start glowing. We’ve used a diode D5 & current limiting the resistor R1 to charge battery.
In this circuit, 230V mains power supply is stepped down by means of transformer T1 which ‘ll give us 9V AC supply. Now transformer output is rectified by means of a bridge rectifier made with the help of diode D1 – D4. C1 is used to remove the AC components & IC 17806 is utilized to provide us regulated 6V power supply.
Summary of LDR circuit for automatic light system:
- This circuit is based on Light Dependent Resistor (LDR) which ‘ll turn on LED’s when ever darkness is sensed in room. You can put this small lamp in your kid’s room as a simple emergency lamp to avoid panic situation in case of sudden power failure. As it provides ample of light in the room so that your child can easily come out of room.
- Important feature of this LDR based automatic lamp circuitis that it automatically become on by sensing the darkness so no need to search for a switch.
- And it is powered by a rechargeable battery which charges automatically by the AC power supply so no need to worry about battery backup.
- You can use adapter readily available in market or make your own power supply by means of transformer & bridge rectifier.
- And when light comes it automatically off the lamp so need to off the lamp manually
An LDR is stand for Light dependent Resistor, as its name refers, it is a device that is used a light detector. It works as a resistor when light is emitting on it. Its resistance will decreases when light is emitting on LDR and resistance is increase when there is on Light. The resistivity in absence of light reaches to Mega-Ohms.
IC7806 is a voltage regulator integrated circuit. You can find different linear voltage regulator IC in this range (78’XX’). The source voltage in any type of circuit is not fixed it can be change according to your circuit design.
Resistor is a forbearing component used to control current in a circuit. Its resistance is provided by ratio of voltage given across its terminals to the current passing through this circuit. Thus a particular value of the resistor, for fixed voltage, limits the current through it.