Automatic Day/Night switch for AC Lamp Circuit.

Switching ON outdoor light or any other light in the evening and turning it OFF in the morning is usually every Home’s daily routine. Sometimes we forget to turn it OFF in morning and a lot of energy is wasted and also life of the light bulb is decreased as, it is unnecessary ON in presence of daylight, or sometimes we forget to turn it ON in the evening causing inconvenience.
This little circuit can do this task for you i.e. this circuit will automatically turn ON AC Light at evening and turn OFF the AC Light in the morning.

This circuit is basically a ‘dark activated AC Light circuit’ based around LDR, when there is dark It turns ON AC Light and when it detects Light, It turns OFF the AC Light.

LDR or Light Dependent Resistor is the heart of this circuit
LDR (Light Dependent Resistor) – It is a kind of light-sensitive resistor, whose value of resistance varies according the intensity of light falling upon it.
In a LDR; when intensity of light falling upon it is more or high, then the value of the resistor of LDR becomes less or Low,
and when the intensity of light falling upon it is low(or it is dark), then the value of resistor of LDR goes High.


We will see working of this circuit in two parts. as shown in the above image:-

Part 1:- First part is the Power Supply Unit which converts 230V AC to 12DC.
This power supply unit is designed using the least number of components, such as Zener diode, X rated capacitor, rectifier diode, and few resistors.
X rated capacitor and Zener diode is used to get the desired output voltage.

Part 2:- This is the LDR based dark activated circuit.
(R3 + R4) and R5 forms a voltage divider network; when more light falls upon LDR, the value of R5 becomes less and more current flows in direction of i1 and less current in direction of i2 . [i= i1 + i2] .here we can say that i2 is the base current of transistor T1. In this condition when intensity of light is high, most of the part of current ‘i’ will be drained toward i1 and very less amount of current or almost zero current will flow in direction of i2. When i2 is very low transistor won’t turn ON as base current is very LOW.
When intensity of Light starts decreasing, For example during Dusk when Sun is setting the intensity of light slowly decreases and become dark, during this time period when intensity of light is slowly decreasing the value of resistor R5 will slowly increase, this will cause i1 to decrease and i2 to increase, after some time the value of i2 will become sufficient to turn ON transistor T1. we are using NPN transistor S8050 to drive a 12V relay switch, so when T1 turns ON Relay also get turned ON as a result connected ac lamp will also get turned ON.

To avoid flickering during threshold point we are using C3 at the base of T1.
Here threshold point means the point when i2 reaches the minimum base current. For example, at evening when intensity of light slowly decrease and i2 slowly increase; at a certain point when i2 has just reached the value of minimum required base current, the connected AC light may flicker for some time because at evening light is not decreasing in a perfectly linear way.
Most of the time i2 may be fluctuating a little bit, which will be not an issue most of the time, but at the threshold point, it can cause transistor to turn ON and Off rapidly for some time which will cause AC Lamp to flicker. after some time when i2 becomes much higher than threshold current, the flickering of Light will stop because at this point value of i2 will not fall below the threshold current.
To avoid this flickering we have added capacitor C3 at the base of transistor.
The current i2 will also charge capacitor C3 and this capacitor will discharge through R15.
When i2 become absent or value of i2 drops, C3 will provide base current to the transistor for some time, this will avoid transistor to turn On and Off at threshold point, thus flickering of Lamp will be eliminated.

2,315 thoughts on “Automatic Day/Night switch for AC Lamp Circuit.”

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