Automatic Night Lamp

The circuit automatic night lamp is for controlling the light in the sitout of our house. The light is switched ‘ON’ when it is dark and is switched ‘OFF’ when everybody in the house goes to sleep.

Circuit Description of Automatic Night Lamp

The circuit Automatic night lamp consists of two LDR controlled switches and a gate interfaced relay circuit. LDR1 senses the presence of sunlight outside the house, while LDR2 senses the light inside the house. Transistors T1 and T2 conduct only when both the LDRs are in light. At this time, the voltage at the collector is 0 or low state. When both the LDRs are in dark, T1 and T2 are in cut-off state and the collectors are at high state.

circuit diagram of automatic control for sitout light

These two collectors are the input of XOR gate. When one of the transistor is cut off and the other is at ‘ON’ state, there is a 1, 0 input to the XOR gate or vice-versa. In both the case the output is at high state and the transistor T3 is cut off, hence there is no current flow through the transistor T4. At this state, the relay is not energized. We can connect the relay (two-pole, two-way) such as to ‘ON’ the sitout light.

When both the LDRs are in light or in dark, the output of the XOR gate will be low. As a result transistor T3 and T4 conducts and the relay is energized. Now the automatic night lamp will be switched ‘OFF’.

LDRs should be placed in a convenient place free from damage. LDR1 should be kept outside the room in a dark place. At night, LDR2 should be kept inside the room. It should be placed outside during day time.

The main advantage of this circuit automatic night lamp is that one can do some saving in his electricity bill as the light will remain off during day time.

The resistance R1 can be replaced with potentiometer with a series resistance of 4.7KΩ to control the light conditions.


Resistor (all ¼-watt, ± 5% Carbon)

R1, R4= 10 KΩ

R2, R5 = 220 KΩ

R3, R6 = 1.2 KΩ

R7 = 6.8 KΩ

R8 = 680 Ω

R9 = 470 Ω


IC1 = 7486

T1, T2 = BC147

T3, T4 = AC127

D1 = 1N4001


RL1 = 12V, 200Ω Relay