Auto on LED circuit

A sensor-based lighting circuit that works automatically in the dark is discussed in this post. This is a simple circuit that even a beginner or novice can assemble easily. This circuit is very popular among science students, as they assemble it and show it in school exhibitions. As such, this is a school project and can be a great, enjoyable project nowadays.   

Let us discuss the circuit and explore the possibilities of assembling it. Yet another benefit of this circuit is that it is cost-effective since very few components go into the circuit, and it also works on a 5 V power supply. 

Know your circuit 

The circuit schematic of the auto light gadget is shown below. By going through the circuit, you can see how simple it is.   A single transistor and the LDR ( Light Dependent Resistor) are the main components at the heart of the circuit. Of course, the LEDs light the way. The circuit works on a 5V DC power supply, or one can even use dry cells for the DC source conveniently. 

Apart from the transistor, LDR and LEDs, only two resistors are needed to complete the circuit. 

Know your design

The base bias of the transistor 2N2222 is kept at zero as long as, during daytime, the LDR is exposed to bright light such as the sun shining. The resistor value falls to zero when exposed to daylight. The LEDs do not get power as the transistor is not conducting during that period. As soon as the light on the LDR is removed or falls to zero, the LDR value increases and allows the base-bias voltage to swing to the required level, which makes the transistor conduct and the LEDs glow. All four LEDs, D1 to D4, illuminate and give light. 

Parts list

T1=2N2222 - NPN transistor ........................................1 no. 

R1=LDR (Light Dependent Resistor) 1000lux...................1 no.  

D1 to D4= LEDs .........................................................4 nos.

R2 = 1k 1/4 W Resistor      ..........................................1 no. 

R3= 50K 1/4 W Resistor ..............................................1 no. 

V1 = 5 V DC supply .....................................................1 no. 

Know your assembling

The circuit can be easily assembled on a general-purpose printed board or Vero board. Arrange the LDR sensor side to face upwards, so that light falls on the sensor portion. One can use one's own ingenuity to place the circuit in a convenient box or enclosure, so that it functions normally. 

Conclusion

Try this simple circuit, assemble it and put it in use so that other new beginners, students and electronics enthusiasts learn the circuit design, working and putting into other uses as one imagines.