Shown here is a low power consuming LED lamp circuit which starts to grow after light intensity goes below a threshold level of detection. This is not a switch based circuit, thus needs no human interference in order to turn it ON or OFF. Circuit is based on IC CA3140 which is a operational amplifier which combine the voltage of PMOS transistor with high voltage bipolar transistor on single chip, Light depending resistor(LDR) whose resistance dependent upon the amount of light falling on it and few more components.
Fig. 1: Prototype Of LED Lamp Using CA3140 IC on Breadboard
The LED lamp circuit uses an operational amplifier CA3140 whose pins 2 and 3 act as inverting and non inverting inputs.
Two potential divider circuits are formed: (1) LDR and VR1 form one potential divider which will provide a variable voltage at the inverting input that is pin 2;
(2) the second potential divider is built around pin 3 with the help of R1 and R2 which provides half of the supply voltage to non inverting pin that is pin3.
As we know, in presence of light or in day time LDR shows low resistance, therefore voltage at pin 2 is more than the voltage at pin 3 .Thus, output at pin 6 is low hence transistor will be in cut off mode means LED will not glow.
But in darker ambiences, resistance of LDR is high. Hence voltage at inverting input pin 2 of the IC CA3140 decreases than the non inverting input pin 3 as a result output pin 6 goes high which makes transistor to conduct and the LED connected to it start glowing.