LDR Controlled Transistor switching is a type of switching in which ON-OFF (switching) of transistor is controlled by LDR (Light Dependent Resistor). That’s means, switching is done in according to presence of light around the LDR sensor. This type of switching arrangement is used in various automatic switching like street light automatic switching, emergency lamp etc.
The arrangement shown in image : 1 is an inverted type. Here, when light is falls on LDR sensor, its resistances decreases which results in decreasing in voltage drop across it. Due to which, voltage across the Base of the transistor decreases and when it drops below 0.7V (Si) or 0.3V (Ge),then npn transistor is turn OFF ( Reverse in case of pnp transistor). Here output is connected to LED light as a load. When light will fall on LDR, output LED will be OFF.
In non-inverted (shown in image :2), when light falls on the LDR sensor, its resistance will be decreases which results in drop of voltage across it. Less voltage drop across LDR sensor result in more voltage across resistor which turns on the npn transistor. Hence, in this case, when light fall on LDR sensor, LED will start grow.
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