Question

You are given the following components in a Light Sensor Circuit:

Light Dependent Resistor (LDR)	Bright condition (500 lux)	Dark Condition (50 lux)
Resistance values (reference)	1 kΩ	5 kΩ

Light Emitting Diode (LED)	Turn on Voltage Rating	Turn on Current Rating
Characteristics (White)	4.0 V	20 mA

Your goal is to design a light sensor circuit which turns on the LED when it is in a dark room, and turn off the LED when the room is lit. The LDR has a resistance value which changes according to the presence of light in the room, as shown in the first table. The LED turns on when it is given the voltage and current rating in the second table.

Answer 1

Schematic diagram:

The circuit diagram shows how you can make a light detector. An LDR or “Light Dependent Resistor” is a resistor where the resistance decreases with the strength of the light. LDRs are also called photoresistors as they are made of high resistance semiconductor material. When light hits the device, the photons give electrons energy. This makes them jump into the conductive band and thereby conduct electricity.

Working:

When it’s light, the LDR has low resistance. This makes the voltage at the base of the transistor too low to turn the transistor ON.

Therefore, no current will go from the collector to the emitter of the transistor. All the current will instead pass through the LDR and the potentiometer.

When it’s dark, the LDR has high resistance. This makes the voltage at the base of the transistor higher. High enough to turn the transistor ON.

Because the transistor is turned on, current flows through the transistor. It flows from the positive battery terminal, through R1, the LED, and the transistor down to the negative battery terminal.

This makes the LED light up.