Light Detector with Sensitivity Control Circuit LM741

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Introduction

Many alarm and security devices of today’s era adept light detector devices for their circuits. Because it helps to trigger those alarms to detect the greater light intensity to aware the user. These light detector devices usually use LDR (light-dependent resistor), also known as photoresistors. Because the LDR has a long response time and it just takes several seconds to adjust conductivity after exposure to light. The LDR transforms changes in the incident light into changes in the resistance. Thus, they can be made from various materials. To understand fully, in this tutorial, we are going to “Light Detector With Sensitivity Control Circuit”

Hardware Required

S.no	Component	Value	Qty
1.	LDR (Light Dependent Resistor)	–	1
2.	Operational Amplifier IC	LM741	1
3.	NPN Transistor	BC547	1
4.	LED	–	1
5.	Potentiometer	10KΩ	1
6.	Resistor	220Ω, 2KΩ, 10KΩ	1, 1, 2
7.	Battery	9V	1

LM741 Op Amp

The circuit is also using a 741 operational amplifier IC whose specifications and pinouts are:

Specifications

Power Supply: Requires a Minimum voltage of 5V and can withstand up to 18V
Input Impedance: About 2 MΩ
Output impedance: About 75 Ω
Voltage Gain: 200,000 for low frequencies (200 V / mV)
Maximum Output Current: 20 mA
Recommended Output Load: Greater than 2 KΩ
Input Offset: Ranges between 2 mV and 6 mV
Slew Rate: 0.5V/µS (It is the rate at which an Op-Amp can detect voltage changes)

Pinouts

PINS	Details
1 ( Offset N1)	Offset pin 1 is for external input offset voltage change
2 ( Inverting Input)	Inverting input for applying the inverting input voltage
3 (Non-Inverting Input)	Non-inverting input pin for applying a non-inverting input voltage
4 (-Vcc)	Vcc- is for applying negative DC supply
5 (Offset N2)	Offset Null 2 is for external input offset voltage adjustment
6 (Output)	Output pi is for obtaining output voltage
7 (+Vcc)	Vcc+ is for applying a positive DC supply
8 (NC)	This pin is for no connection i.e. it is to be kept open

Circuit Diagram

Working Explanation

In this Light Detector With Sensitivity Control Circuit, we are using a Wheatstone bridge circuit that helps us in finding the value of the unknown from a quadrilateral having three known resistances and applying a voltage between a pair of opposite corners. The voltage difference from the Wheatstone bridge applies to the input pin of the 741 operational amplifier IC. The op-amp amplifies the difference between inverting pin and the non-inverting pin and provides an output at pin 6 of an IC. We fed this output into transistor Q1 which is working as a switch and the LED glows, depending on the ON and OFF conditions of this transistor.