How to make Wheatstone bridge based light controller?

Let us make light intensity controller based on ambient light available. Circuit is simple and uses Wheatstone Bridge concept with a LDR to sense the ambient light and control the intensity of a LED connected at output.

Watch out this video to see the project in action.

Light Dependent Resistor (LDR) changes its resistance based on intensity of light falling on it. When light intensity is high the resistance is very low in order of some ohms and as light intensity decreases its resistance increases in order of some kilo Ohms or even mega Ohms.

We have used this property to make a Wheatstone bridge based project which detects light and switches off the LED light connected to it and when light falling on it decreases, it turns on the LED light. This can be used to switch ON or switch OFF street lights and other lights based on sunlight intensity falling on it, hence they will switch ON automatically when it’s dark and switches OFF automatically when sunlight falls on it. To drive such high power lights some addition is required in this circuit.

Circuit Diagram & PCB Layout:

Circuit Working:

Here resistor R1, LDR1, R2 and R3 makes a Wheatstone bridge whose output is fed to IC1. Resistor R2 and R3 creates a voltage divider circuit.

IC1 is working as amplifier which amplifies the difference in voltage between point A and B of Wheatstone Bridge.

This is required as the voltage difference at point A and B may be high when the bridge is unbalanced but the current is very less to drive any light or load.

This IC has two input pins – 2 & 3. Pin 2 is marked as “-“ and pin3 marked as “+”.

IC1 compares the voltages at its both input and based on their difference it gives output as some voltage near to IC supply voltage i.e. 9V or Zero voltage.

Below table gives describes the condition at its input and corresponding output.

Table - 1

WB Voltage	IC1 Input Condition	IC1 Output
VB< VA	Pin 2 voltage < Pin 3 voltage	~ 9V
VB> VA	Pin 2 voltage > Pin 3 voltage	Zero Volt

Output of IC is not enough to drive LED1 directly hence we need to amplify it further using transistor.

Output of the IC1 is fed to a transistor T1 through resistor R4.

Resistor R4 limits the current to transistor input which amplifies the current that is enough to drive the LED1 connected to it.

Another resistor R5 is used here to limit the current and voltage across LED1 as white LEDs need ~3V and current between 10mA to 20mA for safe operation.

When input to transistor is Zero, it’s OFF and LED1 is also OFF.

When input to transistor is 9V, it turns ON and drives the LED1 also ON.

Extending table -1 to fit the condition of transistor T1 and LED1 based on above conditions.

Table - 2

WB Voltage	IC1 Input Condition	IC1 Output	Transistor and LED Conditions
VB< VA	Pin 2 voltage < Pin 3 voltage	~ 9V	ON
VB> VA	Pin 2 voltage > Pin 3 voltage	Zero Volt	OFF

Circuit Operation:

Voltage at point B of Wheatstone bridge can be calculated using as

V_B = 9V * R2 / (R2 + R3)

     = 9 * 5600/(5600+5600) Volts

     = 4.5V

This voltage is fixed as value of resistor R2 and R3 is fixed and will not change based on any condition of light falling on it.

Now we know when light falls at LDR its resistance is very low in order of some Ohms, so let’s assume the resistance is 500 Ohms when light is falling on it.

In this condition let’s calculate voltage at point A.

V_A (Light) = 9V * LDR Resistance / (LDR resistance + R1)

                = 9 * 500 / (500 + 5600)

                = 0.73V

When there is no light on LDR1 the resistance increases to some kilo Ohms, let’s take as 20Kilo Ohms i.e. 20000 Ohms and calculate voltage at point A in dark condition.

V_A (Dark) = 9V * LDR Resistance / (LDR Resistance + R1)

                   = 9 * 20000 / (20000 + 5600)

                   = 7.03V

Now lets us extend table 2 and put these values during light and dark conditions.

Table-3:

WB Voltage	IC1 Input Condition	Light Condition	VB and VA comparison	IC1 Output	Transistor and LED Conditions
VB< VA	Pin 2 voltage < Pin 3 voltage	Dark	VB (4.5V) < VA (7.03V)	~ 9V	ON
VB> VA	Pin 2 voltage > Pin 3 voltage	Light	VB (4.5V) > VA (0.73V)	Zero Volt	OFF

Download the Eaglecad PCB file from Google Drive, else you can assemble on breadboard or general purpose PCB.

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