The term Wheatstone bridge is invented and named by Charles Wheatstone. The main purpose of this circuit is to calculate the value of unknown resistance. But, the current digital multimeters gives a simple way to calculate a resistance. The applications of the Wheatstone bridge mainly involves, it can be used in amplifier circuits to interface different sensors and transducers with modern operational amplifiers. The Wheatstone bridge circuit is designed with two serial and parallel resistances in between two terminals like voltage and ground. When the Wheatstone bridge is stable, then the GND terminal creates a zero voltage difference b/n the two parallel branches. A Wheatstone bridge circuit comprises of two i/p terminals and two o/p terminals, that are arranged in a diamond shape using four transistors.Please follow the below link for projects: Electrical and electronic projects for honors

## Wheatstone Bridge Working

The Wheatstone bridge circuit is mainly used for measuring the electrical resistance. The circuit of the Wheatstone bridge is built in the form of bridge using one variable resistor, two unknown resistors and one known resistor. By using we can measure the value of the unknown resistance

### Wheatstone Bridge Circuit Arrangement

The arrangement of the Wheatstone bridge circuit is shown below. This circuit is built with four arms, they are AB, CD, AD and BC and includes of four resistances namely P, Q, R and S.From these resistances, P and Q are fixed and known resistances. A galvanometer is placed between the two terminals B and D through a switch1 i.e, S1. The voltage source is associated with the two terminals A and C through a switch2 i.e, S2. A variable resistor S arranged b/n the two terminals C and D. When the value of the resistor S adjusts then the potential at the D terminal differs. For ex: currents ‘I1’ and ‘I2’ is flowing through the points ABC and ADC. When the resistance value of arm CD differs, then the current ‘I2‘will also vary.

### Derivation of Wheatstone Bridge

From the above Wheatstone bridge circuit, the two currents I1 & I2 are

I1=V/P+Q

I2=V/R+S

Now point ‘B’ potential with respect to point ‘C’ is the voltage drop across the transistor ‘Q’, then the equation becomes

I1Q= VQ/P+Q …………………………..(1)

Now point ‘D’ potential with respect to point ‘C’ is the voltage drop across the resistor ‘S’, then the equation becomes

I2S=VS/R+S …………………………..(2)

From the above two equations I1Q and I2S we get,

I2S=I1Q

VS/R+S = VQ/P+Q `

S/R+S = Q/P+Q

R+S/S=P+Q/Q

R/S+1=P/Q+1

P/Q+1=R/S+1

P/Q=R/S

R=S.P/Q

In the above equation P/Q=R/S, the values of P/Q & S are known values. So the value of R can be easily determined.

#### Unbalanced Wheatstone Bridge

The circuit diagram of the unbalanced Wheatstone bridge is shown below. Calculate the output voltage across the two points C&D and the resistor ‘R4’ value is required to balance the Wheatstone bridge circuit.

**Example of Wheatstone Bridge**

In the above unbalanced circuit, ACB is the first series arm

Vc= (R2/(R1+R2)).Vs

Where, R1=60ohms

R2=100ohms

Vs=100

Substitute R1, R2 and Vs values in the above equation

Vc= (100/(60+100)) X 100

= 62 volts

In the above unbalanced circuit, ADB is the second series arm

VD = R4/(R3+R4) X Vs

VD= 180/ (360+180) X 100

=33 Volts

The voltage across C and D points can be calculated as

Vout= VC-VD

Vout= 62-33 = 29 volts.

The resistor R4 value is mandatory to balance the Wheatstone bridge is given as:

R4= R2 R3/R1

100 X 360/60

600 ohms.

So, finally we can notice that, the circuit of the Wheatstone bridge consist of two input and two output terminals such as A and B, C and D. When the Wheatstone bridge circuit is balanced, the voltage across the o/p terminals is 0V. When the circuit is unbalanced, then the output voltage may be either positive or negative based on the direction of unbalance.

#### Application of Wheatstone Bridge

One of the application of the Wheatstone bridge is a light detector circuit using a Wheatstone bridge circuit.

The applications of the balanced bridge circuits mainly involve in various electronic applications to measure variations in light intensity, pressure or strain. The various kinds of resistive sensors are used in a Wheatstone bridge circuit are light dependent resistors, Thermistors, potentiometers and strain gauges, etc.

The applications of a Wheatstone bridge circuit are used to detect electrical and mechanical quantities. But, the simple application of the Wheatstone bridge is a measurement of the light using photoresistive device. In the Wheatstone bridge circuit, a LDR is positioned in the place of one of the resistors.A Light Dependent Resistor is a one kind of passive resistive sensor, used to change the levels of visible light into a change in a voltage and resistance.

Light dependent resistor is used to measure and monitor the level of light intensity .The resistance of the LDR is high in dim light or dark light about 900ohms at a 100 Lux of a light intensity and low to around 30ohms in a bright light.By connecting the LDR to the Wheatstone bridge circuit, we can measure & monitor the deviations in the light levels.

This is all about what is Wheatstone bridge, Wheatstone bridge circuit diagram, working principle with the application. We believe that you have got a better understanding of this concept. Furthermore, any queries or doubts regarding this article or electronics project kits, please give your feedback by commenting in the comment section below.