Wheatstone Brigde

A Wheatstone bridge circuit as shown below is used as a sensor to measure very small voltage in the order of millivolts. It is often used when greater sensitivity is required. The Wheatstone bridge consists of 4 resistors. When all the resistances are equal, no voltage is seen across the bridge. When used as a sensor, resistors R1-R3 are of the same value while the resistance of resistor R4 which is the sensor will vary according according to the change in resistance. R4 can be a photocell whose resistance vary with light intensity, strain guage whose resistance varies with length or a flex sensor that changes resistance based on the degree of bend.

A Wheatstone bridge consist of two voltage divider circuit. As shown in the circuit above, the voltage across R3 is given by:

while the voltage across R4 is given by:

Therefore, the voltage across the Wheatstone bridge is given by:

Since R1 = R2 = R3 = R,

Therefore, if R equals R4, the output voltage is zero; if R is greater than R4, the output voltage is negative; if R is less than R4, the output voltage is positive. The changes in the output voltage is in the order of mV and therefore sensitivity is improved using a Wheatstone bridge configuration. A voltmeter is used to read this voltage. If this output voltage is to be read by a microcontroller, an operational amplifier will be required to amplify the voltage.