The Wheatstone Bridge

If we wish to measure an unknown electrical resistance, we can use a circuit called a Wheatstone Bridge (Fig 1).
It works by balancing two legs of a bridge circuit with one of the components being the unknown one.

Fig 1

For the configuration in Fig 1, R2 is a variable resistor, Rx is the unknown, and R1 & R3 should be the same value. The bridge is balanced by varying R2, until the output voltage = 0v.
Set up the following circuit (Fig 2) on a breadboard.
Connect a Voltmeter across the centre of the bridge and use it to measure the voltage in order to ‘balance’ the bridge.I have used a Light Dependant Resistor (LDR) as Rx but other types of Resistors can also be used.
When turning the variable resistor you should aim to get the Voltmeter to read 0v – this means your bridge is balanced. Obviously if you are using an LDR, you will need to monitor the Voltmeter as changes in light levels will alter the balance of the bridge (why do you think this is so? What ways could this circuit be used?).

Fig 2

Breadboard layout shown in Fig 3, and 4. Connect your Voltmeter and use the variable resistor to set the voltage to 0v.

In addition to a Prototype board (Breadboard), wire for connections and a power source, you will need the following components to build this circuit:
2 x 10KΩ Fixed Resistor1 x 100KΩ Preset Potentiometer1 x miniature LDR
You will also require a Digital Multimeter.

Fig 3

Fig 4

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