DIY Wheatstone Bridge

A Wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component. The primary benefit of the circuit is its ability to provide extremely accurate measurements (in contrast with something like a simple voltage divider). Its operation is similar to the original potentiometer.

The Wheatstone bridge circuit mainly consists of two known resistors, one variable resistor, one unknown resistor (whose value to be calculated) and a Galvanometer (detect and measure small amounts of current in an electrical circuit). In the place of the resistors and galvanometer I have used small bulbs which I got from old decorative lighting strip. The resistance of all the bulbs are same. Alternatively, You can use torch light bulbs.

The Wheatstone bridge works on the principle of null deflection, i.e. the ratio of their resistances are equal and no current flows through the circuit. Under normal conditions, the bridge is in an unbalanced condition where current flows through the galvanometer. The bridge is said to be in a balanced condition when no current flows through the galvanometer. This condition can be achieved by adjusting the known resistance and variable resistance.

In the figure, Rx is the fixed, yet unknown, resistance to be measured. R1, R2, and R3 are resistors of known resistance and the resistance of R2 is adjustable. The resistance R2 is adjusted until the bridge is "balanced" and no current flows through the galvanometer Vg. At this point, the potential difference between the two midpoints (B and D) will be zero. Therefore the ratio of the two resistances in the known leg (R2 / R1) is equal to the ratio of the two resistances in the unknown leg (Rx / R3). If the bridge is unbalanced, the direction of the current indicates whether R2 is too high or too low.

Detecting zero current with a galvanometer can be done to extremely high precision. Therefore, if R1, R2, and R3 are known to high precision, then Rx can be measured to high precision. Very small changes in Rx disrupt the balance and are readily detected.

For more information visit this link: -https://en.wikipedia.org/wiki/Wheatstone_bridge

For the implementation of this principle, I used 4 bulbs in the place of resistors. Also add on 1 bulb in the place of Galvanometer.

• Cardboard
• A4 white paper Sheet
• Plastic box (size= 16cm X 8cm X3.5cm)
• Adhesive Glue
• Five Bulbs from Decorative light strip
• Female double row header connectors (total = 5)
• ON/OFF Switch
• One 9V battery
• Wires
• Thermocol Sheet
• Screws

Tools:

• Scissors
• Wire Stripping Plier
• Vernier Caliper Digital
• Soldering Iron
• Screwdriver
• Cardboard Cutter
• Adhesive tape
• Printer
• Multimeter

Take a print out of the circuit diagram of Wheatstone Bridge (PDF Attached)on a white paper sheet. Cut the circuit diagram (Size = 16cm X 8cm).

Also cut a cardboard of same size as that of circuit diagram. Paste the Circuit diagram on the cardboard with the help of adhesive glue. At the back of cardboard paste the thermocol sheet and cut the extra material.

Now measure the dimensions of female header connector with the help of Vernier caliper. According to the size of header connectors cut the cavities on the resistor’s symbol and for the ON/OFF Switch shown on circuit diagram.

Insert the header connectors in each cavities (total = 5). Put the header connectors and Switch in the cavities and paste them with the help of adhesive glue. For the battery, cut a profile according to size of the battery. Now solder the wires of header connectors, switch and battery wires according to the given circuit diagram.

To support the battery paste a cardboard piece at the bottom of battery holder profile. Put the whole circuit assembly in the plastic box (size= 16.2cm X 8.2cm X3.5cm) and you can add a label and instructions to use the circuit inside the box. Insert the bulbs in the header connectors keeping in mind the positive and negative terminal of the battery. Now, put the battery in the circuit.

Switch ON the circuit. The four Bulbs are glowing and the bulb at the place of galvanometer not glowing as there is no current flowing through it. It means the Wheatstone Bridge is balanced. It satisfies the equation given that the ratio of resistances are equal.

If we remove the bulb from R3 position, we can see that the current starts flowing through the bulb (very dim light) in the place of Galvanometer. And the bulbs at the position of R2 and R4 glows with dim light (low value of current). If we remove another bulb from R2 position, we can see that the small amounts of current in the remaining bulbs.

This shows that the Wheatstone Bridge is Unbalanced and does not follow the equation as the ratio of resistances are not equal.

This is a simple Demonstration of how Wheatstone Bridge works. I hope you like this Project.

Thanks for Watching.