Tricky Circuit

Two dividers, one difference

A bridge that notices tiny changes

A Wheatstone bridge is four resistors arranged as two voltage dividers, each producing a fraction of the supply voltage. Comparing their midpoint voltages reveals a resistance change; the trim resistor adjusts the balance. A kilohm (kΩ) is 1000 ohms, and a millivolt (mV) is one thousandth of a volt.

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EXPERIMENT BENCH

Change the circuit settings, predict the readings, then check a challenge. These are learning models, with the assumptions explained in each lesson.

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Why it works

Ideal unloaded bridge with two fixed upper resistors of 10 kΩ. No measuring-amplifier load, temperature drift or conversion from resistance to force.

Vs is supply, Rs sensor resistance and Rt trim resistance. Vleft = Vs × Rt/(10000 + Rt); Vright = Vs × Rs/(10000 + Rs), using ohms. Difference = Vright − Vleft; zero means balanced, not unpowered.

Difficulty

  1. Build · Build it

    Power on at 5 V with sensor and trim both 10 kΩ. Obtain a difference of 0 V.

    Need a hint?

    Vs = 5 V; Rs = Rt = 10 kΩ → 0 V

  2. Explore · Change one thing

    Keep 5 V and trim 10 kΩ; set sensor to 15 kΩ. Read +0.50 V difference.

    Need a hint?

    Vs = 5 V; Rs = 15 kΩ; Rt = 10 kΩ → +0.50 V

  3. Solve it · Solve the challenge

    At 5 V and sensor 7 kΩ, adjust trim so the difference magnitude is at most 5 mV.

    Need a hint?

    Vs = 5 V; Rs = Rt = 7 kΩ → 0 V

Extra reading and circuit recipes

A bridge that notices tiny changes

A useful pattern

At balance, both midpoints can sit at 2.5 V while their difference is 0 V. A difference measurement can hide the shared voltage.

Beyond the model

Real bridge sensors can turn stretching or temperature changes into resistance changes. An amplifier is often needed to read a small difference.

A useful trap to avoid

Zero bridge difference does not mean zero current through its four resistors.

At balance, the supply doubles and both resistor ratios stay fixed. The difference…

  1. doubles to 5 V
  2. remains 0 V
  3. changes sign
Read more

Both midpoint voltages scale together and remain equal; subtracting them still gives zero.

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