A bridge that notices small changes
Balance two divider midpoints, then introduce a known mismatch.
Difficulty: Breadboard. Estimated build time: about 25 minutes. 4-line bill of materials. Compare supplier offers when available. Make a four-resistor Wheatstone bridge.
Prepare the bench
Use an isolated, regulated 5 V source with a 20 mA current limit and a digital voltmeter with approximately 10 MΩ input resistance. These are bench tools, not included in the shopping list. Disconnect power before changing connections. Confirm breadboard rail continuity and check for supply shorts before powering up.
Build two matching sides
Make two 10 kΩ / 10 kΩ dividers across the same 5 V supply. Call the left midpoint L and the right midpoint R. Each midpoint should be near 2.5 V relative to ground. Then put the voltmeter’s red lead on R and black lead on L; a balanced bridge reads near zero.
Add one small change
Power off. Add 1 kΩ in series with the lower resistor on the right, so that branch is now 11 kΩ. Keep R at the junction of the upper 10 kΩ and lower 11 kΩ. Power on: R is about 2.619 V, L about 2.500 V, and R−L about +119 mV.
Interpret the small difference
The differential signal sits on a common-mode voltage near 2.5 V. Reversing meter leads reverses the sign. Resistor tolerance can cause a nonzero baseline; record it rather than calling it sensor output. Never use an earth-grounded scope clip on a midpoint as if it were ground.