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How to Build a Bridge balance indicator Circuit Diagram. This Indicator circuit provides an accurate comparison of two voltages by indicating their degree of balance (or imbalance). Detecting small variations near the null point is difficult with the basic Wheatstone bridge alone. Amplification of voltage differences near the null point will improve circuit accuracy and ease of use. The 1N914 diodes in the feedback loop result in high sensitivity near the point of balance (R1/R2 = R3/R4). 

When the bridge is unbalanced the amplifiers closed-loop gain is approximately Rp/r, where r is the parallel equivalent of Rl and R3. The resulting gain equation is G = RF(1/R1 + 1/R3). During an unbalanced condition the voltage at point A is different from that at point B.

This difference voltage (V^), amplified by the gain factor G, appears as an output voltage, As the bridge approaches a balanced condition (R1/R2 = R3/R4), V^ approaches zero. As V^ approaches zero the 1N914 diodes in the feedback loop lose their forward bias and their resistance increases, causing the total feedback resistance to increase. This increases circuit gain and accuracy in detecting a balanced condition. The figure shows the effect of approaching balance on circuit gain. The visual indicator used at the output of the OP-07 could be a sensitive voltmeter or oscilloscope.