Applications such as photoelectric control, temperature detection and moisture sensing require a circuit that can accurately detect a given resistance ratio. A simple technique that uses an op amp as a sensing element can provide 0,5% accuracy with low parts cost. The reed-relay contacts close when the resistance of the sensor Rp equals 47% of the standard Rs. Adjusting either Rl or R2 provides a variable threshold; the threshold is controlled by varying R3. For the most part, the type of resistors used for Rl and R2 determines the accuracy and stability of the circuit.
With metal-film resistors, less than 0.5% change in ratio sensing occurs over the commercial temperature range (0 to 70 C) with ac input variations from 105 to 135 V.