Resistance ratio detector circuit

Power supply control equipment, temperature detection equipment and humidity detectors require a circuit that can accurately detect the presence of a resistive ratio. A simple method is to use an operational amplifier as the sensor, which has an accuracy of up to 0.5% and the cost of parts is very low. The contacts of the reed relay are closed when the resistance of the inductor Rp is equal to 47% of the standard inductor R3.