The circuit shown in the figure can monitor resistance in a noisy or otherwise hostile environment. A 1:1 transformer provides isolation and a good common-mode rejection ratio (CMRR). The resistance across the secondary winding is reflected to the primary, where it forms a voltage divider with resistor R1 (see the figure). This divider produces a reducedamplitude clock signal that's coupled through C2, rectified by D1 and D2, and filtered by C3 and R2. The result, delivered at the inputs of the quad voltage monitor (IC1), is a dc voltage that varies with the monitored resistance as shown in the table.
The monitored resistance can be in the form of a single resistor, like an NTC resistor for measuring temperature, or a set of resistors and switches, as shown in the example circuit. The slow response of the rectifier and filter helps to mask the effect of any switch bounce.
The component values shown support a 5-V, 0.1-MHz to 1-MHz clock signal and resistance values from 0.5 kO to 10 kO. IC1's voltage-monitoring thresholds (1.8 V, 2.5 V, 3.3 V, and 5 V) are suitable for this application, and T1 can be any 1:1 pulse transformer with an inductance of approximately 0.5 mH. You can decode the IC1 output with suitable discrete logic, or feed it to a microcontroller as required by the specific application.