If you send a DC voltage signal to carry the information for a long distance, then the signal will suffer attenuation with unpredictable characteristic introduced by the medium, for example the cable resistance that vary with temperature changes. If you carry the information with frequency, not a DC voltage level, then the frequency will be stable event it’s amplitude characteristic is distorted by nonlinear process of the medium. More interesting, you can send the frequency information over a general voice radio transmitter to send the information for much longer distance. The circuit described in the schematic diagram below produce an audio frequency which varies proportional to temperature being measured. The temperature sensing uses LM35 sensor chip.
If you don’t need the optical isolation then you can omit 4N28 opto-coupler, and connect 1k resistor directly to pin 3 of LM131 IC, and tap the output signal from that junction. The output will produce 20Hz to 1500Hz signal corresponding with 2-150 Celsius degree. This frequency signal carrying temperature information should be easily transmitted using narrow-band standard voice communication radio. [Circuit's schematic diagram source: National Semiconductor Application Notes]
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