Decoding two wire sensor circuit

Jules said:

I have found this unique circuit which uses the same pair of long leads (<1000m) for the power supply and frequency data transfer from a capacitative humidity sensor. The text with it says to switch off the power when making a measurement., then reconnect it. Is this possible automatically? I don't want to use PIC to do this. Ideally, I'd like the power supply to be on for 55 seconds every minute, with data collection (frequency) in the next 5 seconds.
I also want the frequency to be converted back to a voltage measurement.
Can a 74C14 do this DA conversion (the reverse of the AD conversion the remote humiity circuit is doing)

Please see this corcuit:

https://www.electro-tech-online.com/custompdfs/2005/07/40402di-1.pdf

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Jules said:

You mentioned there were other ways of having just two wires for power and signal. Are there any links to these circuits, or are they design ideas only?

The classic example is TV aerial amplifiers, the amplifier is mounted next to the aerial on the roof, and it's power supply is down near the TV. The UHF signal comes down the coax, AC coupled at each end with capacitors (blocking the DC, and passing the AC), the DC is fed to the same coax inner via a small RF choke (passes the DC, blocks the AC) - at the amplifier end a similar choke feeds the DC supply to the amplifier.

The Sky Digiboxes do something similar as well, feeding 9V up the coax to feed a 'magic eye', and feeding the UHF signal UP the coax to the TV, but also sending the IR remote control data back DOWN the coax, modulated on an RF carrier. So, power and signal UP the cable, and a seperate signal DOWN the cable.

I have no experience of PIC, so that's why I hope there is a non-PIC control circuit to switch between power in and signal out! I wouldn't know where to start, and have no PIC programming. If it would be a hugely complicated CMOS switching circuit or similar alternative, then I guess I'd need the microprocessor solution, but the simple remote powered analogue sensor, producing a digital output on very long wires is quite an interesting idea and I'd like to run with it a bit.
Any thoughts?

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