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Decoding two wire sensor circuit

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Jules

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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.pdf
 
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

There are many ways of sending data down the power wires, this is just one of them.

I don't think you could use a 74C14 to convert it back?, you need a frequency to voltage converter, a 555 in monostable mode can do this, or you can get specific frequency/voltage converter chips. You would then require the control logic to make it all work - although you don't want to use a PIC, it would simplify it to a single chip, both measuring the frequency and controlling it all.
 
remote humidity sensor

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?
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?
 
Re: remote humidity sensor

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?

What do you ultimately want to do with the data?, just display it on a voltmeter?, or something more useful?.
 
Humidity sensor data

I want a 0 - 1 voltage output to correspond to RH %, where 1 volt is 100%. This is applied to one channel of a chart recorder, so I can get either daily, weekly or monthly hard copy records. I already have temp on a separtate channel - the two variables are often recorded together on the same chart. The problem with RH is that the sensor can't be at the end of long wires, because of capacitance. The sensor I have in mind, HS1101 (Humirel) varies from 180pf to 220 pf for a 100% change in humidity. The application notes use it as a variable cap in a frequency circuit, which is either measured directly or changed to voltage. This can use a long wire. The remote sensor circuit I found and posted seemed to fit what I needed, but has the control end left open. (separating power from measuring) This is where my problem lies. If there is a non-PIC way to go. I'd like to try it out.
 
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