BRILLIANT reply! Thanks! This is part of my thinking - its not an *extremely* hostile environment, its heated and with a fan etc, but its still relatively exposed, and I'd prefer to keep the remote hardware relatively simple. I'd still like to have a PIC at the remote end, if only for the possibilities an RS232 link would give, but I would still like one pair to be a simple 'fail-safe' solution. (We have various other bits built into the cabinet, like a door microswitch - a LED to say "hey, someones opened this door! hopefully they should be there" would be nice, and can be done via RS232. Not sure why I didn't think of remote-powering the PIC! That would be the best solution! So, plan: One pair connected with a relay and wallwart. One end of the pair (at my end) is connected to 5V. If the PIC input is high, its powered, if not, the relay is de-energized and powers off, hence the PIC input is floating. Thats the 'fail-safe' approach. Actually, couldn't I take 5V from the above pair, and use it to power the PIC. What about the differences in ground? Any way to get around that? Probably not.... OK, so: second pair: 5V and ground from my end to the PIC, which has a temperature sensor (and whatever else I come up with) attached to it. Third pair: RS232 link back to a PIC at my end, with opto isolators at the RX ends of each wire. I'd rather not bother with a thermistor, and just attach a SPI one to the PIC and read that via RS232. If the powers off, the least of my concerns is the temperature ;) On 25/10/2007, Dr Skip wrote: > > 1 pr could run power, and yes, the fault possibilities are many, which is > why > simple is better. > > I should have also mentioned that you could just run a wall wart to power > a > relay and have it short the line. Then, all of your 'electronics' and > power are > on your side... open=power off, closed=power on. I'm not sure why a > resistor > isn't good enough though... It's a perfect solution, especially since you > have > serial resistance in the cable. > > To measure the cable resistance, short one end and measure across the > wires at > your end... > > You could also just put a resistive divider on the power, or out of a > transformer, or some combination of both, to divide by 10 (for example), > and > put it right on the line. Now run a voltmeter at your end and monitor > voltage > directly... If cable resistance is high, measure across a resistance at > your end. > > Use 2 pr and set up one with a thermistor of appropriate range (accounting > for > cable R). Now everything at the remote end is passive and simple, you get > temp > and V, and you can do what you want at your end and upgrade to your > delight any > time you want to... ;-) DMM today, PIC and graphing and alarms and sirens > tomorrow... Even both (for when the fancy stuff breaks). And the remote > end is > simple and pretty foolproof. > > If you remote power it, and direct reading the thermistor is too rough > (the > cable R is too great), remote power a 555 with the thermistor as part of > the RC > circuit at the other end. Send the output down the line and measure > frequency. > [make higher temps=higher freq, audio range, and it's also an overtemp > siren HAHA] > > Put a scope on the line and look at waveform quality on the first pr this > way > too. It'll impress the boss. ;-) > > This project just BEGS for simple hardware... > > > Ben Wheare wrote: > > I did consider a PIC on one end, and a RS232 link to monitor > temperature, > > but what about when the power goes? > > I'd (ideally) need to build in some sort of battery backup, and with the > > wide variations of temperatures (just below freezing in the winter, all > the > > way up to 60/70degC in the summer), a battery isn't a really good choice > > IMO. > > Not only that, if I didn't get a response from the remote PIC, it could > be > > due to a dodgy cable, wiring fault, the PIC might've hung, etc etc. > > Of course, I have 8 pairs, no reason why I couldn't use one as a simple > AC > > monitoring system using a wallwart, and another one connected to a PIC > for > > serial comms. > > > > Some questions: > > 1) Could I use an RS232 link at say 300 baud over such a link - is there > > anything I would need to take into account? Can I seriously just connect > > each end of the pair to the TX and RX pins on two PICs, or would using > say a > > MAX232 as well help? > > 2) I'm not alltogether keen on putting a wallwart plus resistor straight > to > > the cable for the simple AC monitoring, I'd prefer to have something > > definitely limiting the current (yes I know this is what a resistor > does), > > but considering measuring the resistance of the cable is a bit > impratical, > > I'd rather just put a semiconductor that is designed to limit output > current > > to some value and use that between the AC and the cable. > > > > > > On 25/10/2007, Jinx wrote: > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist