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