The problem comes with making it trip a relay to shut itself off. I guess with would have to be a normally open circuit that gets held closed while conditions are a-okay and goes open when thermistor screams or loses power/fails to read.

then you have a relay that is being held closed for hours and could burn out at any time, thus shutting down a perfectly good print. probably better to just monitor your printer occasional, maybe a simple audible alarm to alert you of a condition in need of your attention.

lets face it, this technology is far from mature enough to hit print and forget like a laserjet. lol. i check on mine every 20-30 minutes, and i dont start a print if i know i have to go to sleep/out soon. i have had a few filament breaks so far, so i am designing and will be incorporating some sort of micro roller switch alarm bolt on for the mk4 extruder, complete with a coil spring filament strain relief for dealing with those pesky brittle filaments. could work an overtemp alarm for the PEEK into it with relative ease. but that project is superseeded by several others at the moment, like getting my rostock up and printing as a backup should anything happen to the SD3. haha.

Simpler electromechanical solution, which thus removes the need to ensure that a watchdog is inplace on the Sang to ensure the firmware hasn't locked up causing the issue....

http://www.ebay.com/itm/1-pcs-Temperatu … 0613907818

Stick it against the PEEK barrel a bit further up than the nozzle end, inline with the heater cartridge.

http://www.ebay.com.au/itm/KSD301-120-C … 0915910431

Stick that one inline on your Bed.

Those are pretty big. I'm not sure if they'd get to 200C internally, even with the outside of the PEEK at 250C+. What you really care about is the temperature at the threads of the PEEK, and the only way to get that is drill the PEEK, insert thermistor.

Simpler electromechanical solution, which thus removes the need to ensure that a watchdog is inplace on the Sang to ensure the firmware hasn't locked up causing the issue....

http://www.ebay.com/itm/1-pcs-Temperatu … 0613907818

Stick it against the PEEK barrel a bit further up than the nozzle end, inline with the heater cartridge.

http://www.ebay.com.au/itm/KSD301-120-C … 0915910431

Stick that one inline on your Bed.

Those are convenient temperatures! Great idea!

I like this one for the hotend, I would put it right on the brass barrel if there is room.

http://www.ebay.com/itm/2PCS-KSD301-Tem … 1c28219e18

http://www.ebay.com/itm/1-pcs-Temperatu … 0613907818

Stick it against the PEEK barrel a bit further up than the nozzle end, inline with the heater cartridge.

http://www.ebay.com.au/itm/KSD301-120-C … 0915910431

Stick that one inline on your Bed.

The Sanguinololu has extra analog pins that you might be able to use for an additional thermistor.  I don't know how you would incorporate that into firmware however.

Even better!

It'd just be a few lines. You'd probably want a resistor and a capacitor - the Sanguinololo uses a 1-pole RC filter to smooth things out.

Firmware is pretty easy - look for the code that holds the overtemperature fault trip, and add a line to check your new thermistor.

Perhaps the techno wizards on this forum could come up with a solution to trip the dc to the board, as you suggest. I think it's a great way to handle the problem.

This is starting to sound like a frankenstein but I think it will work. What are the traces on the board rated for? Can I feed the entire Sanguinololu board through a mosfet? The mosfet is rated for 30A or something sick like that.