Faking a thermistor with a test rig


#1

Thought I'd share this. Today I wanted to do some more realistic testing beyond the Virtual Printer which is part of OctoPrint on a test rig I'm using. I had a spare controller board for my printer so I thought I'd use it with OctoPrint.

Unfortunately, OctoPrint has a minimum-temperature check that it wants to see upon handshaking with the printer. Since the controller board was naked, the Terminal screen was showing a temperature reported of -14ºC so OctoPrint didn't like that.

Having researched the configuration.h file from the Marlin fork, I knew that it's a 100K thermistor. So I soldered a 100K ohm resistor to a connector, traced where that was on the pins of the actual printer and made this happy; OctoPrint now reports room temperature and continues to "print".

Of course, I had to run the gcode through my "dry run" code in my GcodeEdit (Go language) program to remove all the heatup and extrusions commands but I've got a great test rig now for this.


#2

Wrong :wink: OctoPrint has no such thing. The printer's firmware however usually has.


#3

I stand corrected.  


#4

Was about to say, mintemp and maxtemp are firmware safety features. The firmware will lock up and kill itself if it detects a temperature outside of certain ranges as that can indicate either a disconnected, or short circuited thermistor.

Some firmwares have a fake thermistor for testing naked boards, that's what I use. Check your firmware's configuration docs, and it should also have a "report this value to the host" temperature value as well.


#5

Thanks, this ultimately won't matter for the Smoothieboard in the near future. If I do jump into the Marlin config I'll check that out. As this is just a test rig, this should be good enough for the moment, to be honest.