Using Melzi's expansion header to fix heated bed temp circuit.


I short-circuited my Melzi Ardentissimo's heated bed temp sensor circuit by accidentally connecting it to the actual heated bed circuit.  In an effort to repair it, I built an alternative heated bed sensor circuit use an analogue pin in expansion header, following instructions found on another forum.
Here's what was done:
Unfortunately this setup doesn't seem to work, as the voltage readings between any of the analogue pins and GND are of less than 1V Meanwhile, if the healthy extruder temp circuit is similarly tested, the reading was of around 4.5V.
Also, the thermistor is fine and the new wires were successfully tested for continuity.

Anyway, before I toss this board out and get a new one, I'd like to exclude any software configuration mistakes I might have made:
  • Could the problem be in the firmware configuration?
  • Do I have to, somehow, enable the expansion header pins in the firmware before using them?
  • Are there any tests I could run through repetier-host to determine if the pin I used was also fried by the short-circuit?
Thank you in advance.


  • Changing pin number is all that would be needed. Please note that analog and digital pin numbers differ!
    You should also be able to measure with a multimeter the voltage at the adc changing if thermistor gets hot.
  • From what I could gather, in Melzi's case, the pin labelled A4 is analogue pin 4, right?

    If that is correct, then it's properly configured.

    Before measuring the voltage at the adc (those are some tiny, closely spaced terminals! :D ), I failed to mention that:
    • The temperature curve for the heat bed temp shows ~900°C at all times
    • The voltage in other parts of the circuit and the displayed temperature don't noticeably change when the thermistor is heated.
    Could measuring the voltage of pin 4 at the adc provide new information? I mean, I wouldn't be able to replace it.
  • 900°C indicates a short normally. So you should measure thermistor resistance of bed thermistor if it simply has a short and not 100K at room temperature. Extreme temperatures indicate that pin has 0 or 5v.

    Since I'm no melzi fan (flash is too small for current firmware) I would consider a bigger board as replecement if it looks like another board defect.

    A4 is analog 4 yes.
  • Thank you for helping me!

    Since the thermistor is not shorted and its resistance is indeed ~100KΩ at room temp, I'm going to assume that I don't have the skills required to detect/repair whatever it's causing the problem.

    I just ordered a new board and (for now at least) I'll stick to the only board I know to eliminate new variables. But, for future reference, which board would you recommend?

    Thank you once again.
  • Due based boards like RADDS board with EEPROM replacement. Some boards have no eeprom making it hard to changes vars on the fly or store autoleveling results, so eeprom is an important feature for the board.
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