Thing-O-Matic: Improved EC Thermistor Connector Orientation

Given that the SMD pads fell off the HBP circuit board and I must replace the connector, I figured I may as well also replace the remarkably stiff MBI thermistor cable with a much more flexible CD-ROM audio cable. Although the EC end of the MBI cable looks like a standard CD-ROM audio connector, it’s been rewired. No problem: this is not an audio application and I’m going to do exactly the same thing.

The Extruder Controller, however, doesn’t have a matching connector and the recommended attachment involves simply jamming the connector onto the pin header, per this detail cropped from that photo in the MBI assembly instructions:

MBI EC HBP Thermistor Connector Alignment - Detail
MBI EC HBP Thermistor Connector Alignment - Detail

Here’s a better closeup of my EC, taken from the other side:

MBI Extruder Controller - HBP thermistor connector
MBI Extruder Controller - HBP thermistor connector

The header block breaks out the Arduino’s Analog Input pins, with A6 in the front of that photo. From left to right, the pins under the HBP connector are A6 / +5 V / Gnd. Unfortunately, the connector wiring and alignment puts the thermistor signal on the cable shield, with the Gnd and +5 V wires safely tucked inside. This is, shall we say, suboptimal.

The Gnd connection provides a low-impedance connection to the least-noisy part of the circuit, so putting it on the shield tends to prevent the relatively high-impedance signals within from picking up noise. This isn’t always successful, for a number of reasons, but it’s a Good Idea.

Although probably doesn’t make much difference (it’d just add a bit of noise to the HBP temperature signal), but if I’m going to be rewiring it anyway, the cable shield will be at ground potential with the signal  wire inside. Here’s my cable & connector, rearranged to make that so:

EC HBP thermistor connector - revised
EC HBP thermistor connector - revised

The analog audio connector on the back of old-school CD-ROM drives, back before digital audio output from the drives actually worked, had four pins:

  • Left (white) and Right (red) audio channels on the outer pair
  • Ground (black) on at least one of the central pair

So the red wire will be in the far right-hand socket of the connector shell; depress its locking tab, slide it out of the shell, poke it into the socket between the other two wires, push to click, and you’re set. Conveniently, this puts the +5 V supply on the red wire, which is sorta-kinda standard. Your cable colors may vary; pay attention to the actual wiring and ignore the color code!

Tape the connector in place (with the empty socket now toward the board edge) to prevent the tangle of wires in the Thing-O-Matic’s electronics bay from dislodging it at an inopportune moment:

EC HBP thermistor connector - secured
EC HBP thermistor connector - secured

Admittedly, that arrangement still tucks the +5V wire right next to the signal wire inside the shield, but it’s a step in the right direction.

You could flip the MBI cable around, too, as long as you also rearranged the pins at the HBP end to match.

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